Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 593

b

r

a

h

c

w

Figure 9.4 Three types of shapes

There is an is-a relationship here, because a rectangle, a circle, and a triangle are
all shapes. But code sharing isn't useful in this case because each class implements
its behavior differently. As depicted in Figure 9.4 and Table 9.12, each shape com-
putes its area and perimeter in a totally different way. The w and h represent the
rectangle's width and height; the r represents the circle's radius; and the a , b , and c
represent the lengths of the triangle's three sides.
Since no code is shared among these classes, we should not create a common
superclass to represent their is-a relationship. Java uses single inheritance, and we
don't want to use up our only potential inheritance relationship here. A better solu-
tion would be to write an interface called Shape to represent the common functional-
ity of all shapes: the ability to ask for an area and a perimeter. Our various shape
classes will implement this interface.
To write an interface, we create a new file with the same name as the interface's
name; our Shape interface, for example, would be stored in Shape.java . We give
the interface a header with the keyword interface in place of the word class :

public interface Shape {
...

}

Inside the interface, we write headers for each method that we want a Shape to
contain. But instead of writing method bodies with braces, we simply place a
semicolon at the end of each header. We don't specify how the methods are imple-
mented. Instead, we're requiring that any class that wants to be considered a shape
must implement these methods. In fact, it isn't legal for an interface to contain
method bodies; an interface can only contain method headers and class constants.

Table 9.12 Formulas for Area and Perimeter of Each Shape Type

Rectangle Circle Triangle

Area w * h π r 2 1 s ( s - a )( s - b )( s - c )

a + b + c

where s =

2

Perimeter 2 ( w h ) 2 π r a b c