Software Engineering for Students A Programming Approach

230 Chapter 16 ■ Programming in the large
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As an example, we declare an interface named Displayable. Any class complying
with this interface must include a method named display which displays the object.
The interface declaration in Java is:
public interface Displayable {
void display(Graphics paper);
}
Now we write a new class, named Square, which represents square graphical
objects. We say in the header of the class that it implements Displayable. We include
within the body of the class the method display:
public class Square implements Displayable {
}
private int x, y, size;
public void display(Graphics paper) {
paper.setColor(Color.black);
paper.drawRectangle(x, y, size, size);
}
// other methods of the class Square
As the heading states, this class (and any object created from it) conforms to the
Displayable interface. It means that any object of this class can be passed around a
program and we are confident that, when necessary, it can be displayed by calling its
method display.
SELF-TEST QUESTION
16.5 We wish to write a new class Circle that implements the
Displayable interface. Write the header for the class.
16.8 ● Multiple interfaces
Just as a TV has interfaces both to a power source and to a signal source, so can we
specify that a class implements a number of interfaces.
Java, for example, is a language that provides single inheritance – a class can inherit
from (or be the subclass of) only one class. The class structure is a tree, with the root at
the top, in which a class can have many subclasses but only one superclass. Figure 16.3
shows illustrative classes Circle and Game as subclasses of superclass JFrame. Each class
appears only within a single tree and each class has only a single superclass.
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