Week 7 Java Programming Methods For I.T students.pdf

Java Methods
Week 7
Objective: To apply predefined and user defined methods in Java
programming.

What is a Method in Java?
• Is a block of code or collection of statements or a set of code
grouped together to perform a certain task or operation.
• Used to achieve a reusability of code. This means that a
method can written once and use it many times.

• Provides easy modification and readability of code by adding
or removing a chunk of code.
• It is executed only when we call or invoke it.
• The most important method in java is the main() method.

Types of Method
• Predefined Method
• User-defined Method

Predefined Method
Methods already written
and provided by Java
Organized as a collection
of classes (class libraries)
To use: it needs an
import package
Method Type: data type
of value returned by
method

Predefined Method
Are the method
that is already
defined in Java
class libraries.
Also known as
standard library
method or built-in
method.

User-Defined Methods
The method
written by the user
or programmer
This method is
modified according
to the requirement

Types of User-Defined Method
Ø Non – value returning method
Ø Value returning method
Ø Methods Overloading

• When the method returns nothing, the return
keyword at the end of the method is optional.
• It uses void keyword.
• Similar in structure to value-returning methods
Non - Value Returning Method

• Similar in structure to value-returning methods
• Call to method is always stand-alone statement
• Can use return statement to exit method early
Void Method

Syntax of Creating a method
without a return value
public static type name()
{
// method boy or method signature
}
Access Specifier
Return
Type
Method
Name
Method Header

Example of a
User-Defined
Method
public class Method {
public static void display()
{
System.out.println("Hi Java");
}
public static void main(String[] args) {
// call or invoke the Method
display();
} // end of main
}//end of class

public static void display()
{
System.out.println("Hi Java");
}
// call or invoke the Method
display();
} // end of main
}//end of class

Week 7 Java Programming Methods For I.T students.pdf

• Calculate and return a value
• Used to save value for later calculation or print value
• Uses a return keyword.
Value Returning Method

public static int sum(int x, int y)
{
int sum = x + y;
System.out.println("Sum = "+sum);
return sum;
}
Scanner input = new Scanner(System.in);
int a, b;
System.out.print("Enter value for a:");
a = input.nextInt();
System.out.print("Enter value for b:");
b = input.nextInt();
sum(a,b);
}
}

Variable x, y are arguments or known as the
actual parameters;
Variable a ,b are known as format parameters;

Naming a Method
• remember that the method name must be a verb and start
with a lowercase letter.
• If the method name has more than two words, the first
name must be a verb followed by adjective or noun.
• In the multi-word method name, the first letter of each
word must be in uppercase except the first word.
Example:
• Single-word method name: sum(), area()
• Multi-word method name: computeCircle(),
calculateArea()

Naming a Method
• It is also possible that a method has the same
name as another method name in the same
class, it is known as method overloading

Primitive Type Wrapper Classes as
Parameters
• If a formal parameter is of the primitive data type and the corresponding actual
parameter is a variable, then the formal parameter cannot change the value of the
actual parameter
• Only reference variables can pass values outside the method (except, of course, for
the return value)
• Corresponding to each primitive data type, Java provides a class so that the values of
primitive data types can be wrapped in objects
• The class Integer does not provide a method to change the value of an existing
Integer object
• The same is true of other wrapper classes

Reference Variables as Parameters
• If a formal parameter is a reference variable:
• Copies value of corresponding actual parameter
• Value of actual parameter is address of the object where
actual data is stored
• Both formal and actual parameter refer to same object

Use Reference Variables as Parameters
• Can return more than one value from a method
• Can change the value of the actual object
• When passing address, would save memory space and time,
relative to copying large amount of data

Method Overloading
• Method Overloading: is creating several methods within a class,
with the same name.
• The signature of the method consists of the method name and its
formal parameter list
• Two methods have different signatures if they have either different
names or different format parameter lists
• Note that the signature of a method does not include the return type of
the method

Method Overloading
- by changing the number of arguments

Method Overloading
- by changing the data types

Debugging: Using Drivers and Stubs
• A program may contain a number of methods. In a complex program,
usually, when a method is written, it is tested and debugged alone.
• You can write a separate program to test the method. The program that
tests a method is called a driver program.
• Before writing the complete program, you could write separate driver
programs to make sure that each method is working properly.

Summary
• Sometimes the results calculated by one method are needed in another
method.
• In that case, the method that depends on another method cannot be tested
alone.
• A method stub is a method that is not fully coded.
• For a void method, a method stub might consist of only a method header
and a set of empty braces, {}.
• For a value-returning method it might contain only a return statement with
a plausible return value.

• If the problem is large and complex, it must be broken into subproblems, and if a
subproblem is still complex, it must further be divided into subproblems.
• The subdivision of a problem should continue to the point where the solution is clear and
obvious.
• Once a subproblem is solved, we can continue with the solution of another subproblem
and if all the subproblems of a problem are solved, we can continue with the next level.
• Eventually, the overall solution of the problem must be assembled and tested to ensure that
the programming code accomplishes the required task.

• A Java program is a collection of classes, and a class is a collection of data members
and methods.
• Each class and each method must work properly.
• To accomplish this, as explained in the previous section, once a method is written, it
can be tested using stubs and drivers.
• Since a method can be tested in isolation, it is not necessary to code all the methods in
order.
• Once all the methods are written, the overall program must be tested.

• The technique to solve a problem by subdividing into smaller problems is known as divide
and conquer and top-down design approach.
• These techniques are suitable and work for many kinds of problems, including most of the
problems given in this book and the problems you will encounter as a beginning
programmer.
• To simplify the overall solution of a problem that consists of many subproblems, we write
and test the code one piece at a time.
• Typically, once a subproblem is solved and the code is tested, it is saved as the first
version or a version of the program.
• We continue to add and save the program one piece at a time. Keep in mind that a working
program with fewer features is better than a nonworking one with many features.

Week 7 Java Programming Methods For I.T students.pdf

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