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Java - Basic Syntax
• When we consider a Java program, it can be defined as a collection of objects that communicate via
invoking each other's methods. Let us now briefly look into what do class, object, methods, and
instance variables mean.
• Object − Objects have states and behaviors. Example: A dog has states - color, name, breed as well
as behavior such as wagging their tail, barking, eating. An object is an instance of a class.
• Class − A class can be defined as a template/blueprint that describes the behavior/state that the object
of its type supports.
• Methods − A method is basically a behavior. A class can contain many methods. It is in methods
where the logics are written, data is manipulated and all the actions are executed.
• Variable: A variable is the name for a memory location where you store some data. A variable in
Java must be declared (the type of variable) and defined (values assigned to a variable) before it can
be used in a program.
• Data type: A data type defines which kind of data will store in variables and also defines memory
storage of data. There are two kinds of data types User defined data types and Standard data types.
• Return type: Return type is used to terminate main( ) Method and causes it to return the value to its
type. If a Method is defined as having a return type of void, it should not return a value. If a Method
is defined as having a return type other than void, it should return a value.
Syntax
public class FirstJavaProgram {
public static void main(String[] args){
System.out.println("This is my first program in java");
}//End of main
}//End of FirstJavaProgram Class
Every line of code that runs in Java must be inside a class. In our example, we named the class
FirstJavaProgram. A class should always start with an uppercase first letter.
Note: Java is case-sensitive: " FirstJavaProgram " and " firstJavaprogram " has different meaning.
The name of the java file must match the class name. When saving the file, save it using the class name and
add ".java" to the end of the filename.
Let we see each line of code one by
public class FirstJavaProgram {
This is the first line of our java program. Every java application must have at least one class definition that
consists of class keyword followed by class name. When I say keyword, it means that it should not be
changed, we should use it as it is. However the class name can be anything.
We have made the class public by using public access modifier, we will cover access modifier in a separate
post, all you need to know now that a java file can have any number of classes but it can have only one
public class and the file name should be same as public class name.
The next is main method
public static void main(String[] args) {
This is our next line in the program, lets break it down to understand it:
public: This makes the main method public that means that we can call the method from outside the class.
static: We do not need to create object for static methods to run. They can run itself.
void: It does not return anything.
main: It is the method name. This is the entry point method from which the JVM can run your program.](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-7-320.jpg)
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(String[] args): Used for command line arguments that are passed as strings. We will cover that in a separate
post.
Next the method
System.out.println("This is my first program in java");
This method prints the contents inside the double quotes into the console and inserts a newline after.
Note: The curly braces {} marks the beginning and the end of a block of code.
Note: Each code statement must end with a semicolon
Self-Check -2 Written Test
Directions: Answer all the questions listed below. Use the Answer sheet provided in the next
page:(2 pts each)
1. Which one of the following is true about java programing language?
A. platform independent byte code can be carried to any platform
B. Java supports multithreading
C. is object oriented programing language
D. all
2. The correct phases of java program execution
A. Writing of the program execute the bytecode generated by compiler Compile
the program by javac compiler
B. Writing of the program Compile the program by javac compiler execute the
bytecode generated by compiler
C. Compile the program by javac compiler Writing of the program execute the
bytecode generated by compiler
D. execute the bytecode generated by compiler Writing of the program Compile
the program by javac compiler
3. Java feature that allows concurrent execution of two or more parts of a program for
maximum utilisation of CPU.
A. Portable B. not complex C. Multithreading D. platform independent
4. The type of code generated after JDK compiles the java source code and save it into
a .class file to be can be executed by JVM.
A. Source code B. Byte code C. Machine code D. B&C
5. Which one is true
A. In order to create, compile and run Java program you would need JDK installed on
your computer.
B. Java Development Kit(JDK) includes JRE (Java Runtime Environment), compilers
and various tools like Java debugger](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-8-320.jpg)
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C. Java Runtime environment (JRE) contains Java virtual Machine(JVM), class
libraries but not compiler and debugger
D.All
6. Java - Basic Syntax is
A. Accessmodifier Class ClassName{
Public static Void Main(String[] args){
//body
}}
B. Accessmodifier Class MethodName(){
Public static Void Main(String[] args){
//method
}}
C. Accessmodifier DataType variableName{
Public static Void Main(String[] args){
//variables
}}
D. All
7. Which of the following option leads to the portability and security of Java?
A. Bytecode is executed by JVM
B. The applet makes the Java code secure and portable
C. Use of exception handling
D. Dynamic binding between object
1.3 Understanding Data-Types, Operators and Expressions
Data type: A data type defines which kind of data will store in variables and also defines memory storage of
data.
There are two kinds of data types User defined data types and Standard data types.
The Java language allows you to create and use data types other than the fundamental data types. These
types are called user-defined data types.
In Java language, there are four main data types
• char
• int
• float
• Boolean
1. Character data type
A keyword char is used for character data type. This data type is used to represents letters or
symbols.
A character variable occupies 1 byte on memory.](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-9-320.jpg)
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1. Sequence structure
2. Selection structure
3. Loops/ Repetition/ Iteration
Sequence Structure in java
The sequence structure is built into java statements that execute one after the other in the order in which they
are written—that is, in sequence. Break and continue statements are example of sequence control structure.
The ‘break’ statement causes an immediate exit from the inner most ‘while’, ‘do…while’, ‘for loop’ or from
a ‘switch- case’ statement.
If you are using nested loops (ie. one loop inside another loop), the break statement will stop the execution of
the innermost loop and start executing the next line of code after the block.
Flow Diagram:
Example:
public class FirstJavaProgram {
public static void main(String[] args){
int a = 10; // Local variable declaration:
do{ // do loop execution
System.out.println("value of a: "+a);
a = a + 1;
if( a > 15)
{
break;
}
}while( a < 20 );
return 0;
}
When the above code is compiled and executed, it produces following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-14-320.jpg)
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The goto statement provides an unconditional jump from the goto to a labeled statement in the same
function.
Syntax of a goto statement in java:
goto label;
..
.
label: statement;
Where label is an identifier that identifies a labeled statement. A labeled statement is any statement that is
preceded by an identifier followed by a colon (:).
Flow Diagram:
Example:
public class FirstJavaProgram {
public static void main(String[] args){
int a = 10; // Local variable declaration:
LOOP:do // do loop execution
{
if( a == 15)
{
a = a + 1;// skip the iteration.
goto LOOP;
}
System.out.println("value of a: " +a );
a = a + 1;
}while( a < 20 );
return 0;
}
When the above code is compiled and executed, it produces following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 16](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-15-320.jpg)
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value of a: 17
value of a: 18
value of a: 19
The continue statement is similar to ‘break’ statement but instead of terminating the loop, the continue
statement returns the loop execution if the test condition is satisfied.
Flow Diagram:
Example:
public class FirstJavaProgram {
public static void main(String[] args){
int a = 10; // Local variable declaration:
do{ // do loop execution
if( a == 15)
{
a = a + 1; // skip the iteration.
continue;
}
System.out.println("value of a: " +a );
a = a + 1;
}while( a < 20 );
return 0;
}
When the above code is compiled and executed, it produces following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 16
value of a: 17
value of a: 18
value of a: 19
Selection statements in java](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-16-320.jpg)
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There are basically two types of control statements in java that allows the programmer to modify the regular
sequential execution of statements. They are selection and iteration statements.
The selection statements allow to choose a set of statements for execution depending on a condition.
If statement and switch statement are the two selection statements.
If ... else statement:
Syntax: if (expression or condition)
{
Statement 1;
Statement 2;
}
Else
{
Statement 3;
Statement 4;
}
If the condition is true, statement1 and statement2 is executed; otherwise statement 3 and statement 4 is
executed.
The expression or condition is any expression built using relational operators which either yields true or false
condition.
Flow Diagram:
If the condition evaluates to true, then the if block of code will be executed otherwise else block of code
will be executed. Following program implements the if statement.
public class FirstJavaProgram {
public static void main(String[] args){
int num=12;
if ((num%2) == 0)
System.out.println("is a even number ");
else
System.out.println("a number is odd");
}
The above program accepts a number from the user and divides it by 2 and if the remainder (remainder is
obtained by modulus operator) is zero, it displays the number is even, otherwise it is odd.
you must use the relational operator ‘ ==’ to compare whether remainder is equal to zero or not.](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-17-320.jpg)
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Switch statement
One alternative to nested if statement is the switch statement which allows a variable to be tested for equality
against a list of values. Each value is called a case, and the variable being switched on is checked for each
case.
Syntax:
Switch (variablename)
{
case value1: statement1; break;
case value2: statement2; break;
case value3: statement3; break;
default: statement4;
}
Flow Diagram:
Example:
public class
FirstJavaProgram {
public static void
main(String[] args){
int
choice=3;
switch
(choice)
{
case 1:
System.out.println(""First
item selected!" );
break;
case 2:
System.out.println(""second item selected!" );
break;
case 3: System.out.println(""third item selected!" );
break;
default: System.out.println("Invalid selection!!" ); }
}}
Iteration statements in java
Iteration or loops statements are important statements in java, which helps to accomplish repeatitive
execution of programming statements.
There are three loop statements in java. They are - while loop, do while loop and for loop.
While Loop
The while loop construct is a way of repeating loop body over and over again while a certain condition
remains true. Once the condition becomes false, the control comes out of the loop. Loop body will execute
only if condition is true.
Syntax: While (condition or expression)
{](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-18-320.jpg)
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Statement1;
Statement 2;
}
Flow Diagram:
The flow diagram indicates that a condition is first evaluated. If the condition is true, the loop body is
executed and the condition is re-evaluated. Hence, the loop body is executed repeatedly as long as the
condition remains true. As soon as the condition becomes false, it comes out of the loop and goes to display
the output.
Example:
public class FirstJavaProgram {
public static void main(String[] args){ {
int n=10, i=1, sum=0;
while(i<=n)
{
sum=sum+i;
i++;
}
System.out.println( " sum of the series is "+ sum);
}
Do …While loop
The do... while statement is the same as while loop except that the condition is checked after the execution of
statements in the do..while loop.
Syntax: do{
Statement;
While (test condition);
Statement;
}
Flow Diagram:](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-19-320.jpg)
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Its functionality is exactly the same as the while loop, except that condition in the do-while loop is evaluated
after the execution of statement. Hence in do..while loop, Loop body execute once even if the condition is
false.
Example:
public class FirstJavaProgram {
public static void main(String[] args){
int n=10, i=1, sum=0;
{
int n, i=1, sum=0;
do{
sum=sum+i;
i++;
} while(i<=n);
System.out.println( " sum of the series is "+ sum);
}}
For loop:
The for loop statements (loops or iteration statement) in java allow a program to execute a single statement
multiple times (repeatedly) , given the initial value, the condition, and increment/decrement value.
Java for loop is used to run a block of code for a certain number of times. The syntax of for loop is:
for (initialExpression; testExpression; updateExpression) { // body of the loop }
Here,
• The initialExpression initializes and/or declares variables and executes only once.
• The condition is evaluated. If the condition is true, the body of the for loop is executed.
• The updateExpression updates the value of initialExpression.
syntax:
for ( initial value ; test condition ; increment/decrement)
{
Statement;
}](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-20-320.jpg)
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Flow Diagram:
A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a
specific number of times.
Example: public class FirstJavaProgram {
public static void main(String[] args){
int n=10, i=1, sum=0;
{
for(i=1;i<=n; i++)
{
sum=sum+i;
}
System.out.println( " sum of the series is "+ sum);
}}
Nested Loop in Java
If a loop exists inside the body of another loop, it's called a nested loop. Here's an example of the
nested for loop.
Syntax for Nested For loop:
for ( initialization; condition; increment ) { for ( initialization; condition; increment ) { // statement of
inside loop } // statement of outer loop }
// outer loop
for (int i = 1; i <= 5; ++i) {
// codes // inner loop
for(int j = 1; j <=2; ++j) {
// codes } .. }
class Main
{ public static void main(String[] args) {
int weeks = 3;](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-21-320.jpg)
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When you build modules, you have the ability to hide the internal implementation from the consumers of
your module. The hidden implementation details are usually referred to as being encapsulated, and what you
expose to the consumers of your module is usually called the interface of your module.
Although Java developers have leveraged many different patterns and best practices over the years in order
to write and structure modular and maintainable code, the language has never had native support to create
modular units and build modular applications, until Java 9. With Java 9, Java developers now have the ability
to create smaller units of code with a new construct called Java modules that they can group together like
building blocks in order to compose larger applications. In addition to introducing this feature to the
language, Java 9 also comes with what is probably the biggest overhaul to the core Java code bases itself.
The Java Runtime Environment (JRE) and the Java Development Kit (JDK) have been rewritten to use
the concepts of modularity so that the core Java Platform itself is modularized.
When learning about Java 9 module features, it's important to understand what those new features add to the
language when compared to the other features the language already has. Can't we write well organized code
in Java 8? In fact, one of the benefits of object-oriented programming is indeed the idea of breaking down
functionality into sub-units called objects or classes. We've been writing code like this in Java since version
1. Every Java class contains a portion of the overall application functionality that happens to belongs
together. We have the ability to encapsulate some functionality as internal to a class (as private) and some
others as external (or public).
And then there's something in between with protected, thanks to the concept of packages.
1.6 Using arrays and arrays of objects
An Array is a collection of similar data items which shares a common name within the consecutive memory.
An Array can be any data type, but it should be the collection of similar items.
Each item in an array is termed as ‘element’, but each element in an array can be accessed individually.
The number of element in an array must be declared clearly in the definition.
The size or number of elements in the array can be varied according to the user needs.
Syntax for array declaration:
DataType ArrayName [number of element in the array]
Example: int a[5]; - ‘int’ is the data type.
- ‘a’ is the array name.
- 5 is number of elements or size.
int a[5] means a[0], a[1], a[2], a[3], a[4] or
int Int int int int
a[0] a[1] a[2] a[3] a[4]
- 0, 1, 2, 3, 4 are called subscript which is used to define an array element position and is called
Dimension.
- Array index in java starts from 0 to n-1 if the size of array is n.
An individual element of an array is identified by its own unique index (or subscript).
NOTE: The elements field within brackets [], which represents the number of elements the array is going to
hold, must be a constant value.](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-24-320.jpg)
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Arrays of variables of type “class” are known as "Array of objects". The "identifier" used to refer the array
of objects is a user defined data type.
Self-Check -6 Written Test
Directions: Answer all the questions listed below. Use the Answer sheet provided in the next
page:(2 pts each)
1. Which of these distinctions of modular programs over non modular are true?
a) They are not that easier to understand and explain
b) They are not easier to document
c) They are easier to change
d) Testing and Debugging is complex comparatively
2. Which of these comes under the Modularity principle?
a) Small modules b) Coupling c) Cohesion d) All of the mentioned
3. An array Index starts with.?
a) -1 b) 0 c) 1 d) 2
4. Objects in a sequence that have the same type, is called
a). Arrays b). Operators c). Functions d). Stacks
5. Choose one of the best option from the following Position number contained within a
square brackets e.g. my_array [5], is referred as ____?
a) Sub-scripts b) Post-scripts c) Elements of an array d) None of them](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-25-320.jpg)
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public members are accessible from anywhere where the object is visible.
Private: Access to only members of the same class
Protected: Access to members of same class and its derived classes
For example our bicycle object, we can create the class as
Example:
class Bicycle {
// state or field
private int gear = 5;
// behavior or method
public void braking() {
System.out.println("Working of Braking");
}
}
In the above example, we have created a class named Bicycle. It contains a field named gear and a method
named braking ().
Here, Bicycle is a prototype. Now, we can create any number of bicycles using the prototype. And, all the
bicycles will share the fields and methods of the prototype.
Note: We have used keywords private and public. These are known as access modifiers.
Variables in Java
Types of Variables in Java
There are three types of variables in Java.
1) Local variable
2) Static (or class) variable
3) Instance variable
Static (or class) Variable
Static variables are also known as class variable because they are associated with the class and common for all the
instances of class. For example, If I create three objects of a class and access this static variable, it would be
common for all, the changes made to the variable using one of the object would reflect when you access it through
other objects.
Example of static variable
public class StaticVarExample {
public static String myClassVar="class or static variable";
public static void main(String args[]){
StaticVarExample obj = new StaticVarExample();
StaticVarExample obj2 = new StaticVarExample();
StaticVarExample obj3 = new StaticVarExample();
//All three will display "class or static variable"
System.out.println(obj.myClassVar);](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-27-320.jpg)
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System.out.println(obj2.myClassVar);
System.out.println(obj3.myClassVar);
//changing the value of static variable using obj2
obj2.myClassVar = "Changed Text";
//All three will display "Changed Text"
System.out.println(obj.myClassVar);
System.out.println(obj2.myClassVar);
System.out.println(obj3.myClassVar); }}
Output:
class or static variable
class or static variable
class or static variable
Changed Text
Changed Text
Changed Text
As you can see all three statements displayed the same output irrespective of the instance through which it is being
accessed. That’s is why we can access the static variables without using the objects like this:
System.out.println(myClassVar); Do note that only static variables can be accessed like this. This doesn’t apply for
instance and local variables.
Instance variable
Each instance(objects) of class has its own copy of instance variable. Unlike static variable, instance variables have
their own separate copy of instance variable. We have changed the instance variable value using object obj2 in the
following program and when we displayed the variable using all three objects, only the obj2 value got changed,
others remain unchanged. This shows that they have their own copy of instance variable.
Example of Instance variable
public class InstanceVarExample {
String myInstanceVar="instance variable";
public static void main(String args[]){
InstanceVarExample obj = new InstanceVarExample();
InstanceVarExample obj2 = new InstanceVarExample();
InstanceVarExample obj3 = new InstanceVarExample();
System.out.println(obj.myInstanceVar);
System.out.println(obj2.myInstanceVar);
System.out.println(obj3.myInstanceVar);
obj2.myInstanceVar = "Changed Text";
System.out.println(obj.myInstanceVar);
System.out.println(obj2.myInstanceVar);
System.out.println(obj3.myInstanceVar); }}](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-28-320.jpg)
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Output:
instance variable
Instance variable
instance variable
instance variable
Changed Text
instance variable
Local Variable
These variables are declared inside method of the class. Their scope is limited to the method which means that You
can’t change their values and access them outside of the method.
In this example, I have declared the instance variable with the same name as local variable, this is to demonstrate the
scope of local variables.
Example of Local variable
public class VariableExample {
// instance variable
public String myVar="instance variable";
public void myMethod(){
// local variable
String myVar = "Inside Method";
System.out.println(myVar); }
public static void main(String args[]){
// Creating object
VariableExample obj = new VariableExample();
/* We are calling the method, that changes the
* value of myVar. We are displaying myVar again after
* the method call, to demonstrate that the local
* variable scope is limited to the method itself. */
System.out.println("Calling Method"); obj.myMethod();
System.out.println(obj.myVar); }}
Output:
Calling Method
Inside Method
instance variable
Java Constructor
Constructorlooks like a method but it is in fact not a method. It’s name is same as class name and it does
not return any value. You must have seen this statement in almost all the programs I have shared above:
MyClass obj = new MyClass();](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-29-320.jpg)
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• You would not find it in your source code(the java file) as it would be inserted into the code during
compilation and exists in .class file.
• This process is shown in the diagram below:
If you implement any constructor then you no longer receive a default constructor from Java compiler.
no-arg constructor:
Constructor with no arguments is known as no-arg constructor. The signature is same as default
constructor, however body can have any code unlike default constructor where the body of the constructor is
empty.
Although you may see some people claim that that default and no-arg constructor is same but in fact they are
not, even if you write public Demo() { } in your class Demo it cannot be called default constructor since you
have written the code of it.
Example: no-arg constructor
class Demo{
public Demo() {
System.out.println("This is a no argument constructor");
}
public static void main(String args[]) {
new Demo(); }}
Output:
This is a no argument constructor
Parameterized constructor
Constructor with arguments(or you can say parameters) is known as Parameterized constructor
Example: parameterized constructor
In this example we have a parameterized constructor with two parameters id and name. While creating the
objects obj1 and obj2 I have passed two arguments so that this constructor gets invoked after creation of obj1
and obj2.
public class Employee {
int empId;
String empName;
//parameterized constructor with two parameters
Employee(int id, String name){
this.empId = id;](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-31-320.jpg)
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parameter1/parameter2 - These are values passed to a method. We can pass any number of arguments to a
method.
Calling a Method in Java
In the above example, we have declared a method named addNumbers(). Now, to use the method, we need to
call it.
Here's is how we can call the addNumbers() method.
// calls the method addNumbers();
Example 1: Java Methods
class Main {
// create a method
public int addNumbers(int a, int b) {
int sum = a + b;
// return value return sum;
}
public static void main(String[] args) {
int num1 = 25; int num2 = 15;
// create an object of Main
Main obj = new Main();
// calling method
int result = obj.addNumbers(num1, num2);
System.out.println("Sum is: " + result); } }
Output
Sum is: 40
Java Method Return Type
A Java method may or may not return a value to the function call. We use the return statement to return any
value. For example,](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-33-320.jpg)
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int addNumbers()
{ ... return sum; }
Here, we are returning the variable sum. Since the return type of the function is int. The sum variable should
be of int type. Otherwise, it will generate an error.
Example 2: Method Return Type
class Main {
// create a method
public static int square(int num) {
// return statement
return num * num; }
public static void main(String[] args) {
int result;
// call the method
// store returned value to result
result = square(10);
System.out.println("Squared value of 10 is: " + result); } }
Output:
Squared value of 10 is: 100
In the above program, we have created a method named square(). The method takes a number as its
parameter and returns the square of the number.
Here, we have mentioned the return type of the method as int. Hence, the method should always return an
integer value.
public void square(int a) {
int square = a * a;
System.out.println("Square is: " + square ); }
Method Parameters in Java
A method parameter is a value accepted by the method. As mentioned earlier, a method can also have
any number of parameters. For example,
// method with two parameters
int addNumbers(int a, int b) {
// code }](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-34-320.jpg)
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// method with no parameter
int addNumbers(){
// code }
If a method is created with parameters, we need to pass the corresponding values while calling the method.
For example,
// calling the method with two parameters addNumbers(25, 15);
// calling the method with no parameters addNumbers()
Example 3: Method Parameters
class Main {
// method with no parameter
public void display1() {
System.out.println("Method without parameter");
}
// method with single parameter
public void display2(int a) { System.out.println("Method with a single parameter: " + a);
}
public static void main(String[] args) {
// create an object of Main
Main obj = new Main();
// calling method with no parameter obj.display1();
// calling method with the single parameter obj.display2(24); } }
Output
Method with parameter Method with a single parameter: 24
Standard Library Methods
The standard library methods are built-in methods in Java that are readily available for use.
These standard libraries come along with the Java Class Library (JCL) in a Java archive (*.jar) file with
JVM and JRE.
For example,
print() is a method of java.io.PrintSteam. The print("...") method prints the string inside quotation marks.
sqrt() is a method of Math class. It returns the square root of a number.
Here's a working example:
Example 4: Java Standard Library Method
public class Main {
public static void main(String[] args) {
// using the sqrt() method System.out.print("Square root of 4 is: " + Math.sqrt(4)); } }
Output:
Square root of 4 is: 2.0
Output
Method with parameter Method with a single parameter: 24
ModelClasses andObjects real-life beings to software entities
Let's take an example of developing a pet management system, specially meant for dogs.
You will need various information about the dogs like different breeds of the dogs, the age, size, etc.
You need to model real-life beings, i.e., dogs into software entities](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-35-320.jpg)
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Know you can design program using oop approach
You can design any program using this OOPs approach.
While creating a class, one must follow the following principles.
• // Class Declaration
• public class Dog {
• // Instance Variables
• String breed;
• String size;
• int age;
• String color;
• // method 1
• public String getInfo() {
• return ("Breed is: "+breed+" Size is:"+size+" Age is:"+age+" color is: "+color);
• }
• public static void main(String[] args) {
• Dog maltese = new Dog();
• maltese.breed="Maltese";
• maltese.size="Small";
• maltese.age=2;
• maltese.color="white";
• System.out.println(maltese.getInfo());
• }
• }
• Out put: Breed is: Maltese Size is:Small Age is:2 color is: white
Types of method
Static Methods
Static methods are methods in java that can be called without actually creating an object of that class.
By declaring a method using the static keyword, you can call it without first creating an object because it
becomes part of the class or a class method(a method that belongs to a class rather than to an object).
They are designed with aim to be shared with all objects created with that class.
One reason why you might want to use a static method is when you want a common operation to be
performed through out the class.
Static methods cannot be overridden.
Syntax:
Static keyword followed by return type, followed by method name.
static return_type method_name();
EXAMPLE:
class Car{
static void display(){
System.out.println(“this is my favorite car”);
}
public static void main(String[], args){
.display();](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-39-320.jpg)
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}
}
Instance Methods
Instance methods are methods that require an object of its class to be created before it can be called.
Instance methods belong to object of the class and not the class. Every object individual object created from
the class has its own copy of the instance methods of that class.
They can be overridden.
class JavaExample{
static int i = 100;
static String s = “sawla";
//Static method
static void display() {
System.out.println("i:"+i);
System.out.println("i:"+s); }
//non-static method
void funcn() {
//Static method called in non-static method
display(); }
//static method
public static void main(String args[]) {
JavaExample obj = new JavaExample();
//You need to have object to call this non-static method
obj.funcn();
//Static method called in another static method
display(); }}
2.3 Object constructionin Java class
Java Objects
An object is called an instance of a class. For example, suppose Bicycle is a class
then MountainBicycle, SportsBicycle, TouringBicycle, etc can be considered as objects of the class.
Here is how we can create an object of a class.
className object = new className();
// for Bicycle class
Bicycle sportsBicycle = new Bicycle();
Bicycle touringBicycle = new Bicycle();
We have used the new keyword along with the constructor of the class to create an object. Constructors are
similar to methods and have the same name as the class. For example, Bicycle() is the constructor of
the Bicycle class.](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-40-320.jpg)
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In this example, we have a parent class Teacher and a child class MathTeacher. In the MathTeacher class we
need not to write the same code which is already present in the present class. Here we have college name,
sawla and does() method that is common for all the teachers, thus MathTeacher class does not need to write
this code, the common data members and methods can inherited from the Teacher class.
class Teacher {
String designation = "Teacher";
String college = “sawla";
void does(){
System.out.println("Teaching");
}
}
public class MathTeacher extends Teacher{
String mainSubject = "Maths";
public static void main(String args[]){
MathTeacher obj = new MathTeacher();
System.out.println(obj.college);
System.out.println(obj.designation);
System.out.println(obj.mainSubject);
obj.does();
}
}
• Output:
• sawla
• Teacher
• Maths
• Teaching
Forms/types of Inheritance:
- Single Inheritance
- Multiple Inheritance
- Multilevel Inheritance
- Hierarchical Inheritance
- Hybrid Inheritance
1. Single inheritance: - If a class is derived from a single base class, it is called as single inheritance.
In Single Inheritance, there is only one Super Class and Only one Sub Class Means they have one to one
Communication between them.
2. Multiple Inheritances: - If a class is derived from more than one base class, it is known as multiple
inheritances.](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-43-320.jpg)
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In java programming the Polymorphism principal is implemented with method overriding concept of java.
Polymorphism principal is divided into two sub principal they are:
Static or Compile time polymorphism
Dynamic or Runtime polymorphism
Static Polymorphism:
Polymorphism that is resolved during compiler time is known as static polymorphism. Method overloading
can be considered as static polymorphism example.
Method Overloading: This allows us to have more than one methods with same name in a class that differs in
signature.
class DisplayOverloading
{
public void disp(char c)
{
System.out.println(c);
}
public void disp(char c, int num)
{
System.out.println(c + " "+num);
}
}
public class ExampleOverloading
{
public static void main(String args[])
{
DisplayOverloading obj= newDisplayOverloading();
obj.disp('a');
obj.disp('a',10);
}
}
Output:
a
a 10
Dynamic Polymorphism
It is also known as Dynamic Method Dispatch. Dynamic polymorphism is a process in which a call to an overridden
method is resolved at runtime rather, thats why it is called runtime polymorphism.
Example
class Animal{
public void animalSound(){
System.out.println("Default Sound");
}
public class Dog extends Animal{
public void animalSound(){
System.out.println("Woof");
}
public class Dog extends Animal{
public void animalSound(){
System.out.println("Woof");](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-46-320.jpg)
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}
public static void main(String args[]){
Animal obj = newDog();
obj.animalSound();
}
}
Output:
• Woof
Since both the classes, child class and parent class have the same method animalSound. Which of the method will be
called is determined at runtime by JVM.
Abstraction
One of the most fundamental concept of OOPs is Abstraction. Abstraction is a process where you show only
“relevant” data and “hide” unnecessary details of an object from the user.
The abstract keyword is a non-access modifier, used for classes and methods:
• Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited
from another class).
• Abstract method: can only be used in an abstract class, and it does not have a body. The body is provided
by the subclass (inherited from).
• Syntax Abstract Class
• abstract class A{
• abstract void myMethod();
• void anotherMethod(){
• //Does something
• }
• }
Note 1: There can be some scenarios where it is difficult to implement all the methods in the base class. In such
scenarios one can define the base class as an abstract class which signifies that this base class is a special kind of
class which is not complete on its own.
A class derived from the abstract base class must implement those methods that are not implemented (means they
are abstract) in the abstract class.
Abstract class cannot be instantiated which means you cannot create the object of abstract class. To use this class,
you need to create another class that extends this abstract class provides the implementation of abstract methods,
then you can use the object of that child class to call non-abstract parent class methods as well as implemented
methods(those that were abstract in parent but implemented in child class).
If a child does not implement all the abstract methods of parent class(the abstract class), then the child class must
need to be declared abstract.
Example of Abstract class and Methods
Here we have an abstract class Animal that has an abstract method animalSound(), since the animal sound differs
from one animal to another, there is no point in giving the implementation to this method as every child class must
override this method to give its own implementation details. That’s why we made it abstract.
• //abstract class
• abstract class Animal{
• //abstract method
• public abstract void animalSound();
• }
• public class Dog extends Animal{
•
• public void animalSound(){](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-47-320.jpg)
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• System.out.println("Woof");
• }
• public static void main(String args[]){
• Animal obj = new Dog();
• obj.animalSound();
• }
• }
• Output:
• Woof
Encapsulation
Encapsulation is the method of combining the data and functions inside a class. This hides the data from being
accessed from outside a class directly, only through the Methods inside the class is able to access the information.
Encapsulation is the term given to the process of hiding all the details of an object that do not necessary to its user.
Encapsulation enables a group of properties, methods and other members to be considered a single unit or object.
Features and Advantages of the concept of Encapsulation:
- Makes Maintenance of Application Easier
- Improves the Understandability of the Application
- Enhanced Security
- Protection of data from accidental corruption
- Flexibility and extensibility of the code and reduction in complexity
Encapsulation example in Java
How to
1) Make the instance variables private so that they cannot be accessed directly from outside the class. You can
only set and get values of these variables through the methods of the class.
2) Have getter and setter methods in the class to set and get the values of the fields.
class EmployeeCount
{
private int numOfEmployees = 0;
public void setNoOfEmployees (int count)
{
numOfEmployees = count;
}
public double getNoOfEmployees ()
{
return numOfEmployees;
}
}
public class EncapsulationExample
{
public static void main(String args[])
{
EmployeeCount obj = new EmployeeCount ();
obj.setNoOfEmployees(5613);
System.out.println("No Of Employees: "+(int)obj.getNoOfEmployees());](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-48-320.jpg)
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}
}
Output:
No Of Employees: 5613
The class EncapsulationExample that is using the Object of class EmployeeCount will not able to get the
NoOfEmployees directly. It has to use the setter and getter methods of the same class to set and get the value.
The class EncapsulationExample that is using the Object of class EmployeeCount will not able to get the
NoOfEmployees directly. It has to use the setter and getter methods of the same class to set and get the value.
So what is the benefit of encapsulation in java programming
Well, at some point of time, if you want to change the implementation details of the class EmployeeCount, you can
freely do so without affecting the classes that are using it.
Introduction to Exception Handling
An exception is a special condition that changes the normal flow of program execution.
Exceptional conditions are things that occur in a system that are not expected or are not a part of normal system
operation. When the system handles these exceptional conditions improperly, it can lead to failures and system
crashes.
Exception handling is the method of building a system to detect and recover from exceptional conditions.
Exceptional conditions are any unexpected occurrences that are not accounted for in a system's normal operation.
Some examples of exceptional conditions:
- incorrect inputs from the user
- bit level memory or data corruption
If these exceptional conditions are not properly caught and handled, they can cause an error or failure in the system.
An exception is a problem that arises during the execution of a program. A Java exception is a response to an
exceptional circumstance that arises while a program is running, such as an attempt to divide by zero.
Exceptions provide a way to transfer control from one part of a program to another. Java exception handling is built
upon three keywords: try, catch, and throw.
throw: A program throws an exception when a problem shows up. This is done using a throw keyword.
catch: A program catches an exception with an exception handler at the place in a program where you want
to handle the problem. The catch keyword indicates the catching of an exception.
try: A try block identifies a block of code for which particular exceptions will be activated. It's followed by
one or more catch blocks
Basic syntax:
try{
}
catch(Exception e){
}
Example:
public class ExceptionDemo {
public static void main (String[] args) {
int a=10;
for(int i=3;i>=0;i--)
try{
System.out.println(a/i);](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-49-320.jpg)
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}catch(ArithmeticException e){
System.out.println(e);
}
}
}
Output:
3
5
10
java.lang.ArithmeticException: / by zero
try block contains the code that might throw an exception. Don’t write anything extra in try as statements after the
exception will not get executed if the exception occurred. Try must be immediately followed by catch or finally
block.
public class ExceptionDemo {
public static void main (String[] args) {
int a=10;
for(int i=3;i>=0;i--)
try{
System.out.println(a/i);
}
}
}
Compile time error:
prog.java:5: error: 'try' without 'catch', 'finally' or resource declarations
try{
^
1 error
The catch block is used to catch the exception thrown by statements in the try block. The catch must follow try else
it will give a compile-time error.
public class ExceptionDemo {
public static void main (String[] args) {
int a=10;
for(int i=3;i>=0;i--)
try{
System.out.println(a/i);
}
System.out.println("between try and catch");
catch(ArithmeticException e){
System.out.println(e);
}
}
}](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-50-320.jpg)
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Compile Time Error:
prog.java:5: error: 'try' without 'catch', 'finally' or resource declarations
try{
^
prog.java:9: error: 'catch' without 'try'
catch(ArithmeticException e){
^
2 errors
Things to Remember
Do not keep any code after the statement which is prone to exception. Because if an exception occurred then it will
straight away jump to the catch or finally block, ignoring all other statements in the try block.
class Main {
public static void main (String[] args) {
try
{
System.out.println(4/0);
//will not get printed
System.out.println("end of try!");
}
catch(ArithmeticException e)
{
System.out.println("divide by 0");
}
}
}
Output:
divide by 0
While catching the exception in the catch block, either you can have directly the class of exception or its superclass.
Example: Exact Exception
class Main {
public static void main (String[] args) {
try{
System.out.println(4/0);
}
//ArithmeticException
catch(ArithmeticException e){
System.out.println("divide by 0");
}](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-51-320.jpg)
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Java uses the Observer-Observable Design Pattern to handle events from GUI components, e.g. button clicks and
mouse movements. Components supply methods to add various types of listeners, e.g. "void
addActionListener(ActionListener al)" and "void
addMouseMotionListener(MouseMotionListener mml)". Multiple listeners can be added and all will
be called when the associated event occurs, though there is no guarantee as to the order in which the installed
listeners will be called.
java.awt.event.ActionListener -- Interface used to detect a simple action on a component, such as clicking a button
or hitting Enter on a text field.
java.awt.event.MouseListener -- Interface used for event listeners watching for mouse click events. This event
allows more detailed capture of the mouse click than an ActionListener event, for instance, distinguishing between a
mouse button pressed vs. a mouse button released events. A convenience class, MouseAdapter, is also available
that no-ops all the methods, freeing the developer to only write the code that overrides the desired methods.
java.awt.event.MouseMotionListener -- Interface used for event listeners watching for mouse motion events, e.g.
dragging. A convenience class, MouseMotionAdapter, is also available that no-ops all the methods, freeing the
developer to only write the code that overrides the desired methods.
How to start a GUI-based program
In general, a GUI-based program will start whenever the first GUI component is shown (visibility set to true).
Typically that would be one of the frames in the system, so one would have a line of code like such:
myFrame.setVisible(true);
This a quickie starts for a GUI program would have a main method like such:
public static void main(String[] args) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
try {
(new MyFrameClass("The title")).setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
(Note: The above code correctly instantiates the frame on the GUI thread as specified by the official Java
requirements. The code can be simplified a little bit by using a lambda function instead of the anonymous inner
class implemenation of a Runnable.)
In Model-View-Controller architectures however, it is not recommended that the view, i.e. the main frame, be
started by simply setting its visibility to true. It is much better to define a "start()" method on the frame or
main view class, where all final initializations and checks are performed and whose last line of code is
the setVisible(true) statement.](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-57-320.jpg)
![Sawla Polytechnic College ICT Database Administration Level IV
Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 58
A typical controller in an MVC wills instantiate the model and the view, connecting them together with their
respective adapters. After all that is done, the controller's last act will be to call the view's start method.
NEVER START THE GUI FROM THE CONSTRUCTOR OF THE FRAME! This is simply asking for
trouble as many of the other components in the system are probably not fulling instantiated or intialized yet. But as
soon as the first frame becomes visible, events will start firing, and those listeners will expect fully operational
objects to be present but won't find them, causing, at best, a flurry of null pointer errors.
Java GUI Example
Now in this Swing Java Tutorial, let's understand GUI with Java Swing examples.
Example: To learn Java GUI programming in this Java GUI tutorial
Step 1) Copy the following code into a text editor like notepad
import javax.swing.*;
class SPTC{
public static void main(String args[]){
JFrame frame = new JFrame("Sawla Polytechnic college");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(300,300);
JButton button = new JButton("SPTC_BUTTON");
frame.getContentPane().add(button); // Adds Button to content pane of frame
frame.setVisible(true);
}}
Step 2) Save, Compile, and Run the code using javac and java commands in to cmd.
Step 3) now let's Add a Button to our frame. Copy following code into an editor from given Java GUI Example
import javax.swing.*;
class SPTC{
public static void main(String args[]){
JFrame frame = new JFrame("Sawla Polytechnic college");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(300,300);
JButton button = new JButton("SPTC_BUTTON");
frame.getContentPane().add(button); // Adds Button to content pane of frame
frame.setVisible(true);
}}
Step 4) Execute the code. You will get a big button](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-58-320.jpg)
![Sawla Polytechnic College ICT Database Administration Level IV
Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 59
Step 5) How about adding two buttons? Copy the following code into an editor.
import javax.swing.*;
class SPTC{
public static void main(String args[]){
JFrame frame = new JFrame("Sawla Polytechnic college");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(300,300);
JButton button = new JButton("SPTC_BUTTON");
frame.getContentPane().add(button); // Adds Button to content pane of frame
frame.setVisible(true);
}}
Step 6) Save, Compile, and Run the program.
Step 7) Unexpected output =? Buttons are getting overlapped.
Java Layout Manager
The Layout manager is used to layout (or arrange) the GUI java components inside a container.There are many
layout managers, but the most frequently used are-
Java BorderLayout
A BorderLayout places components in up to five areas: top, bottom, left, right, and center. It is the default layout
manager for every java JFrame](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-59-320.jpg)
![Sawla Polytechnic College ICT Database Administration Level IV
Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 61
//Usually you will require both swing and awt packages
// even if you are working with just swings.
import javax.swing.*;
import java.awt.*;
class gui2 {
public static void main(String args[]) {
//Creating the Frame
JFrame frame = new JFrame("Sawla Polytechnic Students Chat Frame");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(400, 400);
//Creating the MenuBar and adding components
JMenuBar mb = new JMenuBar();
JMenu m1 = new JMenu("FILE");
JMenu m2 = new JMenu("Help");
mb.add(m1);
mb.add(m2);
JMenuItem m11 = new JMenuItem("Open");
JMenuItem m22 = new JMenuItem("Save as");
m1.add(m11);
m1.add(m22);
//Creating the panel at bottom and adding components
JPanel panel = new JPanel(); // the panel is not visible in output
JLabel label = new JLabel("Enter Text");
JTextField tf = new JTextField(10); // accepts upto 10 characters
JButton send = new JButton("Send");
JButton reset = new JButton("Reset");
panel.add(label); // Components Added using Flow Layout
panel.add(tf);
panel.add(send);
panel.add(reset);
// Text Area at the Center
JTextArea ta = new JTextArea();
//Adding Components to the frame.
frame.getContentPane().add(BorderLayout.SOUTH, panel);
frame.getContentPane().add(BorderLayout.NORTH, mb);
frame.getContentPane().add(BorderLayout.CENTER, ta);
frame.setVisible(true);
}
}](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-61-320.jpg)
![Sawla Polytechnic College ICT Database Administration Level IV
Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 63
setSize(250,250);
setLayout(null);
setVisible(true);
}
//2
public void actionPerformed(ActionEvent e){
textField.setText("Hello SPTC Students");
}
public static void main(String args[]){
new EventHandle();
}
}
Output
Image 1 shows the output of our code when the state of the button was unclicked. Image 2 shows the output after the
button is pressed.
Let’s us continue with event handling in java article and look at the logic behind the code and understand
ActionListener in detail.
First of all, we imported all the important packages required to implement the functionalities required. After
importing packages we implemented ActionListener interface to our class EventHandle.
Now, look at the code I’ve divided it into 2 important parts. Int the first part we are registering our button object
with the ActionListener. This is done by calling the addActionListener( ) method and passing current instance using
‘this’ keyword.
Image 1 image 2](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-63-320.jpg)
![Sawla Polytechnic College ICT Database Administration Level IV
Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 65
setVisible(true);
}
public void actionPerformed(ActionEvent e) {
int a = Integer.parseInt(jt1.getText());
int b = Integer.parseInt(jt2.getText());
int c = 0;
if (e.getSource().equals(jb1)) {
c = a + b;
lbl.setText(String.valueOf(c));
}
}
public static void main(String args[]) {
Test t = new Test();
}
}
Output:
Java - Applet Basics
An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java
application because it has the entire Java API at its disposal.
There are some important differences between an applet and a standalone Java application, including
the following −
An applet is a Java class that extends the java.applet.Applet class.
A main() method is not invoked on an applet, and an applet class will not define main().
Applets are designed to be embedded within an HTML page.
When a user views an HTML page that contains an applet, the code for the applet is downloaded to the user's
machine.](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-65-320.jpg)
![Sawla Polytechnic College ICT Database Administration Level IV
Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 75
button.
Name the project as Debugging_Intro and select appropriate project location.
Click on Finish button.
Note that a project with the name Debugging_Intro is being created on the left pane under projects.
Replace the main method with the following method in the main class.
public static int main(String[] args) {
// TODO code application logic here
Scanner sc;
System.out.println("Enter any month in number (Ex: 1 for January, etc)");
sc = new Scanner(System.in);
float month = Integer.parseInt(sc.next());
if (month / 2 == 0) {
System.out.println("The month is Even");
} else {
System.out.println("The month is Odd");
}
// switch statement to return the name of the month
switch (month) {
case 1:
System.out.println("January");
break;
case 2:
System.out.println("February");
break;
case 3:](https://image.slidesharecdn.com/ict-dba4-05-0811-apply-object-oriented-programming-language-skills-220517134708-931218cd/85/ICT-DBA4-05-0811-Apply-Object-Oriented-Programming-Language-Skills-doc-75-320.jpg)
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ICT-DBA4 -05-0811-Apply-Object-Oriented-Programming-Language-Skills.doc
- 1. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 1 NOMINAL DURATION: 240hrs
- 2. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 2 LO1. Apply basic language syntax and layout 1.1 Introduction to Programming Language As we know, to communicate with a person, we need a specific language, similarly to communicate with computers, programmers also need a language is called Programming language. Before learning the programming language, let's understand what is language? What is Language? Language is a mode of communication that is used to share ideas, opinions with each other. For example, if we want to teach someone, we need a language that is understandable by both communicators. 1.1.1 What is a Programming Language? A program is a set of instructions given to a computer to perform a specific operation. or computer is a computational device which is used to process the data under the control of a computer program. A programming language is a computer language that is used by programmers (developers) to communicate with computers. It is a set of instructions written in any specific language ( C, C++, Java, Python) to perform a specific task. A programming language is mainly used to develop desktop applications, websites, and mobile applications. Types of programming language Low-level programming language Low-level language is machine-dependent (0s and 1s) programming language. The processor runs low- level programs directly without the need of a compiler or interpreter, so the programs written in low-level language can be run very fast. Low-level language is further divided into two parts – i. Machine Language Machine language is a type of low-level programming language. It is also called as machine code or object code. Machine language is easier to read because it is normally displayed in binary or hexadecimal form (base 16) form. It does not require a translator to convert the programs because computers directly understand the machine language programs. The advantage of machine language is that it helps the programmer to execute the programs faster than the high- level programming language. ii. Assembly Language
- 3. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 3 Assembly language (ASM) is also a type of low-level programming language that is designed for specific processors. It represents the set of instructions in a symbolic and human-understandable form. It uses an assembler to convert the assembly language to machine language. The advantage of assembly language is that it requires less memory and less execution time to execute a program. High-level programming language High-level programming language (HLL) is designed for developing user-friendly software programs and websites. This programming language requires a compiler or interpreter to translate the program into machine language (execute the program). The main advantage of a high-level language is that it is easy to read, write, and maintain. High-level programming language includes Python, Java, JavaScript, PHP, C#, C++, Objective C, Cobol, Perl, Pascal, LISP, FORTRAN, and Swift programming language. There are several approaches to programming processes in A high-level language. But the two most popular and important ones are object-oriented programming and procedural programming. I. Procedural Oriented programming language Procedural Oriented Programming (POP) language is derived from structured programming and based upon the procedure call concept. It divides a program into small procedures called routines or functions. Procedural Oriented programming language is used by a software programmer to create a program that can be accomplished by using a programming editor like IDE, Adobe Dreamweaver, or Microsoft Visual Studio. The advantage of POP language is that it helps programmers to easily track the program flow and code can be reused in different parts of the program. The advantage of POP language is that it helps programmers to easily track the program flow and code can be reused in different parts of the program. Example: C, FORTRAN, Basic, Pascal, etc. II Object-Oriented Programming language Object-Oriented Programming (OOP) language is based upon the objects. In this programming language, programs are divided into small parts called objects. It is used to implement real-world entities like inheritance, polymorphism, abstraction, etc in the program to makes the program resusable, efficient, and easy-to- use. The main advantage of object-oriented programming is that OOP is faster and easier to execute, maintain, modify, as well as debug. Note: Object-Oriented Programming language follows a bottom-up approach. Example: C++, Java, Python, C#, etc.
- 4. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 4 Differences between object-oriented and procedural programming The objective of procedural programming is to break down a program into a collection of variables, data structures whereas the main aim of object-oriented programming is to break down a programming task into objects. In simple words, procedural programming uses procedures to operate on data structures, while object-oriented uses objects for the purpose. In both programming paradigms, the nomenclature is different though have similar semantics: Procedural Programming Object-oriented Programming Procedure Method Record Object Module Class Procedure call Message Object-oriented programming? Object-oriented programming combines a group of variables (properties) and functions (methods) into a unit called an "object." These objects are organized into classes where individual objects can be grouped together. OOP can help you consider objects in a program's code and the different actions that could happen in relation to the objects. This programming style widely exists in commonly used programming languages like Java, C++ and PHP. These languages help simplify the structure and organization of software programs. Programmers often use OOP when they need to create complex programs. Self-Check -1 Written Test Directions: Answer all the questions listed below. Use the Answer sheet provided in the next page:(2 pts each) 1. What is program? 2. What is programing language? 3. Define low level and high level programing language? 4. Write The Deference between object oriented and Procedural programing language? 5. Write at least two example of object oriented programing language?
- 5. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 5 1.2. Understanding Basic language syntax rules and best practices What are the Basic programming languages? Some of the basic programming languages are: C, C++, JAVA, Visual Studio (Visual basic.Net, C-Sharp(C#), and Visual C++), and so on. Syntax is a rule that specify how valid instructions (constructs) are written. 1.2.1 Introduction to Java programming JAVA was developed by Sun Microsystems Inc in 1991, later acquired by Oracle Corporation. It was developed by James Gosling and Patrick Naughton. It is a simple programming language. Writing, compiling and debugging a program is easy in java. It helps to create modular programs and reusable code. • Simple Java is considered as one of simple language because it does not have complex features like Operator overloading, multiple inheritance, pointers and Explicit memory allocation. • Multithreading Java supports multithreading. Multithreading is a Java feature that allows concurrent execution of two or more parts of a program for maximum utilisation of CPU. • Portable The platform independent byte code can be carried to any platform for execution that makes java code portable. Java Virtual Machine (JVM) This is generally referred as JVM. Before, we discuss about JVM lets see the phases of program execution. Phases are as follows: we write the program, then we compile the program and at last we run the program. 1) Writing of the program is of course done by java programmer like you and me. 2) Compilation of program is done by javac compiler, javac is the primary java compiler included in java development kit (JDK). It takes java program as input and generates java bytecode as output. 3) In third phase, JVM executes the bytecode generated by compiler. This is called program run phase. So, now that we understood that the primary function of JVM is to execute the bytecode produced by compiler. Each operating system has different JVM, however the output they produce after execution of bytecode is same across all operating systems. That is why we call java as platform independent language. Bytecode As discussed above, javac compiler of JDK compiles the java source code into bytecode so that it can be executed by JVM. The bytecode is saved in a .class file by compiler. Java Development Kit(JDK) As the name suggests this is complete java development kit that includes JRE (Java Runtime Environment), compilers and various tools like Java debugger etc. In order to create, compile and run Java program you would need JDK installed on your computer. JVM Vs JRE Vs JDKJRE: JRE is the environment within which the java virtual machine runs. JRE contains Java virtual Machine(JVM), class libraries, and other files excluding development tools such as compiler and debugger. Which means you can run the code in JRE but you can’t develop and compile the code in JRE. JVM: As we discussed above, JVM runs the program by using class, libraries and files provided by JRE. JDK: JDK is a superset of JRE, it contains everything that JRE has along with development tools such as compiler, debugger etc
- 6. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 6 Java is a platform independent language Compiler(javac) converts source code (.java file) to the byte code(.class file). As mentioned above, JVM executes the bytecode produced by compiler. This byte code can run on any platform such as Windows, Linux, Mac OS etc. Which means a program that is compiled on windows can run on Linux and vice- versa? Each operating system has different JVM, however the output they produce after execution of bytecode is same across all operating systems. That is why we call java as platform independent language. JAVA Basic Syntax contains: Object Class Methods Data type Return type Variable
- 7. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 7 Java - Basic Syntax • When we consider a Java program, it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what do class, object, methods, and instance variables mean. • Object − Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behavior such as wagging their tail, barking, eating. An object is an instance of a class. • Class − A class can be defined as a template/blueprint that describes the behavior/state that the object of its type supports. • Methods − A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed. • Variable: A variable is the name for a memory location where you store some data. A variable in Java must be declared (the type of variable) and defined (values assigned to a variable) before it can be used in a program. • Data type: A data type defines which kind of data will store in variables and also defines memory storage of data. There are two kinds of data types User defined data types and Standard data types. • Return type: Return type is used to terminate main( ) Method and causes it to return the value to its type. If a Method is defined as having a return type of void, it should not return a value. If a Method is defined as having a return type other than void, it should return a value. Syntax public class FirstJavaProgram { public static void main(String[] args){ System.out.println("This is my first program in java"); }//End of main }//End of FirstJavaProgram Class Every line of code that runs in Java must be inside a class. In our example, we named the class FirstJavaProgram. A class should always start with an uppercase first letter. Note: Java is case-sensitive: " FirstJavaProgram " and " firstJavaprogram " has different meaning. The name of the java file must match the class name. When saving the file, save it using the class name and add ".java" to the end of the filename. Let we see each line of code one by public class FirstJavaProgram { This is the first line of our java program. Every java application must have at least one class definition that consists of class keyword followed by class name. When I say keyword, it means that it should not be changed, we should use it as it is. However the class name can be anything. We have made the class public by using public access modifier, we will cover access modifier in a separate post, all you need to know now that a java file can have any number of classes but it can have only one public class and the file name should be same as public class name. The next is main method public static void main(String[] args) { This is our next line in the program, lets break it down to understand it: public: This makes the main method public that means that we can call the method from outside the class. static: We do not need to create object for static methods to run. They can run itself. void: It does not return anything. main: It is the method name. This is the entry point method from which the JVM can run your program.
- 8. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 8 (String[] args): Used for command line arguments that are passed as strings. We will cover that in a separate post. Next the method System.out.println("This is my first program in java"); This method prints the contents inside the double quotes into the console and inserts a newline after. Note: The curly braces {} marks the beginning and the end of a block of code. Note: Each code statement must end with a semicolon Self-Check -2 Written Test Directions: Answer all the questions listed below. Use the Answer sheet provided in the next page:(2 pts each) 1. Which one of the following is true about java programing language? A. platform independent byte code can be carried to any platform B. Java supports multithreading C. is object oriented programing language D. all 2. The correct phases of java program execution A. Writing of the program execute the bytecode generated by compiler Compile the program by javac compiler B. Writing of the program Compile the program by javac compiler execute the bytecode generated by compiler C. Compile the program by javac compiler Writing of the program execute the bytecode generated by compiler D. execute the bytecode generated by compiler Writing of the program Compile the program by javac compiler 3. Java feature that allows concurrent execution of two or more parts of a program for maximum utilisation of CPU. A. Portable B. not complex C. Multithreading D. platform independent 4. The type of code generated after JDK compiles the java source code and save it into a .class file to be can be executed by JVM. A. Source code B. Byte code C. Machine code D. B&C 5. Which one is true A. In order to create, compile and run Java program you would need JDK installed on your computer. B. Java Development Kit(JDK) includes JRE (Java Runtime Environment), compilers and various tools like Java debugger
- 9. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 9 C. Java Runtime environment (JRE) contains Java virtual Machine(JVM), class libraries but not compiler and debugger D.All 6. Java - Basic Syntax is A. Accessmodifier Class ClassName{ Public static Void Main(String[] args){ //body }} B. Accessmodifier Class MethodName(){ Public static Void Main(String[] args){ //method }} C. Accessmodifier DataType variableName{ Public static Void Main(String[] args){ //variables }} D. All 7. Which of the following option leads to the portability and security of Java? A. Bytecode is executed by JVM B. The applet makes the Java code secure and portable C. Use of exception handling D. Dynamic binding between object 1.3 Understanding Data-Types, Operators and Expressions Data type: A data type defines which kind of data will store in variables and also defines memory storage of data. There are two kinds of data types User defined data types and Standard data types. The Java language allows you to create and use data types other than the fundamental data types. These types are called user-defined data types. In Java language, there are four main data types • char • int • float • Boolean 1. Character data type A keyword char is used for character data type. This data type is used to represents letters or symbols. A character variable occupies 1 byte on memory.
- 10. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 10 2. Integer data types Integers are those values which have no decimal part and they can be positive or negative. Like 12 or -12. There are 3 data types for integers a. int b. short int c. long int int keyword is used for integers. It takes two bytes in memory. There are two more types of int data type. i. Signed int or short int (2nd type of integer data type) ii. Unsigned int or unsigned short int Signed int: The range of storing value of signed int variable is -32768 to 32767.It can interact with both positive and negative value. Unsigned int: This type of integers cannot handle negative values. Its range is 0 to 65535. Long int As its name implies, it is used to represent larger integers. It takes 4byte in memory and its range is -2147483648 to 2147483648. 3. Float: This type of data type is defined for fractional numbers. There are two further type of data type for fractional numbers. Below table is Displaying: Number of bytes or memory taken by numbers. Decimal places up to which they can represent value. Data type Bytes Decimal places Range of values float 4 6 3.4E -38 to3.4E+38 double 8 15 1.7E - 308 to1.7E +308 long double 10 19 3.4E -4932 to 304E +4932 Constant Constant is a data type whose value is never changes and remains the same through the programme execution. Example: - Area of the circle A=πr2 ; const float π=3.14 - Total number of days in a week=7 ; - Total hours in a day=24 - Body temperature of human=23C0; Here the value of above listed can be treated as a constant through the program execution. The four basic data types in java, their meaning, and their size are: Type Meaning Size (bytes) Size (bits) char a single byte, capable of holding one character 1 byte 8 bits int an integer 2 bytes 16 bits float single-precision floating point number 4 bytes 32 bits double double-precision floating point number 8 bytes 64 bits Byte is the smallest addressable memory unit. Bit, which comes from BInary digiT, is a memory unit that can store either a 0 or a 1. A byte has 8 bits. Operators in Java
- 11. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 11 An operator is a symbol that tells the compiler to perform specific mathematical or logical manipulations. Java is rich in built-in operators and provides following type of operators: Arithmetic Operators Relational Operators Increment Operator Decrement Operator Scope resolution operator Logical Operators Assignment Operators Arithmetic Operators: The following arithmetic operators are supported by Java language: Assume variable A holds 10 and variable B holds 20 then: Operator Description Example + Adds two operands A + B will give 30 - Subtracts second operand from the first A - B will give -10 * Multiply both operands A * B will give 200 / Divide numerator by de-numerator B / A will give 2 % Modulus Operator and remainder of after an integer division B % A will give 0 ++ Increment operator, increases integer value by one A++ will give 11 -- Decrement operator, decreases integer value by one A-- will give 9 RelationalOperators: The following relational operators are supported byJava language: Assume variable A holds 10 and variable B holds 20 then: Operator Description Example == Checks if the value of two operands is equal or not, (A == B) is not true. != Checks if the value of two operands is equal or not, (A != B) is true. > Checks if the value of left operand is greater than the value of right operand, (A > B) is not true. < Checks if the value of left operand is less than the value of right operand, (A < B) is true. >= Checks if the value of left operand is greater than or equal to the value of right operand, (A >= B) is not true. <= Checks if the value of left operand is less than or equal to the value of right operand, (A <= B) is true. Logical/Boolean operator: The following logical operators are supported by Java language: Assume variable A holds 1 and variable B holds 0 then: Operator Description Example && If both the operands are non-zero then condition becomes true. (A && B) is false. || If any of the two operands is non-zero then condition becomes true. (A || B) is true. ! Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false. !(A && B) is true.
- 12. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 12 Assignment Operators: Assignment operator is used for assign/initialize a value to the variable during the program execution. The following assignment operators are supported by Java language: Operator Description Example = Simple assignment operator, Assigns values from right side operands to left side operand C = A + B will assign value of A + B into C += Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand C += A is equivalent to C = C + A -= Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand C -= A is equivalent to C = C - A *= Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand C *= A is equivalent to C = C * A /= Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand C /= A is equivalent to C = C / A %= Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand C %= A is equivalent to C = C % A Increment Operator (++): This operator is used for increment the value of an operand. Example: assume that A=20 and B=12, then ++A=21, ++B=12 and A++=20, B++=12 Decrement Operators (--):This operator is used for decrement the value of an operand. Example: assume that B=14 and C=10, then –B=13, --C=9 and B-- =14, C--=10 Scope resolution operator (::) This operator is used to differentiate a local variable from the global variable. The variable having the same name can have different meaning at different block. Expression: Expressions are formed by combining operators and operands together following the programming language. Compile & Execute Java Program: Let’s look at how to save the file, compile and run the program. Please follow the steps given below: Open a text editor and write the code Save the file as : hello.java Compile it to check if it has error Debug the code. You will be able to see ' Hello World ' printed on the window. Semicolons & Blocks in Java: In Java, the semicolon is a statement terminator. Each individual statement must be ended with a semicolon. For example, following are three different statements: x = y; y = y+1; add(x, y); A block is a set of logically connected statements that are surrounded by opening and closing braces. For example: { System.out.println( "Hello World"); // prints Hello World return 0;}
- 13. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 13 Self-Check -3 Written Test Directions: Answer all the questions listed below. Use the Answer sheet provided in the next page:(2 pts each) 1. Decrement operator, −−, decreases the value of variable by what number? A. 1 B. 2 C. 3 D.4 2. Which of these statements are incorrect? A. Assignment operators are more efficiently implemented by Java run-time system than their equivalent long forms B. Assignment operators run faster than their equivalent long forms C. Assignment operators can be used only with numeric and character data type D.None of the mentioned 3. Can 8 byte long data type be automatically type cast to 4 byte float data type? A. True B. False 4. To change the order in which expressions areevaluated …………………. areplaced around the expression that is to be evaluated first. A) ampersand B) equals C) parentheses D) greater than 5. What will bethe result of the expression of 8/8. A) 16 B) 1 C) 8 D) 64 6. If j and k are int typevariables, what will bethe result of the expression j%k when j=10 and k=3? A) 0 B) 1 C) 2 D) 3 7. Which of the following assignment operator does not exist in Java? A. > B. %= C >>> D < 8. Modulus operator, %, can be applied to which of these? A. Both Integers and floating - point numbers B. Integers C. Floating - point numbers D. None of the mentioned 1.4 Using the Appropriate Language Syntax for Sequence, Selection and Iteration Constructs Control Structures in java is a Statement that used to control the flow of execution in a program. There are three types of Control Structures:
- 14. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 14 1. Sequence structure 2. Selection structure 3. Loops/ Repetition/ Iteration Sequence Structure in java The sequence structure is built into java statements that execute one after the other in the order in which they are written—that is, in sequence. Break and continue statements are example of sequence control structure. The ‘break’ statement causes an immediate exit from the inner most ‘while’, ‘do…while’, ‘for loop’ or from a ‘switch- case’ statement. If you are using nested loops (ie. one loop inside another loop), the break statement will stop the execution of the innermost loop and start executing the next line of code after the block. Flow Diagram: Example: public class FirstJavaProgram { public static void main(String[] args){ int a = 10; // Local variable declaration: do{ // do loop execution System.out.println("value of a: "+a); a = a + 1; if( a > 15) { break; } }while( a < 20 ); return 0; } When the above code is compiled and executed, it produces following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15
- 15. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 15 The goto statement provides an unconditional jump from the goto to a labeled statement in the same function. Syntax of a goto statement in java: goto label; .. . label: statement; Where label is an identifier that identifies a labeled statement. A labeled statement is any statement that is preceded by an identifier followed by a colon (:). Flow Diagram: Example: public class FirstJavaProgram { public static void main(String[] args){ int a = 10; // Local variable declaration: LOOP:do // do loop execution { if( a == 15) { a = a + 1;// skip the iteration. goto LOOP; } System.out.println("value of a: " +a ); a = a + 1; }while( a < 20 ); return 0; } When the above code is compiled and executed, it produces following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 16
- 16. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 16 value of a: 17 value of a: 18 value of a: 19 The continue statement is similar to ‘break’ statement but instead of terminating the loop, the continue statement returns the loop execution if the test condition is satisfied. Flow Diagram: Example: public class FirstJavaProgram { public static void main(String[] args){ int a = 10; // Local variable declaration: do{ // do loop execution if( a == 15) { a = a + 1; // skip the iteration. continue; } System.out.println("value of a: " +a ); a = a + 1; }while( a < 20 ); return 0; } When the above code is compiled and executed, it produces following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 16 value of a: 17 value of a: 18 value of a: 19 Selection statements in java
- 17. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 17 There are basically two types of control statements in java that allows the programmer to modify the regular sequential execution of statements. They are selection and iteration statements. The selection statements allow to choose a set of statements for execution depending on a condition. If statement and switch statement are the two selection statements. If ... else statement: Syntax: if (expression or condition) { Statement 1; Statement 2; } Else { Statement 3; Statement 4; } If the condition is true, statement1 and statement2 is executed; otherwise statement 3 and statement 4 is executed. The expression or condition is any expression built using relational operators which either yields true or false condition. Flow Diagram: If the condition evaluates to true, then the if block of code will be executed otherwise else block of code will be executed. Following program implements the if statement. public class FirstJavaProgram { public static void main(String[] args){ int num=12; if ((num%2) == 0) System.out.println("is a even number "); else System.out.println("a number is odd"); } The above program accepts a number from the user and divides it by 2 and if the remainder (remainder is obtained by modulus operator) is zero, it displays the number is even, otherwise it is odd. you must use the relational operator ‘ ==’ to compare whether remainder is equal to zero or not.
- 18. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 18 Switch statement One alternative to nested if statement is the switch statement which allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each case. Syntax: Switch (variablename) { case value1: statement1; break; case value2: statement2; break; case value3: statement3; break; default: statement4; } Flow Diagram: Example: public class FirstJavaProgram { public static void main(String[] args){ int choice=3; switch (choice) { case 1: System.out.println(""First item selected!" ); break; case 2: System.out.println(""second item selected!" ); break; case 3: System.out.println(""third item selected!" ); break; default: System.out.println("Invalid selection!!" ); } }} Iteration statements in java Iteration or loops statements are important statements in java, which helps to accomplish repeatitive execution of programming statements. There are three loop statements in java. They are - while loop, do while loop and for loop. While Loop The while loop construct is a way of repeating loop body over and over again while a certain condition remains true. Once the condition becomes false, the control comes out of the loop. Loop body will execute only if condition is true. Syntax: While (condition or expression) {
- 19. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 19 Statement1; Statement 2; } Flow Diagram: The flow diagram indicates that a condition is first evaluated. If the condition is true, the loop body is executed and the condition is re-evaluated. Hence, the loop body is executed repeatedly as long as the condition remains true. As soon as the condition becomes false, it comes out of the loop and goes to display the output. Example: public class FirstJavaProgram { public static void main(String[] args){ { int n=10, i=1, sum=0; while(i<=n) { sum=sum+i; i++; } System.out.println( " sum of the series is "+ sum); } Do …While loop The do... while statement is the same as while loop except that the condition is checked after the execution of statements in the do..while loop. Syntax: do{ Statement; While (test condition); Statement; } Flow Diagram:
- 20. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 20 Its functionality is exactly the same as the while loop, except that condition in the do-while loop is evaluated after the execution of statement. Hence in do..while loop, Loop body execute once even if the condition is false. Example: public class FirstJavaProgram { public static void main(String[] args){ int n=10, i=1, sum=0; { int n, i=1, sum=0; do{ sum=sum+i; i++; } while(i<=n); System.out.println( " sum of the series is "+ sum); }} For loop: The for loop statements (loops or iteration statement) in java allow a program to execute a single statement multiple times (repeatedly) , given the initial value, the condition, and increment/decrement value. Java for loop is used to run a block of code for a certain number of times. The syntax of for loop is: for (initialExpression; testExpression; updateExpression) { // body of the loop } Here, • The initialExpression initializes and/or declares variables and executes only once. • The condition is evaluated. If the condition is true, the body of the for loop is executed. • The updateExpression updates the value of initialExpression. syntax: for ( initial value ; test condition ; increment/decrement) { Statement; }
- 21. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 21 Flow Diagram: A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times. Example: public class FirstJavaProgram { public static void main(String[] args){ int n=10, i=1, sum=0; { for(i=1;i<=n; i++) { sum=sum+i; } System.out.println( " sum of the series is "+ sum); }} Nested Loop in Java If a loop exists inside the body of another loop, it's called a nested loop. Here's an example of the nested for loop. Syntax for Nested For loop: for ( initialization; condition; increment ) { for ( initialization; condition; increment ) { // statement of inside loop } // statement of outer loop } // outer loop for (int i = 1; i <= 5; ++i) { // codes // inner loop for(int j = 1; j <=2; ++j) { // codes } .. } class Main { public static void main(String[] args) { int weeks = 3;
- 22. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 22 int days = 7; // outer loop prints weeks for (int i = 1; i <= weeks; ++i) { System.out.println("Week: " + i); // inner loop prints days for (int j = 1; j <= days; ++j) { System.out.println(" Day: " + j); } } } } output Week: 1 Day: 1 Day: 2 Day: 3 Day:4 ..... .. .... Week: 2 Day: 1 Day: 2 Day: 3 In the above example, the outer loop iterates 3 times and prints 3 weeks. And, the inner loop iterates 7 times and prints the 7 days. Self-Check -4 Written Test Directions: Answer all the questions listed below. Use the Answer sheet provided in the next page:(2 pts each) 1. The while loop repeats a set of code while the condition is not met? A) True B) False 2. What is true about a break? A. Break stops the execution of entire program B. Break halts the execution and forces the control out of the loop C. Break forces the control out of the loop and starts the execution of next iteration D. Break halts the execution of the loop for certain time frame 3. What is true about do statement? A. do statement executes the code of a loop at least once B. do statement does not get execute if condition is not matched in the first iteration C. do statement checks the condition at the beginning of the loop D. do statement executes the code more than once always 4. Which of the following is used with the switch statement? A) Continue B) Exit C) break D) do 5. From where break statement causes an exit? a) Only from innermost loop
- 23. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 23 b) Terminates a program c) Only from innermost switch d) From innermost loops or switches 6. Which of the following is not a valid flow control statement? a) exit() b) break c) continue d) return 1.5 Using modular programming approach Many programs are too long or complex to write as a single unit. Programming becomes much simpler when the code is divided into small functional units (modules). Modular programming is a programming style that breaks down program functions into modules, each of which accomplishes one function and contains all the source code and variables needed to accomplish that function. Modular programs are usually easier to code, compile, debug, and change than large and complex programs. The benefits of modular programming are: Efficient Program Development Programs can be developed more quickly with the modular approach since small subprograms are easier to understand, design, and test than large and complex programs. Multiple Use of Subprograms Code written for one program is often useful in others. Ease of Debugging and Modifying Modular programs is generally easier to compile and debug than monolithic (large and complex) programs. 1.5.1 Modularity in Java If you've been a developer for any length of time, you'll have very likely realized that the word module is perhaps one of the most over used terms in software development. A module can mean anything ranging from a group of code entities, components, or UI types, to framework elements to complete reusable libraries. Sometimes, we use the word to imply multiple meanings in the same context! There is a good reason for that. When writing code, we typically try to break the code base down into smaller units in order to manage complexity. For anything more than very simple programs, having a monolithic code base is not a good idea. That's why modular programming is a generally favored software design approach. There are two important goals that modularity in software development usually achieves, which are as follows: Divide and conquer approach What do you do when you need to solve a large and seemingly insurmountable problem? You break it down! You'll very likely split it into smaller problems and solve them individually. The principles of modularity encourages separating large code bases into smaller encapsulated units of functionality that are then composed to work together as a bigger unit. This aligns well with the approach we humans usually take to solve large problems. Also, once you've got a bunch of smaller modules with specialized concerns, you can use those to solve various other problems. Thus, we also achieve reusability! When you build modules, you have the ability to hide the internal implementation from the consumers of your module. The hidden implementation details are usually referred to as being encapsulated, and what you expose to the consumers of your module is usually called the interface of your module. Achieving encapsulationand well-defined interfaces
- 24. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 24 When you build modules, you have the ability to hide the internal implementation from the consumers of your module. The hidden implementation details are usually referred to as being encapsulated, and what you expose to the consumers of your module is usually called the interface of your module. Although Java developers have leveraged many different patterns and best practices over the years in order to write and structure modular and maintainable code, the language has never had native support to create modular units and build modular applications, until Java 9. With Java 9, Java developers now have the ability to create smaller units of code with a new construct called Java modules that they can group together like building blocks in order to compose larger applications. In addition to introducing this feature to the language, Java 9 also comes with what is probably the biggest overhaul to the core Java code bases itself. The Java Runtime Environment (JRE) and the Java Development Kit (JDK) have been rewritten to use the concepts of modularity so that the core Java Platform itself is modularized. When learning about Java 9 module features, it's important to understand what those new features add to the language when compared to the other features the language already has. Can't we write well organized code in Java 8? In fact, one of the benefits of object-oriented programming is indeed the idea of breaking down functionality into sub-units called objects or classes. We've been writing code like this in Java since version 1. Every Java class contains a portion of the overall application functionality that happens to belongs together. We have the ability to encapsulate some functionality as internal to a class (as private) and some others as external (or public). And then there's something in between with protected, thanks to the concept of packages. 1.6 Using arrays and arrays of objects An Array is a collection of similar data items which shares a common name within the consecutive memory. An Array can be any data type, but it should be the collection of similar items. Each item in an array is termed as ‘element’, but each element in an array can be accessed individually. The number of element in an array must be declared clearly in the definition. The size or number of elements in the array can be varied according to the user needs. Syntax for array declaration: DataType ArrayName [number of element in the array] Example: int a[5]; - ‘int’ is the data type. - ‘a’ is the array name. - 5 is number of elements or size. int a[5] means a[0], a[1], a[2], a[3], a[4] or int Int int int int a[0] a[1] a[2] a[3] a[4] - 0, 1, 2, 3, 4 are called subscript which is used to define an array element position and is called Dimension. - Array index in java starts from 0 to n-1 if the size of array is n. An individual element of an array is identified by its own unique index (or subscript). NOTE: The elements field within brackets [], which represents the number of elements the array is going to hold, must be a constant value.
- 25. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 25 Arrays of variables of type “class” are known as "Array of objects". The "identifier" used to refer the array of objects is a user defined data type. Self-Check -6 Written Test Directions: Answer all the questions listed below. Use the Answer sheet provided in the next page:(2 pts each) 1. Which of these distinctions of modular programs over non modular are true? a) They are not that easier to understand and explain b) They are not easier to document c) They are easier to change d) Testing and Debugging is complex comparatively 2. Which of these comes under the Modularity principle? a) Small modules b) Coupling c) Cohesion d) All of the mentioned 3. An array Index starts with.? a) -1 b) 0 c) 1 d) 2 4. Objects in a sequence that have the same type, is called a). Arrays b). Operators c). Functions d). Stacks 5. Choose one of the best option from the following Position number contained within a square brackets e.g. my_array [5], is referred as ____? a) Sub-scripts b) Post-scripts c) Elements of an array d) None of them
- 26. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 26 LO2. Apply basic OOP principles in the target language 2.1 Object Oriented Programming in java Java is an object oriented language because it provides the features to implement an object oriented model. Java is an object-oriented programming language. The core concept of the object-oriented approach is to break complex problems into smaller objects. It is used to implement real-world entities like inheritance, polymorphism, abstraction; etc in the program to makes the program resusable, efficient, and easy-to-use. An object is any entity that has a state and behavior. For example, a bicycle is an object. It has States: idle, first gear, etc Behaviors: braking, accelerating, etc. Before we learn about objects, let's first know about classes in Java. 2.2 Introduction to Java Class A class can be considered as a blueprint using which you can create as many objects as you like. For example, here we have a class Website that has two data members (also known as fields, instance variables and object states). This is just a blueprint, it does not represent any website, however using this we can create Website objects (or instances) that represents the websites. We have created two objects, while creating objects we provided separate properties to the objects using constructor. A class is a blueprint for the object. Before we create an object, we first need to define the class. We can think of the class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows, etc. Based on these descriptions we build the house. House is the object. Since many houses can be made from the same description, we can create many objects from a class. OOPS is about developing an application around its data, i.e. objects which provides the access to their properties and the possible operations in their own way. A class in java is an encapsulation of data members and functions that manipulate the data. In terms of variables, a class would be the type, and an object would be the variable. 2.2.1 Classesand Objects A class is a type, and an object of this class is just a variable. Before we can use an object, it must be created. Classes are generally declared using the keyword class. Syntax Acces modifier class ClassName { // fields // methods } Here, fields (variables) and methods represent the state and behavior of the object respectively. Fields are used to store data Methods are used to perform some operations Where class_name is a valid identifier for the class, object_names is an optional list of names for objects of this class. The body of the declaration can contain members that can be either data or function declarations, or optionally access modifier. These modifier modify the access: Public: Access to all code in the program.
- 27. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 27 public members are accessible from anywhere where the object is visible. Private: Access to only members of the same class Protected: Access to members of same class and its derived classes For example our bicycle object, we can create the class as Example: class Bicycle { // state or field private int gear = 5; // behavior or method public void braking() { System.out.println("Working of Braking"); } } In the above example, we have created a class named Bicycle. It contains a field named gear and a method named braking (). Here, Bicycle is a prototype. Now, we can create any number of bicycles using the prototype. And, all the bicycles will share the fields and methods of the prototype. Note: We have used keywords private and public. These are known as access modifiers. Variables in Java Types of Variables in Java There are three types of variables in Java. 1) Local variable 2) Static (or class) variable 3) Instance variable Static (or class) Variable Static variables are also known as class variable because they are associated with the class and common for all the instances of class. For example, If I create three objects of a class and access this static variable, it would be common for all, the changes made to the variable using one of the object would reflect when you access it through other objects. Example of static variable public class StaticVarExample { public static String myClassVar="class or static variable"; public static void main(String args[]){ StaticVarExample obj = new StaticVarExample(); StaticVarExample obj2 = new StaticVarExample(); StaticVarExample obj3 = new StaticVarExample(); //All three will display "class or static variable" System.out.println(obj.myClassVar);
- 28. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 28 System.out.println(obj2.myClassVar); System.out.println(obj3.myClassVar); //changing the value of static variable using obj2 obj2.myClassVar = "Changed Text"; //All three will display "Changed Text" System.out.println(obj.myClassVar); System.out.println(obj2.myClassVar); System.out.println(obj3.myClassVar); }} Output: class or static variable class or static variable class or static variable Changed Text Changed Text Changed Text As you can see all three statements displayed the same output irrespective of the instance through which it is being accessed. That’s is why we can access the static variables without using the objects like this: System.out.println(myClassVar); Do note that only static variables can be accessed like this. This doesn’t apply for instance and local variables. Instance variable Each instance(objects) of class has its own copy of instance variable. Unlike static variable, instance variables have their own separate copy of instance variable. We have changed the instance variable value using object obj2 in the following program and when we displayed the variable using all three objects, only the obj2 value got changed, others remain unchanged. This shows that they have their own copy of instance variable. Example of Instance variable public class InstanceVarExample { String myInstanceVar="instance variable"; public static void main(String args[]){ InstanceVarExample obj = new InstanceVarExample(); InstanceVarExample obj2 = new InstanceVarExample(); InstanceVarExample obj3 = new InstanceVarExample(); System.out.println(obj.myInstanceVar); System.out.println(obj2.myInstanceVar); System.out.println(obj3.myInstanceVar); obj2.myInstanceVar = "Changed Text"; System.out.println(obj.myInstanceVar); System.out.println(obj2.myInstanceVar); System.out.println(obj3.myInstanceVar); }}
- 29. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 29 Output: instance variable Instance variable instance variable instance variable Changed Text instance variable Local Variable These variables are declared inside method of the class. Their scope is limited to the method which means that You can’t change their values and access them outside of the method. In this example, I have declared the instance variable with the same name as local variable, this is to demonstrate the scope of local variables. Example of Local variable public class VariableExample { // instance variable public String myVar="instance variable"; public void myMethod(){ // local variable String myVar = "Inside Method"; System.out.println(myVar); } public static void main(String args[]){ // Creating object VariableExample obj = new VariableExample(); /* We are calling the method, that changes the * value of myVar. We are displaying myVar again after * the method call, to demonstrate that the local * variable scope is limited to the method itself. */ System.out.println("Calling Method"); obj.myMethod(); System.out.println(obj.myVar); }} Output: Calling Method Inside Method instance variable Java Constructor Constructorlooks like a method but it is in fact not a method. It’s name is same as class name and it does not return any value. You must have seen this statement in almost all the programs I have shared above: MyClass obj = new MyClass();
- 30. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 30 If you look at the right side of this statement, we are calling the default constructor of class myClass to create a new object (or instance). We can also have parameters in the constructor, such constructors are known as parametrized constructors. To understand the working of constructor, lets take an example. lets say we have a class MyClass. When we create the object of MyClass like this: MyClass obj = new MyClass() The new keyword here creates the object of class MyClass and invokes the constructor to initialize this newly created object. Types of Constructors There are three types of constructors: Default, No-arg constructor and Parameterized. Default constructor • If you do not implement any constructor in your class, Java compiler inserts a default constructor into your code on your behalf. This constructor is known as default constructor.
- 31. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 31 • You would not find it in your source code(the java file) as it would be inserted into the code during compilation and exists in .class file. • This process is shown in the diagram below: If you implement any constructor then you no longer receive a default constructor from Java compiler. no-arg constructor: Constructor with no arguments is known as no-arg constructor. The signature is same as default constructor, however body can have any code unlike default constructor where the body of the constructor is empty. Although you may see some people claim that that default and no-arg constructor is same but in fact they are not, even if you write public Demo() { } in your class Demo it cannot be called default constructor since you have written the code of it. Example: no-arg constructor class Demo{ public Demo() { System.out.println("This is a no argument constructor"); } public static void main(String args[]) { new Demo(); }} Output: This is a no argument constructor Parameterized constructor Constructor with arguments(or you can say parameters) is known as Parameterized constructor Example: parameterized constructor In this example we have a parameterized constructor with two parameters id and name. While creating the objects obj1 and obj2 I have passed two arguments so that this constructor gets invoked after creation of obj1 and obj2. public class Employee { int empId; String empName; //parameterized constructor with two parameters Employee(int id, String name){ this.empId = id;
- 32. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 32 this.empName = name; } void info(){ System.out.println("Id: "+empId+" Name: "+empName); } Java method A Java method is a collection of statements that are grouped together to perform an operation. When you call the System.out.println() method, for example, the system actually executes several statements in order to display a message on the console. Now you will learn how to create your own methods with or without return values, invoke a method with or without parameters, and apply method abstraction in the program design. A method is a block of code that performs a specific task. Suppose you need to create a program to create a circle and color it. You can create two methods to solve this problem: a method to draw the circle a method to color the circle Dividing a complex problem into smaller chunks makes your program easy to understand and reusable. In Java, there are two types of methods: User-defined Methods: We can create our own method based on our requirements. Standard Library Methods: These are built-in methods in Java that are available to use. Let's first learn about user-defined methods. Declaring a Java Method The syntax to declare a method is: returnType methodName() { // method body } Here, returnType - It specifies what type of value a method returns For example if a method has an int return type then it returns an integer value. If the method does not return a value, its return type is void. methodName - It is an identifier that is used to refer to the particular method in a program. method body - It includes the programming statements that are used to perform some tasks. The method body is enclosed inside the curly braces { }. For example, int addNumbers() { // code } In the above example, the name of the method is adddNumbers(). And, the return type is int. We will learn more about return types later in this tutorial. This is the simple syntax of declaring a method. However, the complete syntax of declaring a method is modifie returnType nameOfMethod (parameter1, parameter2, ...) { // method body } Here, modifier - It defines access types whether the method is public, private, and so on. To learn more, visit Java Access Specifier. static - If we use the static keyword, it can be accessed without creating objects. For example, the sqrt() method of standard Math class is static. Hence, we can directly call Math.sqrt() without creating an instance of Math class.
- 33. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 33 parameter1/parameter2 - These are values passed to a method. We can pass any number of arguments to a method. Calling a Method in Java In the above example, we have declared a method named addNumbers(). Now, to use the method, we need to call it. Here's is how we can call the addNumbers() method. // calls the method addNumbers(); Example 1: Java Methods class Main { // create a method public int addNumbers(int a, int b) { int sum = a + b; // return value return sum; } public static void main(String[] args) { int num1 = 25; int num2 = 15; // create an object of Main Main obj = new Main(); // calling method int result = obj.addNumbers(num1, num2); System.out.println("Sum is: " + result); } } Output Sum is: 40 Java Method Return Type A Java method may or may not return a value to the function call. We use the return statement to return any value. For example,
- 34. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 34 int addNumbers() { ... return sum; } Here, we are returning the variable sum. Since the return type of the function is int. The sum variable should be of int type. Otherwise, it will generate an error. Example 2: Method Return Type class Main { // create a method public static int square(int num) { // return statement return num * num; } public static void main(String[] args) { int result; // call the method // store returned value to result result = square(10); System.out.println("Squared value of 10 is: " + result); } } Output: Squared value of 10 is: 100 In the above program, we have created a method named square(). The method takes a number as its parameter and returns the square of the number. Here, we have mentioned the return type of the method as int. Hence, the method should always return an integer value. public void square(int a) { int square = a * a; System.out.println("Square is: " + square ); } Method Parameters in Java A method parameter is a value accepted by the method. As mentioned earlier, a method can also have any number of parameters. For example, // method with two parameters int addNumbers(int a, int b) { // code }
- 35. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 35 // method with no parameter int addNumbers(){ // code } If a method is created with parameters, we need to pass the corresponding values while calling the method. For example, // calling the method with two parameters addNumbers(25, 15); // calling the method with no parameters addNumbers() Example 3: Method Parameters class Main { // method with no parameter public void display1() { System.out.println("Method without parameter"); } // method with single parameter public void display2(int a) { System.out.println("Method with a single parameter: " + a); } public static void main(String[] args) { // create an object of Main Main obj = new Main(); // calling method with no parameter obj.display1(); // calling method with the single parameter obj.display2(24); } } Output Method with parameter Method with a single parameter: 24 Standard Library Methods The standard library methods are built-in methods in Java that are readily available for use. These standard libraries come along with the Java Class Library (JCL) in a Java archive (*.jar) file with JVM and JRE. For example, print() is a method of java.io.PrintSteam. The print("...") method prints the string inside quotation marks. sqrt() is a method of Math class. It returns the square root of a number. Here's a working example: Example 4: Java Standard Library Method public class Main { public static void main(String[] args) { // using the sqrt() method System.out.print("Square root of 4 is: " + Math.sqrt(4)); } } Output: Square root of 4 is: 2.0 Output Method with parameter Method with a single parameter: 24 ModelClasses andObjects real-life beings to software entities Let's take an example of developing a pet management system, specially meant for dogs. You will need various information about the dogs like different breeds of the dogs, the age, size, etc. You need to model real-life beings, i.e., dogs into software entities
- 36. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 36 Moreover, the million dollar question is, how you design such software? Here is the solution-First, let's do an exercise. You can see the picture of three different breeds of dogs below. Stop here right now! List down the differences between them. Some of the differences you might have listed out maybe breed, age, size, color, etc. If you think for a minute, these differences are also some common characteristics shared by these dogs. These characteristics (breed, age, size, color) can form a data members for your object. common characteristics
- 37. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 37 common behaviors of these dogs Next, list out the common behaviors of these dogs like sleep, sit, eat, etc. So these will be the actions of our software objects. So far we have defined following things, Class – Dogs Data members or objects- size, age, color, breed, etc. Methods- eat, sleep, sit and run.
- 38. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 38 Now, for different values of data members (breed size, age, and color) in Java class, you will get different dog objects.
- 39. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 39 Know you can design program using oop approach You can design any program using this OOPs approach. While creating a class, one must follow the following principles. • // Class Declaration • public class Dog { • // Instance Variables • String breed; • String size; • int age; • String color; • // method 1 • public String getInfo() { • return ("Breed is: "+breed+" Size is:"+size+" Age is:"+age+" color is: "+color); • } • public static void main(String[] args) { • Dog maltese = new Dog(); • maltese.breed="Maltese"; • maltese.size="Small"; • maltese.age=2; • maltese.color="white"; • System.out.println(maltese.getInfo()); • } • } • Out put: Breed is: Maltese Size is:Small Age is:2 color is: white Types of method Static Methods Static methods are methods in java that can be called without actually creating an object of that class. By declaring a method using the static keyword, you can call it without first creating an object because it becomes part of the class or a class method(a method that belongs to a class rather than to an object). They are designed with aim to be shared with all objects created with that class. One reason why you might want to use a static method is when you want a common operation to be performed through out the class. Static methods cannot be overridden. Syntax: Static keyword followed by return type, followed by method name. static return_type method_name(); EXAMPLE: class Car{ static void display(){ System.out.println(“this is my favorite car”); } public static void main(String[], args){ .display();
- 40. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 40 } } Instance Methods Instance methods are methods that require an object of its class to be created before it can be called. Instance methods belong to object of the class and not the class. Every object individual object created from the class has its own copy of the instance methods of that class. They can be overridden. class JavaExample{ static int i = 100; static String s = “sawla"; //Static method static void display() { System.out.println("i:"+i); System.out.println("i:"+s); } //non-static method void funcn() { //Static method called in non-static method display(); } //static method public static void main(String args[]) { JavaExample obj = new JavaExample(); //You need to have object to call this non-static method obj.funcn(); //Static method called in another static method display(); }} 2.3 Object constructionin Java class Java Objects An object is called an instance of a class. For example, suppose Bicycle is a class then MountainBicycle, SportsBicycle, TouringBicycle, etc can be considered as objects of the class. Here is how we can create an object of a class. className object = new className(); // for Bicycle class Bicycle sportsBicycle = new Bicycle(); Bicycle touringBicycle = new Bicycle(); We have used the new keyword along with the constructor of the class to create an object. Constructors are similar to methods and have the same name as the class. For example, Bicycle() is the constructor of the Bicycle class.
- 41. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 41 Here, sportsBicycle and touringBicycle are the names of objects. We can use them to access fields and methods of the class. As you can see, we have created two objects of the class. We can create multiple objects of a single class in Java. Note: Fields and methods of a class are also called members of the class. Self-Check -5 Written Test Directions: Answer all the questions listed below. Use the Answer sheet provided in the next page:(2 pts each) 1. Properties are implemented using ___ in Java. a) Methods b) Variables c) Interfaces d) All the above 2. Which is the file extension used for a public Java class source code? a) .j b) .class c) .java d) None 3. After compilation, ajava -Class is kept in a ___ file in Java programming. a) .java b) .cls c) .class d) .interface 4. In Java, the keyword used to declare a class is ___. a) Class b) Java c) class d) java 5. A Java class can contain___. a) Variables b) Methods, Constructors c) Inner Classes d) All the above 6. Creating an object from a class is also called ____. a) Initializing b) Instantiating c) Interfacing d) None of the above 7. The keyword used to create a new object in Java is ___. a) class b) java c) new d) create 8. Choose the correct statements about choosing a name for a class in Java. a) The class name can start with only a letter or underscore or dollar sign. b) The class name can contain numbers c) The class name can not start with a number d) All the above 9. An object is created at __ time in Java. a) Compile-time b) Run time c) Assembling time d) None of the above 10. A Java constructor is like a method without ___. a) statements b) return type c) argument list d) None 11. The name of a constructor and the name of a class are ___. a) Same as with class name b) Different with class name c) a&b 12. All Java methods must have a return type. (TRUE / FALSE) a) true b) false 13. Java method signature is a combination of ___. a) Return type b) Method name c) Argument List d) All the above
- 42. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 42 2.4 Object Oriented Programming features in java These features include inheritance, polymorphism, encapsulation and Abstraction . These four features are the main OOPs Concepts that you must learn to understand the Object Oriented Programming in Java Inheritance The process by which one class acquires the properties and functionalities of another class is called inheritance. Inheritance provides the idea of reusability of code and each sub class defines only those features that are unique to it, rest of the features can be inherited from the parent class. Inheritance is a process of defining a new class based on an existing class by extending its common data members and methods. Inheritance allows us to reuse of code, it improves reusability in your java application. The parent class is called the base class or super class. The child class that extends the base class is called the derived class or sub class or child class. Syntax: Inheritance in Java To inherit a class we use extends keyword. Here class A is child class and class B is parent class. class A extends B { } Inheritance Example
- 43. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills ByAmanuelKassa Page 43 In this example, we have a parent class Teacher and a child class MathTeacher. In the MathTeacher class we need not to write the same code which is already present in the present class. Here we have college name, sawla and does() method that is common for all the teachers, thus MathTeacher class does not need to write this code, the common data members and methods can inherited from the Teacher class. class Teacher { String designation = "Teacher"; String college = “sawla"; void does(){ System.out.println("Teaching"); } } public class MathTeacher extends Teacher{ String mainSubject = "Maths"; public static void main(String args[]){ MathTeacher obj = new MathTeacher(); System.out.println(obj.college); System.out.println(obj.designation); System.out.println(obj.mainSubject); obj.does(); } } • Output: • sawla • Teacher • Maths • Teaching Forms/types of Inheritance: - Single Inheritance - Multiple Inheritance - Multilevel Inheritance - Hierarchical Inheritance - Hybrid Inheritance 1. Single inheritance: - If a class is derived from a single base class, it is called as single inheritance. In Single Inheritance, there is only one Super Class and Only one Sub Class Means they have one to one Communication between them. 2. Multiple Inheritances: - If a class is derived from more than one base class, it is known as multiple inheritances.
- 44. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 44 3. Multilevel Inheritance: When a derived class is created from another derived class, then that inheritance is called as multi level inheritance. 4. HierarchicalInheritance: If a number of classes are derived from a single base class, it is called as hierarchical inheritance. This means a Base Class will have Many Sub Classes or a Base Class will be inherited by many Sub Classes. 5. Hybrid Inheritance: This is a Mixture of two or More Inheritance types. I.e.: Any combination of single, hierarchical and multi-level inheritances is called as hybrid inheritance.
- 45. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 45 Polymorphism in Java The process of representing one form in multiple forms is known as Polymorphism. Polymorphism is derived from 2 greek words: poly and morphs. The word "poly" means many and "morphs" means forms. So polymorphism means many forms. Real life example of polymorphism in Java Suppose if you are in class room that time you behave like a student, when you are in market at that time you behave like a customer, when you at your home at that time you behave like a son or daughter, Here one person present in different-different behaviors. How to achieve Polymorphism in Java ?
- 46. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 46 In java programming the Polymorphism principal is implemented with method overriding concept of java. Polymorphism principal is divided into two sub principal they are: Static or Compile time polymorphism Dynamic or Runtime polymorphism Static Polymorphism: Polymorphism that is resolved during compiler time is known as static polymorphism. Method overloading can be considered as static polymorphism example. Method Overloading: This allows us to have more than one methods with same name in a class that differs in signature. class DisplayOverloading { public void disp(char c) { System.out.println(c); } public void disp(char c, int num) { System.out.println(c + " "+num); } } public class ExampleOverloading { public static void main(String args[]) { DisplayOverloading obj= newDisplayOverloading(); obj.disp('a'); obj.disp('a',10); } } Output: a a 10 Dynamic Polymorphism It is also known as Dynamic Method Dispatch. Dynamic polymorphism is a process in which a call to an overridden method is resolved at runtime rather, thats why it is called runtime polymorphism. Example class Animal{ public void animalSound(){ System.out.println("Default Sound"); } public class Dog extends Animal{ public void animalSound(){ System.out.println("Woof"); } public class Dog extends Animal{ public void animalSound(){ System.out.println("Woof");
- 47. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 47 } public static void main(String args[]){ Animal obj = newDog(); obj.animalSound(); } } Output: • Woof Since both the classes, child class and parent class have the same method animalSound. Which of the method will be called is determined at runtime by JVM. Abstraction One of the most fundamental concept of OOPs is Abstraction. Abstraction is a process where you show only “relevant” data and “hide” unnecessary details of an object from the user. The abstract keyword is a non-access modifier, used for classes and methods: • Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class). • Abstract method: can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from). • Syntax Abstract Class • abstract class A{ • abstract void myMethod(); • void anotherMethod(){ • //Does something • } • } Note 1: There can be some scenarios where it is difficult to implement all the methods in the base class. In such scenarios one can define the base class as an abstract class which signifies that this base class is a special kind of class which is not complete on its own. A class derived from the abstract base class must implement those methods that are not implemented (means they are abstract) in the abstract class. Abstract class cannot be instantiated which means you cannot create the object of abstract class. To use this class, you need to create another class that extends this abstract class provides the implementation of abstract methods, then you can use the object of that child class to call non-abstract parent class methods as well as implemented methods(those that were abstract in parent but implemented in child class). If a child does not implement all the abstract methods of parent class(the abstract class), then the child class must need to be declared abstract. Example of Abstract class and Methods Here we have an abstract class Animal that has an abstract method animalSound(), since the animal sound differs from one animal to another, there is no point in giving the implementation to this method as every child class must override this method to give its own implementation details. That’s why we made it abstract. • //abstract class • abstract class Animal{ • //abstract method • public abstract void animalSound(); • } • public class Dog extends Animal{ • • public void animalSound(){
- 48. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 48 • System.out.println("Woof"); • } • public static void main(String args[]){ • Animal obj = new Dog(); • obj.animalSound(); • } • } • Output: • Woof Encapsulation Encapsulation is the method of combining the data and functions inside a class. This hides the data from being accessed from outside a class directly, only through the Methods inside the class is able to access the information. Encapsulation is the term given to the process of hiding all the details of an object that do not necessary to its user. Encapsulation enables a group of properties, methods and other members to be considered a single unit or object. Features and Advantages of the concept of Encapsulation: - Makes Maintenance of Application Easier - Improves the Understandability of the Application - Enhanced Security - Protection of data from accidental corruption - Flexibility and extensibility of the code and reduction in complexity Encapsulation example in Java How to 1) Make the instance variables private so that they cannot be accessed directly from outside the class. You can only set and get values of these variables through the methods of the class. 2) Have getter and setter methods in the class to set and get the values of the fields. class EmployeeCount { private int numOfEmployees = 0; public void setNoOfEmployees (int count) { numOfEmployees = count; } public double getNoOfEmployees () { return numOfEmployees; } } public class EncapsulationExample { public static void main(String args[]) { EmployeeCount obj = new EmployeeCount (); obj.setNoOfEmployees(5613); System.out.println("No Of Employees: "+(int)obj.getNoOfEmployees());
- 49. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 49 } } Output: No Of Employees: 5613 The class EncapsulationExample that is using the Object of class EmployeeCount will not able to get the NoOfEmployees directly. It has to use the setter and getter methods of the same class to set and get the value. The class EncapsulationExample that is using the Object of class EmployeeCount will not able to get the NoOfEmployees directly. It has to use the setter and getter methods of the same class to set and get the value. So what is the benefit of encapsulation in java programming Well, at some point of time, if you want to change the implementation details of the class EmployeeCount, you can freely do so without affecting the classes that are using it. Introduction to Exception Handling An exception is a special condition that changes the normal flow of program execution. Exceptional conditions are things that occur in a system that are not expected or are not a part of normal system operation. When the system handles these exceptional conditions improperly, it can lead to failures and system crashes. Exception handling is the method of building a system to detect and recover from exceptional conditions. Exceptional conditions are any unexpected occurrences that are not accounted for in a system's normal operation. Some examples of exceptional conditions: - incorrect inputs from the user - bit level memory or data corruption If these exceptional conditions are not properly caught and handled, they can cause an error or failure in the system. An exception is a problem that arises during the execution of a program. A Java exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero. Exceptions provide a way to transfer control from one part of a program to another. Java exception handling is built upon three keywords: try, catch, and throw. throw: A program throws an exception when a problem shows up. This is done using a throw keyword. catch: A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The catch keyword indicates the catching of an exception. try: A try block identifies a block of code for which particular exceptions will be activated. It's followed by one or more catch blocks Basic syntax: try{ } catch(Exception e){ } Example: public class ExceptionDemo { public static void main (String[] args) { int a=10; for(int i=3;i>=0;i--) try{ System.out.println(a/i);
- 50. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 50 }catch(ArithmeticException e){ System.out.println(e); } } } Output: 3 5 10 java.lang.ArithmeticException: / by zero try block contains the code that might throw an exception. Don’t write anything extra in try as statements after the exception will not get executed if the exception occurred. Try must be immediately followed by catch or finally block. public class ExceptionDemo { public static void main (String[] args) { int a=10; for(int i=3;i>=0;i--) try{ System.out.println(a/i); } } } Compile time error: prog.java:5: error: 'try' without 'catch', 'finally' or resource declarations try{ ^ 1 error The catch block is used to catch the exception thrown by statements in the try block. The catch must follow try else it will give a compile-time error. public class ExceptionDemo { public static void main (String[] args) { int a=10; for(int i=3;i>=0;i--) try{ System.out.println(a/i); } System.out.println("between try and catch"); catch(ArithmeticException e){ System.out.println(e); } } }
- 51. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 51 Compile Time Error: prog.java:5: error: 'try' without 'catch', 'finally' or resource declarations try{ ^ prog.java:9: error: 'catch' without 'try' catch(ArithmeticException e){ ^ 2 errors Things to Remember Do not keep any code after the statement which is prone to exception. Because if an exception occurred then it will straight away jump to the catch or finally block, ignoring all other statements in the try block. class Main { public static void main (String[] args) { try { System.out.println(4/0); //will not get printed System.out.println("end of try!"); } catch(ArithmeticException e) { System.out.println("divide by 0"); } } } Output: divide by 0 While catching the exception in the catch block, either you can have directly the class of exception or its superclass. Example: Exact Exception class Main { public static void main (String[] args) { try{ System.out.println(4/0); } //ArithmeticException catch(ArithmeticException e){ System.out.println("divide by 0"); }
- 52. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 52 } } Output: divide by 0 Java GUI Programming One of the big advances that Java made when it was introduced in 1995 was the inclusion of standardized "graphical user interface" ("GUI") libraries. This was a tremendous leap forward over the bewildering array of incompatible third-party libraries that were the only avenues previously open to GUI developers. The Java GUI library was also one of the first major software packages to explicitly describe its architecture in terms of the then-brand-new language of Design Patterns. GUI (Graphical User Interface) in Java is an easy-to-use visual experience builder for Java applications. It is mainly made of graphical components like buttons, labels, windows, etc. through which the user can interact with an application. GUI plays an important role to build easy interfaces for Java applications. Main Packages One confusing aspect of the Java GUI system however is a legacy issue where the system underwent a large-scale upgrade in 1997, so some of the classes involved are spread over the (relatively) older java.awt packages and the newer javax.swing packages. In general, if the class name starts with "J", then it is in the Swing package. Since some functionality appears to be duplicated in the Swing packages, such as frames and buttons, always use the Swing component over the older AWT components when there is a choice. java.awt -- Contains the main super classes for the GUI components plus a number of utility type classes, such as Color and Point. java.awt.event -- Contains the classes and interfaces for managing events from the GUI components, e.g. button clicks and mouse movements. javax.swing -- Contains most of the visible GUI components that are used such as buttons, text fields, frames and panels. Class Hierarchy Java uses the Composite Design Pattern to create GUI components that can also serve as containers to hold more GUI components. Here is a UML diagram showing the class relations between a few of the more commonly used GUI components:
- 53. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 53 Above, the commonly used container components are in orange while the non-container components are in blue. Note that technically, all javax.swing components are capable of holding other Components, but in practice, only JFrame, JApplet, and JPanel, plus a few not shown above (JSplitPane, JTabbedPane and JScrollPane) are commonly used for that purpose.
- 54. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 54 Commonly Used Classes Simple Components javax.swing.JLabel -- A piece of non-editable text on the screen. javax.swing.JButton -- A button that can be clicked. Clicking the button will cause its actionPerformed event to be fired which will call all installed ActionListener's actionPerformed methods. javax.swing.JTextField -- A single line of user-editable text. javax.swing.JTextArea -- A box with multiple lines of text displayed. Can be set for editable or non- editable. javax.swing.JRadioButton, javax.swing.JCheckBox -- Small round or square boxes that can be clicked on to indicate the selection of something, such as an option of some sort. Clicking the radio button or check box will fire a particular type of event. Radio buttons can be grouped together using an invisible (to the user) class called javax.swing.ButtonGroup to create a set where only one button can be set at a time. javax.swing.JComboBox -- A drop-list of items. A JComboBox holds a set of Objects, not just Strings. The displayed texts are the return values of the item's toString() method. Thus a JComboBox can be used to hold arbitrary entities without the GUI knowing what those entities are. Events are fired only when the selected item changes, though the currently selected item and its index can always be retrieved.
- 55. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 55 Container Components javax.swing.JFrame -- a stand-alone "window" or "frame" with a title and the usual abilities to be moved, resized, minimized, maximized and closed. For most people, this is the top-level container in their system. Thus a JFrame cannot hold another JFrame. While a JFrame can hold any type of component, typically, a JFrame holds JPanels, which are arranged to group together sets of the components used in the window (frame). The default layout manager of a JFrame is the BorderLayout. A JFrame has a couple of unique and important methods that are worth pointing out: void getContentPane().add(Component c) -- adds a component to the frame. Note that a JFrame acts slightly differently than a JPanel here because JFrames have a little-used feature of multiple layers (panes) of components that can be quickly switched in and out of visibility. There are overridden forms of this method inherited from java.awt.Container that allow parameters for the layout manager to be passed along, such as specifying the component's position in a BorderLayout. void setDefaultCloseOperation(int option) -- sets the behavior of the frame when it is closed. See the web page on Setting the Behavior When Closing a JFrame. javax.swing.JApplet -- Allows a Java program to be run as a component in a web page. Note that an applet and a frame are not the same thing and are not interchangeable, though there are some work-arounds for that which allow the same code to run as either a stand-alone application or an applet in a web page. javax.swing.JPanel -- The basic building block for laying components out in a frame or inside of other panels. A JAva GUI consists of a frame with panels holding panels holding panels, etc. Note that "panels" here could also be scroll panes, split panes and/or tabbed panes (see below). The default layout manager for a JPanel is the FlowLayout. javax.swing.JScrollPane -- Allows you to display a single panel with scroll bars on the sides, allowing more or larger components to be displayed than will fit on the screen. javax.swing.JSplitPane -- Allows two panels to be displayed with either a vertical or horizonatal, user- moveable bar separating them. javax.swing.JTabbedPane -- Allows multiple panels to be displayed in a "tabbed" format. Commonly Used Methods in GUI Containers void add(Component c) -- adds another component to the container to be laid out as per the currently installed layout manager. There are overridden forms of this method inherited from java.awt.Container that allow parameters for the layout manager to be passed along, such as specifying the component's position in a BorderLayout. void setLayout(LayoutManager lm) -- installs a new layout manager into the container. Utility Classes java.awt.Graphics --This abstract class is rarely instantiated by the developer's code. A machine- dependent instance of this class is handed to the paintComponent method of a visible GUI component during the screen painting process -- it is NOT instantiated by the developer's code but rather by the Java GUI sub- system. The developer can writed code that will use that supplied Graphics object instance to draw lines, shapes and images onto the screen. For advanced graphics work, it should be noted that the supplied Graphics object can always be safely downcast to the more capable java.awt.Graphcs2D class.
- 56. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 56 NEVER CALL paintComponent DIRECTLY FROM YOUR OWN CODE. Likewise, NEVER INSTANTIATE A Graphics OBJECT--the Java GUI system will automatically hand your code the Graphics object that it needs. java.awt.Color -- Represents a color. Has static fields predefined to common colors plus can be expressed in a number of different formats, such as 3-bit RGB values. java.awt.Point -- Represents an point on 2-dimensional plane as specified by integer-valued Cartesian coordinates.. Has fields to retrieve the x and y coordinates plus methods for operations such as calculating the distance between two points. For floating point-valued Cartesian coordinates, use the java.awt.geom.Point2D class. Layout Managers Java containers use the Strategy Design Pattern to implement automatic layout management of GUI components on the screen. The management of the size and positions of the contained components is delegated to a "layout manager", which can be set at design time or dynamically changed. Here are a couple of common layouts: java.awt.BorderLayout -- Separates the component into 5 distinct areas, 4 non-resizeable areas on the edges ("North", "East", "South", and "West") and one re-sizeable area in the center ("Center"). Each area can only contain 1 component, so typically, panels are used in each area to hold multiple components. A typical statement that adds a component to a container that is known to have a BorderLayout installed would be myContainer.add(myComponent, BorderLayout.NORTH); java.awt.FlowLayout -- Holds multiple components in a horizontally arranged line that will wrap around if the width of the container is too small. A FlowLayout does not require any additional parameters when a component is added to the container -- the components will display left-to-right in the order in which they were added. javax.swing.BoxLayout -- Holds components in either a vertical or horizontal arrangement that is fixed, independent of the size of container. The width and height of the contained components are all identical and are automatically set tocompletely fill the container. Event Handling in Java GUIs
- 57. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 57 Java uses the Observer-Observable Design Pattern to handle events from GUI components, e.g. button clicks and mouse movements. Components supply methods to add various types of listeners, e.g. "void addActionListener(ActionListener al)" and "void addMouseMotionListener(MouseMotionListener mml)". Multiple listeners can be added and all will be called when the associated event occurs, though there is no guarantee as to the order in which the installed listeners will be called. java.awt.event.ActionListener -- Interface used to detect a simple action on a component, such as clicking a button or hitting Enter on a text field. java.awt.event.MouseListener -- Interface used for event listeners watching for mouse click events. This event allows more detailed capture of the mouse click than an ActionListener event, for instance, distinguishing between a mouse button pressed vs. a mouse button released events. A convenience class, MouseAdapter, is also available that no-ops all the methods, freeing the developer to only write the code that overrides the desired methods. java.awt.event.MouseMotionListener -- Interface used for event listeners watching for mouse motion events, e.g. dragging. A convenience class, MouseMotionAdapter, is also available that no-ops all the methods, freeing the developer to only write the code that overrides the desired methods. How to start a GUI-based program In general, a GUI-based program will start whenever the first GUI component is shown (visibility set to true). Typically that would be one of the frames in the system, so one would have a line of code like such: myFrame.setVisible(true); This a quickie starts for a GUI program would have a main method like such: public static void main(String[] args) { java.awt.EventQueue.invokeLater(new Runnable() { public void run() { try { (new MyFrameClass("The title")).setVisible(true); } catch (Exception e) { e.printStackTrace(); } } }); } (Note: The above code correctly instantiates the frame on the GUI thread as specified by the official Java requirements. The code can be simplified a little bit by using a lambda function instead of the anonymous inner class implemenation of a Runnable.) In Model-View-Controller architectures however, it is not recommended that the view, i.e. the main frame, be started by simply setting its visibility to true. It is much better to define a "start()" method on the frame or main view class, where all final initializations and checks are performed and whose last line of code is the setVisible(true) statement.
- 58. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 58 A typical controller in an MVC wills instantiate the model and the view, connecting them together with their respective adapters. After all that is done, the controller's last act will be to call the view's start method. NEVER START THE GUI FROM THE CONSTRUCTOR OF THE FRAME! This is simply asking for trouble as many of the other components in the system are probably not fulling instantiated or intialized yet. But as soon as the first frame becomes visible, events will start firing, and those listeners will expect fully operational objects to be present but won't find them, causing, at best, a flurry of null pointer errors. Java GUI Example Now in this Swing Java Tutorial, let's understand GUI with Java Swing examples. Example: To learn Java GUI programming in this Java GUI tutorial Step 1) Copy the following code into a text editor like notepad import javax.swing.*; class SPTC{ public static void main(String args[]){ JFrame frame = new JFrame("Sawla Polytechnic college"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setSize(300,300); JButton button = new JButton("SPTC_BUTTON"); frame.getContentPane().add(button); // Adds Button to content pane of frame frame.setVisible(true); }} Step 2) Save, Compile, and Run the code using javac and java commands in to cmd. Step 3) now let's Add a Button to our frame. Copy following code into an editor from given Java GUI Example import javax.swing.*; class SPTC{ public static void main(String args[]){ JFrame frame = new JFrame("Sawla Polytechnic college"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setSize(300,300); JButton button = new JButton("SPTC_BUTTON"); frame.getContentPane().add(button); // Adds Button to content pane of frame frame.setVisible(true); }} Step 4) Execute the code. You will get a big button
- 59. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 59 Step 5) How about adding two buttons? Copy the following code into an editor. import javax.swing.*; class SPTC{ public static void main(String args[]){ JFrame frame = new JFrame("Sawla Polytechnic college"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setSize(300,300); JButton button = new JButton("SPTC_BUTTON"); frame.getContentPane().add(button); // Adds Button to content pane of frame frame.setVisible(true); }} Step 6) Save, Compile, and Run the program. Step 7) Unexpected output =? Buttons are getting overlapped. Java Layout Manager The Layout manager is used to layout (or arrange) the GUI java components inside a container.There are many layout managers, but the most frequently used are- Java BorderLayout A BorderLayout places components in up to five areas: top, bottom, left, right, and center. It is the default layout manager for every java JFrame
- 60. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 60 Java FlowLayout FlowLayout is the default layout manager for every JPanel. It simply lays out components in a single row one after the other. Java GridBagLayout It is the more sophisticated of all layouts. It aligns components by placing them within a grid of cells, allowing components to span more than one cell. Step 8) How about creating a chat frame like below?
- 61. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 61 //Usually you will require both swing and awt packages // even if you are working with just swings. import javax.swing.*; import java.awt.*; class gui2 { public static void main(String args[]) { //Creating the Frame JFrame frame = new JFrame("Sawla Polytechnic Students Chat Frame"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setSize(400, 400); //Creating the MenuBar and adding components JMenuBar mb = new JMenuBar(); JMenu m1 = new JMenu("FILE"); JMenu m2 = new JMenu("Help"); mb.add(m1); mb.add(m2); JMenuItem m11 = new JMenuItem("Open"); JMenuItem m22 = new JMenuItem("Save as"); m1.add(m11); m1.add(m22); //Creating the panel at bottom and adding components JPanel panel = new JPanel(); // the panel is not visible in output JLabel label = new JLabel("Enter Text"); JTextField tf = new JTextField(10); // accepts upto 10 characters JButton send = new JButton("Send"); JButton reset = new JButton("Reset"); panel.add(label); // Components Added using Flow Layout panel.add(tf); panel.add(send); panel.add(reset); // Text Area at the Center JTextArea ta = new JTextArea(); //Adding Components to the frame. frame.getContentPane().add(BorderLayout.SOUTH, panel); frame.getContentPane().add(BorderLayout.NORTH, mb); frame.getContentPane().add(BorderLayout.CENTER, ta); frame.setVisible(true); } }
- 62. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 62 How to Implement Event handling In Java? While using different programs on your PC or android mobile applications, ever wondered what code is executed after clicking a button or a switch? Most of the programs and mobile applications are written in Java. Java has special ways of handling these situations know as event handling. In this post, we will dive deeper into the concept of Event Handling in Java and understand it’s working in great detail. Source and Event Listeners List Of Event Listeners Delegation Event Model So lets us get started then, Event Handling in Java Source and Events While understanding the concept of event handling you might have come across terms such as sources, events, etc. Sources and events are some of the basic terms which are to be understood before we look at event handling. Event When you press a button in your program or Android application the state of the button changes from ‘Unclicked’ to ‘Clicked’.This change in the state of our button is called an Event. Events are generated based on how you interact with the GUI. For example- entering some text through the keyboard, moving your cursor, scrolling, etc generates events. Source In Java, nearly everything is an object. The button you press is an object too. Sorce is the object which generates an event. In other words, a source is an object which undergoes state change. It also provides information about the event to the listener. We will talk about the listener in the other half of this post. Now that we know what is source and event, lets move on to the next part of this event handling in Java article, Listeners Now we know about the events and the sources. This is a good time to talk about the listeners. Listeners are also called as event handlers as they are the ones responsible to handle events occurring at the source. Listeners are interfaces and different types of listeners are used according to the event. For the understanding purpose, we will be looking at the ActionListener as it is the most commonly used event listener and see how exactly it handles the events. Example Let see the following code import java.awt.*; import java.awt.event.*; class EventHandle extends Frame implements ActionListener{ TextField textField; EventHandle() { textField = new TextField(); textField.setBounds(60,50,170,20); Button button = new Button("Quote"); button.setBounds(90,140,75,40); //1 button.addActionListener(this); add(button); add(textField);
- 63. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 63 setSize(250,250); setLayout(null); setVisible(true); } //2 public void actionPerformed(ActionEvent e){ textField.setText("Hello SPTC Students"); } public static void main(String args[]){ new EventHandle(); } } Output Image 1 shows the output of our code when the state of the button was unclicked. Image 2 shows the output after the button is pressed. Let’s us continue with event handling in java article and look at the logic behind the code and understand ActionListener in detail. First of all, we imported all the important packages required to implement the functionalities required. After importing packages we implemented ActionListener interface to our class EventHandle. Now, look at the code I’ve divided it into 2 important parts. Int the first part we are registering our button object with the ActionListener. This is done by calling the addActionListener( ) method and passing current instance using ‘this’ keyword. Image 1 image 2
- 64. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 64 Java swing program to add two numbers In this program, You will learn how to add two numbers using swing in java. class Test extends JFrame implements ActionListener { JButton jb1; JTextField jt1, jt2; JLabel lbl; } Example: How to add two numbers using swing in java. import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import javax.swing.JButton; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JTextField; class Test extends JFrame implements ActionListener { JButton jb1; JTextField jt1, jt2; JLabel lbl; Test() { jt1 = new JTextField(); jt1.setBounds(90, 50, 150, 30); add(jt1); jt2 = new JTextField(); jt2.setBounds(90, 80, 150, 30); add(jt2); lbl = new JLabel("Result :"); lbl.setBounds(90, 140, 150, 30); add(lbl); jb1 = new JButton("+"); jb1.setBounds(90, 200, 100, 30); add(jb1); jb1.addActionListener(this); setLayout(null); setSize(600, 400);
- 65. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 65 setVisible(true); } public void actionPerformed(ActionEvent e) { int a = Integer.parseInt(jt1.getText()); int b = Integer.parseInt(jt2.getText()); int c = 0; if (e.getSource().equals(jb1)) { c = a + b; lbl.setText(String.valueOf(c)); } } public static void main(String args[]) { Test t = new Test(); } } Output: Java - Applet Basics An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal. There are some important differences between an applet and a standalone Java application, including the following − An applet is a Java class that extends the java.applet.Applet class. A main() method is not invoked on an applet, and an applet class will not define main(). Applets are designed to be embedded within an HTML page. When a user views an HTML page that contains an applet, the code for the applet is downloaded to the user's machine.
- 66. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 66 A JVM is required to view an applet. The JVM can be either a plug-in of the Web browser or a separate runtime environment. The JVM on the user's machine creates an instance of the applet class and invokes various methods during the applet's lifetime. Applets have strict security rules that are enforced by the Web browser. The security of an applet is often referred to as sandbox security, comparing the applet to a child playing in a sandbox with various rules that must be followed. Other classes that the applet needs can be downloaded in a single Java Archive (JAR) file. Life Cycle of an Applet Four methods in the Applet class gives you the framework on which you build any serious applet − init − This method is intended for whatever initialization is needed for your applet. It is called after the param tags inside the applet tag have been processed. start − This method is automatically called after the browser calls the init method. It is also called whenever the user returns to the page containing the applet after having gone off to other pages. stop − This method is automatically called when the user moves off the page on which the applet sits. It can, therefore, be called repeatedly in the same applet. destroy − This method is only called when the browser shuts down normally. Because applets are meant to live on an HTML page, you should not normally leave resources behind after a user leaves the page that contains the applet. paint − Invoked immediately after the start() method, and also any time the applet needs to repaint itself in the browser. The paint() method is actually inherited from the java.awt. A "Hello, World" Applet Following is a simple applet named HelloWorldApplet.java − import java.applet.*; import java.awt.*; public class HelloWorldApplet extends Applet { public void paint (Graphics g) { g.drawString ("Hello World", 25, 50); } } These import statements bring the classes into the scope of our applet class − java.applet.Applet java.awt.Graphics Without those import statements, the Java compiler would not recognize the classes Applet and Graphics, which the applet class refers to. The Applet Class Every applet is an extension of the java.applet.Applet class. The base Applet class provides methods that a derived Applet class may call to obtain information and services from the browser context. These include methods that do the following − Get applet parameters Get the network location of the HTML file that contains the applet Get the network location of the applet class directory Print a status message in the browser Fetch an image
- 67. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 67 Fetch an audio clip Play an audio clip Resize the applet Additionally, the Applet class provides an interface by which the viewer or browser obtains information about the applet and controls the applet's execution. The viewer may − Request information about the author, version, and copyright of the applet Request a description of the parameters the applet recognizes Initialize the applet Destroy the applet Start the applet's execution Stop the applet's execution The Applet class provides default implementations of each of these methods. Those implementations may be overridden as necessary. The "Hello, World" applet is complete as it stands. The only method overridden is the paint method. Invoking an Applet An applet may be invoked by embedding directives in an HTML file and viewing the file through an applet viewer or Java-enabled browser. The <applet> tag is the basis for embedding an applet in an HTML file. Following is an example that invokes the "Hello, World" applet − <html> <title>The Hello, World Applet</title> <hr> <applet code = "HelloWorldApplet.class" width = "320" height = "120"> If your browser was Java-enabled, a "Hello, World" message would appear here. </applet> <hr> </html> Note − You can refer to HTML Applet Tag to understand more about calling applet from HTML. The code attribute of the <applet> tag is required. It specifies the Applet class to run. Width and height are also required to specify the initial size of the panel in which an applet runs. The applet directive must be closed with an </applet> tag. If an applet takes parameters, values may be passed for the parameters by adding <param> tags between <applet> and </applet>. The browser ignores text and other tags between the applet tags. Non-Java-enabled browsers do not process <applet> and </applet>. Therefore, anything that appears between the tags, not related to the applet, is visible in non-Java-enabled browsers. The viewer or browser looks for the compiled Java code at the location of the document. To specify otherwise, use the codebase attribute of the <applet> tag as shown − <applet codebase = "https://amrood.com/applets" code = "HelloWorldApplet.class" width = "320" height = "120"> If an applet resides in a package other than the default, the holding package must be specified in the code attribute using the period character (.) to separate package/class components. For example − <applet = "mypackage.subpackage.TestApplet.class" width = "320" height = "120">
- 68. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 68 Getting Applet Parameters The following example demonstrates how to make an applet respond to setup parameters specified in the document. This applet displays a checkerboard pattern of black and a second color. The second color and the size of each square may be specified as parameters to the applet within the document. CheckerApplet gets its parameters in the init() method. It may also get its parameters in the paint() method. However, getting the values and saving the settings once at the start of the applet, instead of at every refresh, is convenient and efficient. The applet viewer or browser calls the init() method of each applet it runs. The viewer calls init() once, immediately after loading the applet. (Applet.init() is implemented to do nothing.) Override the default implementation to insert custom initialization code. The Applet.getParameter() method fetches a parameter given the parameter's name (the value of a parameter is always a string). If the value is numeric or other non-character data, the string must be parsed. The following is a skeleton of CheckerApplet.java − import java.applet.*; import java.awt.*; public class CheckerApplet extends Applet { int squareSize = 50; // initialized to default size public void init() {} private void parseSquareSize (String param) {} private Color parseColor (String param) {} public void paint (Graphics g) {} } Here are CheckerApplet's init() and private parseSquareSize() methods − public void init () { String squareSizeParam = getParameter ("squareSize"); parseSquareSize (squareSizeParam); String colorParam = getParameter ("color"); Color fg = parseColor (colorParam); setBackground (Color.black); setForeground (fg); } private void parseSquareSize (String param) { if (param == null) return; try { squareSize = Integer.parseInt (param); } catch (Exception e) { // Let default value remain }
- 69. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 69 } The applet calls parseSquareSize() to parse the squareSize parameter. parseSquareSize() calls the library method Integer.parseInt(), which parses a string and returns an integer. Integer.parseInt() throws an exception whenever its argument is invalid. Therefore, parseSquareSize() catches exceptions, rather than allowing the applet to fail on bad input. The applet calls parseColor() to parse the color parameter into a Color value. parseColor() does a series of string comparisons to match the parameter value to the name of a predefined color. You need to implement these methods to make this applet work. Specifying Applet Parameters The following is an example of an HTML file with a CheckerApplet embedded in it. The HTML file specifies both parameters to the applet by means of the <param> tag. <html> <title>Checkerboard Applet</title> <hr> <applet code = "CheckerApplet.class" width = "480" height = "320"> <param name = "color" value = "blue"> <param name = "squaresize" value = "30"> </applet> <hr> </html> Note − Parameter names are not case sensitive. Application Conversionto Applets It is easy to convert a graphical Java application (that is, an application that uses the AWT and that you can start with the Java program launcher) into an applet that you can embed in a web page. Following are the specific steps for converting an application to an applet. Make an HTML page with the appropriate tag to load the applet code. Supply a subclass of the JApplet class. Make this class public. Otherwise, the applet cannot be loaded. Eliminate the main method in the application. Do not construct a frame window for the application. Your application will be displayed inside the browser. Move any initialization code from the frame window constructor to the init method of the applet. You don't need to explicitly construct the applet object. The browser instantiates it for you and calls the init method. Remove the call to setSize; for applets, sizing is done with the width and height parameters in the HTML file. Remove the call to setDefaultCloseOperation. An applet cannot be closed; it terminates when the browser exits. If the application calls setTitle, eliminate the call to the method. Applets cannot have title bars. (You can, of course, title the web page itself, using the HTML title tag.) Don't call setVisible(true). The applet is displayed automatically. Event Handling Applets inherit a group of event-handling methods from the Container class. The Container class defines several methods, such as processKeyEvent and processMouseEvent, for handling particular types of events, and then one catch-all method called processEvent. In order to react to an event, an applet must override the appropriate event-specific method. import java.awt.event.MouseListener;
- 70. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 70 import java.awt.event.MouseEvent; import java.applet.Applet; import java.awt.Graphics; public class ExampleEventHandling extends Applet implements MouseListener { StringBuffer strBuffer; public void init() { addMouseListener(this); strBuffer = new StringBuffer(); addItem("initializing the apple "); } public void start() { addItem("starting the applet "); } public void stop() { addItem("stopping the applet "); } public void destroy() { addItem("unloading the applet"); } void addItem(String word) { System.out.println(word); strBuffer.append(word); repaint(); } public void paint(Graphics g) { // Draw a Rectangle around the applet's display area. g.drawRect(0, 0, getWidth() - 1, getHeight() - 1); // display the string inside the rectangle. g.drawString(strBuffer.toString(), 10, 20); }
- 71. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 71 public void mouseEntered(MouseEvent event) { } public void mouseExited(MouseEvent event) { } public void mousePressed(MouseEvent event) { } public void mouseReleased(MouseEvent event) { } public void mouseClicked(MouseEvent event) { addItem("mouse clicked! "); } } Now, let us call this applet as follows − <html> <title>Event Handling</title> <hr> <applet code = "ExampleEventHandling.class" width = "300" height = "300"> </applet> <hr> </html> Initially, the applet will display "initializing the applet. Starting the applet." Then once you click inside the rectangle, "mouse clicked" will be displayed as well. Displaying Images An applet can display images of the format GIF, JPEG, BMP, and others. To display an image within the applet, you use the drawImage() method found in the java.awt.Graphics class. Following is an example illustrating all the steps to show images − import java.applet.*; import java.awt.*; import java.net.*; public class ImageDemo extends Applet { private Image image; private AppletContext context; public void init() { context = this.getAppletContext(); String imageURL = this.getParameter("image"); if(imageURL == null) {
- 72. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 72 imageURL = "java.jpg"; } try { URL url = new URL(this.getDocumentBase(), imageURL); image = context.getImage(url); } catch (MalformedURLException e) { e.printStackTrace(); // Display in browser status bar context.showStatus("Could not load image!"); } } public void paint(Graphics g) { context.showStatus("Displaying image"); g.drawImage(image, 0, 0, 200, 84, null); g.drawString("www.javalicense.com", 35, 100); } } Now, let us call this applet as follows − <html> <title>The ImageDemo applet</title> <hr> <applet code = "ImageDemo.class" width = "300" height = "200"> <param name = "image" value = "java.jpg"> </applet> <hr> </html> Playing Audio An applet can play an audio file represented by the AudioClip interface in the java.applet package. The AudioClip interface has three methods, including − public void play() − Plays the audio clip one time, from the beginning. public void loop() − Causes the audio clip to replay continually. public void stop() − Stops playing the audio clip. To obtain an AudioClip object, you must invoke the getAudioClip() method of the Applet class. The getAudioClip() method returns immediately, whether or not the URL resolves to an actual audio file. The audio file is not downloaded until an attempt is made to play the audio clip. Following is an example illustrating all the steps to play an audio − import java.applet.*; import java.awt.*; import java.net.*;
- 73. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 73 public class AudioDemo extends Applet { private AudioClip clip; private AppletContext context; public void init() { context = this.getAppletContext(); String audioURL = this.getParameter("audio"); if(audioURL == null) { audioURL = "default.au"; } try { URL url = new URL(this.getDocumentBase(), audioURL); clip = context.getAudioClip(url); } catch (MalformedURLException e) { e.printStackTrace(); context.showStatus("Could not load audio file!"); } } public void start() { if(clip != null) { clip.loop(); } } public void stop() { if(clip != null) { clip.stop(); } } } Now, let us call this applet as follows − <html> <title>The ImageDemo applet</title> <hr> <applet code = "ImageDemo.class" width = "0" height = "0"> <param name = "audio" value = "test.wav"> </applet> <hr> </html>
- 74. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 74 LO3. Debug code 3.1 Using an IntegratedDevelopment Environment An integrated development environment (IDE) is a software application that provides comprehensive facilities to computer programmers for software development. An IDE normally consists of a source code editor, build automation tools,compiler or interpreter and a debugger. Ie: An integrated development environment (IDE) is a programming environment that consisting of a code editor, a compiler, a debugger, and a graphical user interface (GUI) builder. 3.2 Using Programdebugging techniques to detectand resolve errors This topic describes methods of debugging routines in Language Environment. Debug tools are designed to help you to detect errors early in your routine. Debug Tool also provides facilities for setting breakpoints and altering the contents and values of variables. 3.2.1 Debugging in Net Beans - Introduction What is Debugging? In computers, debugging means running (executing) the programming code to perform an action and display the result. Debugging is a necessary process in almost any new software development process. Debugging tools (called debuggers) help identify coding errors at various development stages. Some programming language packages include a facility for checking the code for errors as it is being written. To debug a programming code, start with a problem, isolate the source of the problem (compile), and then fix it. When you write programs, things don't always work correctly. That is, in fact, one of the reasons why we need to test our classes - we want to make sure they work correctly. However, when things don't work correctly, it can be difficult to figure out exactly what the problem is so that we can fix it. NetBeans, like most other programming environments, provides a particular tool for this called a debugger. Debugging can be defined as the process used for examining the code for errors. Debugging is carried out by setting breakpoints in code and then using debugger to run it. We can execute our code one line at a time and examine the state of our application in order to discover any problems. When we start a debugging session in NetBeans, all of the relevant debugger windows appear automatically at the bottom of our screen. We can debug an entire project, any executable class, and any JUnit tests. Practice Example: Make sure that NetBeans 6.7 IDE is open on your desktop Go to File on the left top side of the menu bar and select New Project. In the pop menu that appears, select Java under categories and Java Application under Projects. Click Next >
- 75. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 75 button. Name the project as Debugging_Intro and select appropriate project location. Click on Finish button. Note that a project with the name Debugging_Intro is being created on the left pane under projects. Replace the main method with the following method in the main class. public static int main(String[] args) { // TODO code application logic here Scanner sc; System.out.println("Enter any month in number (Ex: 1 for January, etc)"); sc = new Scanner(System.in); float month = Integer.parseInt(sc.next()); if (month / 2 == 0) { System.out.println("The month is Even"); } else { System.out.println("The month is Odd"); } // switch statement to return the name of the month switch (month) { case 1: System.out.println("January"); break; case 2: System.out.println("February"); break; case 3:
- 76. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 76 System.out.println("March"); break; case 4: System.out.println("April"); break; case 5: System.out.println("May"); break; case 6: System.out.println("June"); break; case 7: System.out.println("July"); break; case 8: System.out.println("August"); case 9: System.out.println("September"); break; case 10: System.out.println("October"); break; case 11: System.out.println("November"); break; case 12: System.out.println("December"); break; default: System.out.println("Invalid month."); break; } //while loop to calculate time to new year int j = 0; int i = month; while (i != 0) { i += 1; j++; System.out.println("Calculating time to New Year..."); } System.out.println("Total months left for New Year:" + j); } Note: Remember that you can format your code by right clicking anywhere inside the class and selecting
- 77. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 77 Format. It will align your code properly. Notice that there are few compilation errors in the program. Note: The compiler is always running behind the scene. Thus, there is no "Compile" option in this environment to choose from. The very 1st compilation error is in the main method. Change int to void in the main method to remove the compilation error. The main method is always void as it doesn't return any value. Note: The main method is always: public: so that it can be accessed outside the class. static: so that it can be invoked without creating any objects. void: as it doesn't return any value. The 2nd compilation error is in the declaration of switch statement. This error is because of the fact that switch statement does not work with float. At the start of the main method, change variable month to the data type int from float. Note: A switch works with the byte, short, char, and int primitive data types. To remove other errors, right click in the source editor window and select Fix Imports.
- 78. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 78 Make sure that java.util.Scanner appears under import statements in the pop up box. Click OK. Note that now there are no compilation errors in the program. Save the class and click on the Run icon on the top of the menu bar or by right clicking on the project and select Run. When the program prompts for the input, enter 8 in the Output pane at the bottom of the IDE and hit Enter. Note that despite the entered month as even (August), the output being displayed is The month is Odd and August September. And then the program enters into an infinite loop.
- 79. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 79 To exit from the infinite loop, click on the close button at the bottom most margin of the IDE. We're now going to track down the cause of the problem using the debugger. Do not solve the problem just by looking at the code. The point of this exercise is to learn how to use the debugger to solve more difficult problems than this one. We want to step through the program one step at a time, in order to figure out what is wrong with the code. To do this, we can first set a breakpoint in the code. A breakpoint is simply a flag that tells NetBeans "Stop here if you're debugging." A break point is a debugging mechanism which tells the program that when you reach this point you want to stop the program. To set a break point you go to the line of code that you wish to set it on and then click on the grey border on the left. This will put a breakpoint on this line. Let's trace the program to see why these errors occurs by placing a breakpoints on the if statement, switch statement and while loop. You can do this by clicking on the left grey stripe of the editor window. (Click again
- 80. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 80 to remove the break point.) Click on the Debug Main Project icon on the top of the menu bar. Once again, enter 8 in the Output pane at the bottom of the IDE and hit Enter.
- 81. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 81 To walk through the program step by step, you'll use the Step Over icon in the toolbar in the Debug pane of this window, which is located on the top of the IDE. Click on Step Over and continue with the clicking of Step Over and you will find that: a. instead of if statement in the main method, else statement is being executed. b. along with case 8 in switch statement case 9 is also being executed. c. the program never exits the while loop (value of i keeps increasing!) and therefore this loop is being executed infinite times. In fact, click on the Variables tab at the bottom of the IDE and you can observe the values of i goes from 8, to 9, to 10, to 11, etc., not going towards zero at all! This is a classic infinite loop! This process of variable watching allows you to observe the behavior of your program easily.
- 82. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 82 As we have now figured out the source of problem, stop the debugging by clicking on Finish icon on the top of the IDE. We found at the time of debugging that the if condition is not properly written. To fix this error, replace the '/' operator in the if statement with '%' operator. The if statement should look like this: Note: We know that '/' is the division operator. On the other hand, the '%' is the remainder operator, which divides
- 83. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 83 one operand by another and returns the remainder as its result. For example, if 8 is divided by 2, the '/' operator will return 4 and the '%' operator will return 0. While debugging we noticed that along with case 8 in switch statement, case 9 is also being executed. This is because of the fact that there is no break; statement in case 8 and therefore soon after execution of case 8, the control shifts to case 9. To fix this error, include the break; in case 8 of the switch statement. Lastly, we have also discovered while debugging that the while loop executes infinitely. Look closely and you will find that as we need to calculate months remaining for new year, the i != 0 doesn't make any sense. It must rather be i != 12. Therefore to get rid of this error, replace i != 0 with i != 12 in the while loop. Save your project and Run the project again by clicking on the Run icon on the top of the menu bar or by right clicking on the project and select Run. Enter any month of your choice and now you will get the correct output without any more errors! :) The purpose of this tutorial was to make you familiar with debugging by using a simple JAVA program! It is expected that you learn how to use the debugger to solve more difficult problems than this one !!
- 84. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 84 LO4. Document activities 4.1. Guidelines for Developing Maintainable Code Guidelines for developing maintainable code adhering to a set of coding standard is followed 4.2. The Use of Internal DocumentationStandards and Tools 4.2.1. Documentationtechniques At the completion of the design process, a comprehensive package of documentation is assembled. Detail documentation of the system should be created during each phase of the design process. The package should contain a clear description of every facet of the new system in sufficient detail to support its development, including these aspects: . Examples of output. . Descriptions and record layouts of each file. . Data-flow diagrams. . A data dictionary. . Identification of individual programs. High-level documentation provides a picture of the overall structure of the system, including input, processing, storage, and output. The general flow of information throughout the major processes of the system can be depicted in either a data-flow diagram or a systems flowchart The purpose of detail documentation is to provide a programmer with sufficient information to write the program. The documentation should include report layouts for input and output, record layouts that define the files, and the relationship among the components of the programs. 4.2.2. Programand documentation standards Documentation standard is the structure and presentation of documents on a software development. 4.2.3. Internal documentation techniques There are two kinds of documentations 1) System documentation 2) User documentation System documentation is detailed information about a system’s design specifications, its internal workings, and its functionality. System documentation is further divided into internal and external documentation. - Internal documentation is part of the program source code or is generated at compile time. - External documentation includes the outcome of all of the structured diagramming techniques such as DFD and ERD. User documentation is written or visual information about an application system, how it works and how to use it. The kinds of user documents are reference guide, user’s guide, release description, system administrator’s guide and acceptance sign-off.
- 85. Sawla Polytechnic College ICT Database Administration Level IV Applying Object-Oriented Programming Language Skills By Amanuel Kassa Page 85 LO5. Test code 5.1. Conducting Simple Tests to Confirm the Coding ProcessMeetsDesign Specifications Simple tests are developed and conducted to confirm the coding process meets design specification 5.1.1. Testing techniques The tests performed are documented. The detailed specifications produced during the design phase are translated into hardware, communications, and executable software. The design must be translated into a machine-readable form. The code generation step performs this task. If the design is performed in a detailed manner, code generation can be accomplished without much complication. Different high level programming languages are used for coding. With respect to the type of application, the right programming language is chosen. Depending on the size and complexity of the system, coding can be an involved, intensive activity. Once coding is begun, the testing process can begin and proceed in parallel. As each program module is produced, it can be tested individually, then as a part of a larger program, and then as part of larger system. The deliverables and outcome from the coding are the code and program documentation. 5.1.2. User manual A user manual is, also known as a user guide, is a technical communication document intended to give assistance to people using a particular system. It is a step-by-step describes how the users can use the system. Generally the description is in detail keeping in view the fact that the target users using the system have limited knowledge about it. 5.1.3. Printing documents of the program You may print document of the program code. 5.2. Implementation of Required Corrections Corrections are made to the code and the documentation as needed