1. DEFINING METHODS &
CONSTRUCTORS
Chapter 6.5:

2. Contents
 Parameter : value given to method so that
it can do its task
 Can has 0 or more parameter
 Return value: A result that the method has
computed and returns it to the caller
 Can returns 0 or 1 value
 Eg. - pow(2,3)
- calculateArea()
- getBalance( )
- move( )

3. Learning Goals
 A method declaration specifies the code that will
be executed when the method is invoked (called)
 When a method is invoked, the flow of control
jumps to the method and executes its code
 When complete, the flow returns to the place
where the method was called and continues
 The invocation may or may not return a value,
depending on how the method is defined

4. Contents
 If the called method is in the same class, only the
method name is needed
myMethod();
myMethodcompute

5. Learning Goals
 The called method is often part of another class or
object
doIt helpMe
helpMe();obj.doIt();
main

6. Contents
 What specific task will the method perform?
 What input data will it need to perform its task?
 What result will the method produce?
 How input data are processed into result?
 What algorithm will the method use?

7. Learning Goals
 Method Name: calculateArea()
 Task: To calculate the area of a rectangle
 Data Needed (variables)
 length: A variable to store the rectangle's length
 width: A variable to store the rectangle's width
 area: A variable to store result of calculation
 Processing: area = length x width
 Result to be returned: area

8. Contents
public class Rectangle // Class header
{
private double length; // Instance variables
private double width;
public Rectangle(double l, double w) // Constructor method
{
length = l;
width = w;
}
public double calculateArea() // calculate area method
{
double area;
area = length * width;
return area;
} // calculateArea()
} // Rectangle class

9. Learning Goals
 Write an algorithm to calculate the perimeter of a
rectangle.
 Write the method in Java.

10. Contents
 Method Name: calculatePerimeter()
 Task: To calculate the perimeter of a
rectangle
 Data Needed (variables)
 length
 width
 perimeter
 Processing: perimeter = 2 x(length +
width)
 Result to be returned: perimeter

11. Learning Goals
public double calculatePerimeter()
{
double perimeter;
perimeter = 2 * (length + width);
return perimeter;
} // calculatePerimeter()

12. Contents
 Create, or instantiate, two instances of the
Rectangle class:
The objects
(instances) store
actual values.
Rectangle rectangle1 = new Rectangle(30,10);
Rectangle rectangle2 = new Rectangle(25, 20);

13. Learning Goals
 We use a method call to ask each object to tell us
its area:
rectangle1 area 300
rectangle2 area 500Printed output:
System.out.println("rectangle1 area " + rectangle1.calculateArea());
System.out.println("rectangle2 area " + rectangle2.calculateArea());
References to
objects
Method calls

14. Contents
 new ClassName(parameters);
 Example:
 new Rectangle(30, 20);
 new Car("BMW 540ti", 2004);
 Purpose:
 To construct a new object, initialize it with
the construction parameters, and return a
reference to the constructed object.

15. Learning Goals
public class RectangleUser
{
public static void main(String argv[])
{
Rectangle rectangle1 = new Rectangle(30,10);
Rectangle rectangle2 = new Rectangle(25,20);
System.out.println("rectangle1 area " +
rectangle1.calculateArea());
System.out.println("rectangle2 area " +
rectangle2.calculateArea());
} // main()
} // RectangleUser
An application must
have a main() method
Object
Use
Object
Creation
Class
Definition