Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 598

<interface>
Shape

getArea()
getPerimeter()

Circle
radius
Circle(radius)
getArea()
getPerimeter()

Rectangle
width, height
Rectangle(w, h)
getArea()
getPerimeter()

Triangle
a, b, c
Tr iangle(a, b, c)
getArea()
getPerimeter()

Figure 9.5

Hierarchy of shape classes

The major benefit of interfaces is that we can use them to achieve polymorphism.
We can create an array of Shape s, pass a Shape as a parameter to a method, return a
Shape from a method, and so on. The following program uses the shape classes in an
example that is similar to the polymorphism exercises in Section 9.3:

1 // Demonstrates shape classes.
2 public class ShapesMain {
3 public static void main(String[] args) {
4 Shape[] shapes = new Shape[3];
5 shapes[0] = new Rectangle(18, 18);
6 shapes[1] = new Triangle(30, 30, 30);
7 shapes[2] = new Circle(12);
8
9 for ( int i = 0; i < shapes.length; i++) {
10 System.out.println("area = " + shapes[i].getArea() +
11 ", perimeter = " +
12 shapes[i].getPerimeter());
13 }
14 }
15 }

This program produces the following output:

area = 324.0, perimeter = 72.0
area = 389.7114317029974, perimeter = 90.0
area = 452.3893421169302, perimeter = 75.39822368615503

It would be fairly easy to modify our client program if another shape class, such as
Hexagon or Ellipse , were added to the hierarchy. This is another example of the