Core Java

CORE JAVA
PRESENTRED BY:
PRIYANKA PRADHAN
ASSISTANT PROFESSOR
MJPRU,BAREILLY.
Priyanka Pradhan 1

What is Java?
• Java is a high-level programming language originally developed by Sun Microsystems and
released in 1995.
Java is:
 Object Oriented
 Platform independent:
 Simple
 Secure
 Architectural- neutral
 Portable
 Robust
 Multi-threaded
 Interpreted
 High Performance
 Distributed
 Dynamic
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Tools You Will Need
You will need the following softwares:
• Linux 7.1 or Windows xp/7/8 operating system
• Java JDK 8
• Microsoft Notepad or any other text editor
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Setting Up the Path for Windows
Assuming you have installed Java in c:Program
Filesjavajdk directory:
• Right-click on 'My Computer' and select 'Properties'.
• Click the 'Environment variables' button under the
'Advanced' tab.
• Now, alter the 'Path' variable so that it also contains
the path to the Java executable. Example, if the path is
currently set to 'C:WINDOWSSYSTEM32', then
change your path to read
'C:WINDOWSSYSTEM32;c:Program
Filesjavajdkbin'.
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Popular Java Editors
To write your Java programs, you will need a text editor.
• Notepad: On Windows machine, you can use any
simple text editor like Notepad (Recommended for this
tutorial), TextPad.
• Netbeans: A Java IDE that is open-source and free,
which can be downloaded from
http://www.netbeans.org/index.html.
• Eclipse: A Java IDE developed by the eclipse open-
source community and can be downloaded from
http://www.eclipse.org/.
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how to save the file, compile, and run
the program
• Open notepad and add the code.
• Save the file as: MyFirstJavaProgram.java.
• Open a command prompt window and go to the directory where you saved the class. Assume it's
C:.
• Type 'javac MyFirstJavaProgram.java' and press enter to compile your code. If there are no errors in
your code, the command prompt will take you to the next line (Assumption : The path variable is
set).
• Now, type ' java MyFirstJavaProgram ' to run your program.
• You will be able to see ' Hello World ' printed on the window.
C:> javac MyFirstJavaProgram.java
C:> java MyFirstJavaProgram
Hello World
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Basic Syntax
• Case Sensitivity - Java is case sensitive, which means identifier Helloand hello would have different
meaning in Java.
• Class Names - For all class names the first letter should be in Upper Case. If several words are used
to form a name of the class, each inner word's first letter should be in Upper Case.
• Example: class MyFirstJavaClass
• Method Names - All method names should start with a Lower Case letter. If several words are used
to form the name of the method, then each inner word's first letter should be in Upper Case.
Example: public void myMethodName()
• Program File Name - Name of the program file should exactly match the class name. When saving
the file, you should save it using the class name (Remember Java is case sensitive) and append
'.java' to the end of the name (if the file name and the class name do not match, your program will
not compile).
• Example: Assume 'MyFirstJavaProgram' is the class name. Then the file should be saved as
'MyFirstJavaProgram.java'
• public static void main(String args[]) - Java program processing starts from the main() method
which is a mandatory part of every Java program.
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Java Identifiers
• All Java components require names. Names used for classes,
variables, and methods are called identifiers.
• In Java, there are several points to remember about identifiers.
They are as follows:
• All identifiers should begin with a letter (A to Z or a to z), currency
character ($) or an underscore (_).
• After the first character, identifiers can have any combination of
characters.
• A key word cannot be used as an identifier.
• Most importantly, identifiers are case sensitive.
• Examples of legal identifiers: age, $salary, _value, __1_value.
• Examples of illegal identifiers: 123abc, -salary.
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Java Modifiers
It is possible to modify classes, methods, etc., by
using modifiers. There are two categories of
modifiers:
• Access Modifiers: default, public , protected,
private
• Non-access Modifiers: final, abstract, strictfp
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Java Variables
Following are the types of variables in Java:
• Local Variables
• Class Variables (Static Variables)
• Instance Variables (Non-static Variables)
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Java Arrays
• Arrays are objects that store multiple variables
of the same type. However, an array itself is
an object on the heap.
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Java Enums
• Enums were introduced in Java 5.0. Enums restrict a
variable to have one of only a few predefined values.
The values in this enumerated list are called enums.
• With the use of enums it is possible to reduce the
number of bugs in your code.
• For example, if we consider an application for a fresh
juice shop, it would be possible to restrict the glass size
to small, medium, and large. This would make sure that
it would not allow anyone to order any size other than
small, medium, or large.
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Java Keywords
These reserved words may not be used as constant
or variable or any other identifier names.
• abstract
• assert
• boolean
• break
• byte
• case
• catch
• char
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Comments in Java
• Java supports single-line and multi-line comments very similar to C
and C++. All characters available inside any comment are ignored by
Java compiler.
public class MyFirstJavaProgram{
/* This is my first java program.
* This is an example of multi-line comments.
*/
public static void main(String []args){
// This is an example of single line comment
System.out.println("Hello World");
}
}
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Inheritance
• In Java, classes can be derived from classes.
Basically, if you need to create a new class and
here is already a class that has some of the code
you require, then it is possible to derive your new
class from the already existing code.
• This concept allows you to reuse the fields and
methods of the existing class without having to
rewrite the code in a new class. In this scenario,
the existing class is called the superclass and the
derived class is called the subclass.
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Interfaces
• In Java language, an interface can be defined
as a contract between objects on how to
communicate with each other. Interfaces play
a vital role when it comes to the concept of
inheritance.
• An interface defines the methods, a deriving
class (subclass) should use. But the
implementation of the methods is totally up
to the subclass.
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Java – Objects & Classes
Java is an Object-Oriented Language. As a language that has
the Object-Oriented feature, Java supports the following
fundamental concepts:
• Polymorphism
• Inheritance
• Encapsulation
• Abstraction
• Classes
• Objects
• Instance
• Method
• Message Parsing
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Contd…
• Object - Objects have states and behaviors.
Example: A dog has states - color, name, breed as
well as behaviors – wagging the 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 support.
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Objects in Java
• If we consider the real-world, we can find many
objects around us, cars, dogs, humans, etc. All
these objects have a state and a behavior.
• Software objects also have a state and a behavior.
A software object's state is stored in fields and
behavior is shown via methods.
• in software development, methods operate on
the internal state of an object and the object-to-
object communication is done via methods.
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Classes in Java
• A class is a blueprint from which individual objects are created.
public class Dog{
String breed;
int ageC;
String color;
void barking(){
}
void hungry(){
}
void sleeping(){
}
}
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Contd…
A class can contain any of the following variable types.
• Local variables: Variables defined inside methods,
constructors or blocks are called local variables. The
variable will be declared and initialized within the method
and the variable will be destroyed when the method has
completed.
• Instance variables: Instance variables are variables within a
class but outside any method. These variables are initialized
when the class is instantiated. Instance variables can be
accessed from inside any method, constructor or blocks of
that particular class.
• Class variables: Class variables are variables declared
within a class, outside any method, with the static keyword.
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Constructors
• Every class has a constructor.
• If we do not explicitly write a constructor for a class, the Java compiler
builds a default constructor for that class.
• Each time a new object is created, at least one constructor will be invoked.
The main rule of constructors is that they should have the same name as
the class. A class can have more than one constructor.
example of a constructor:
public class Puppy{
public Puppy(){
}
public Puppy(String name){
// This constructor has one parameter, name.
}
}
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Creating an Object
• In Java, the new keyword is used to create new
objects.
There are three steps when creating an object from
a class:
• Declaration: A variable declaration with a
variable name with an object type.
• Instantiation: The 'new' keyword is used to
create the object.
• Initialization: The 'new' keyword is followed by a
call to a constructor. This call initializes the new
object.
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Contd…
example of creating an object:
public class Puppy{
// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
// Following statement would create an object myPuppy
Puppy myPuppy = new Puppy( "tommy" );
}
}
If we compile and run the above program, then it will produce the following result:
Passed Name is :tommy
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Accessing Instance Variables and
Methods
Instance variables and methods are accessed via created
objects.
/* First create an object */
ObjectReference = new Constructor();
/* Now call a variable as follows */
ObjectReference.variableName;
/* Now you can call a class method as follows */
ObjectReference.MethodName();
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how to access instance variables and
methods of a class
public class Puppy{
int puppyAge;
System.out.println("Name chosen is :" + name );
}
public void setAge( int age ){
puppyAge = age;
}
public int getAge( ){
System.out.println("Puppy's age is :" + puppyAge );
return puppyAge;
}
Puppy myPuppy = new Puppy( "tommy" );
myPuppy.setAge( 2 );
myPuppy.getAge( );
System.out.println("Variable Value :" + myPuppy.puppyAge );
}
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Java Package
• it is a way of categorizing the classes and
interfaces. When developing applications in
Java, hundreds of classes and interfaces will
be written, therefore categorizing these
classes is a must as well as makes life much
easier.
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Import Statements
• In Java if a fully qualified name, which includes
the package and the class name is given, then the
compiler can easily locate the source code or
classes. Import statement is a way of giving the
proper location for the compiler to find that
particular class.
• For example, the following line would ask the
compiler to load all the classes available in
directory java_installation/java/io:
• import java.io.*;
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Java – Basic Datatypes
There are two data types available in Java:
• Primitive Datatypes
• Reference/Object Datatypes
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