Interfaces and Nested Classes - Multimedia Introduction to Programming Using Java - page 330

Java syntax: Implements clause
implements interface-name , … , interface-name
Example : implements I, Comparable
Purpose : Placing this implements clause in a class header indicates that
the class implements all the methods of interfaces I and Comparable .

public class C1 implements ActionListener {
void actionPerformed(ActionEvent b)
{ code in method body }

}

The presence of the clause implements ActionListener forces class C1 to pro-
vide an implementation of the methods described in interface ActionListener ;
otherwise, the class definition is syntactically illegal.

12.1.1 The interface as a type

We just showed how a class could implement an interface. We now go into more
detail about what this means. We show that an interface can be the type of a vari-
able, and we discuss the ramifications of an interface being a type. The follow-
ing two sections contain case studies that use interfaces, and you might want to
peruse them in between studying parts of this section.
Figure 12.1 contains an interface I and two classes F and G . We use short
names for the interface and its methods in order to keep diagrams and text man-
ageable. The names are not important to our goal in this section.
To the left in Fig. 12.2 is an object a0 of class G , with the partition for class
G at the bottom, then the partition for class F , and above that the partition for class
Object . A variable ob contains the name a0 .
We want to show how the implementation of interface I within class G
affects the object. To do this most effectively, we replace the lines separating the
components of subclasses by arrows and remove the box around the object —we

Activity
12-2.1

interface I {
void p(C e);

}

public class F { }

public class G extends F implements I {
Button b;
public G() { ... }
void p(C e) { ... }

}
Figure 12.1:

Interface I and classes F and G