More Features of the Java Language

Defining an Interface

The following figure shows that an interface definition has two components: the interface declaration and the interface body. The interface declaration declares various attributes about the interface such as its name and whether it extends another interface. The interface body contains the constant and method declarations within that interface.

The Interface Declaration

The declaration for the Sleeper interface uses the two required elements of an interface declaration--the interface keyword and the name of the interface--plus the public access modifier :

public interface Sleeper {
    . . .
}

An interface declaration can have one other component: the public access specifier and a list of superinterfaces. The full interface declaration looks like this figure:

The public access specifier indicates that the interface can be used by any class in any package. If you do not specify that your interface is public, then your interface will be accessible only to classes that are defined in the same package as the interface.

An interface can extend other interfaces just as a class can extend or subclass another class. However, while a class can extend only one other class, an interface can extend any number of interfaces. The list of superinterfaces is a comma-delimited list of all of the interfaces extended by the new interface. An interface inherits all constants and methods from its superinterfaces, unless the interface hides a constant with another of the same name or redeclares a method with a new method declaration.

The Interface Body

The interface body contains method declarations for all of the methods included in the interface. Here's the body of the Sleeper interface:

public interface Sleeper {
    public void wakeUp();

    public long ONE_SECOND = 1000;
    public long ONE_MINUTE = 60000;
}

The method declaration for wakeUp is followed by a semicolon (;) because an interface does not provide implementations for the methods declared within it. All methods declared in an interface are implicitly public and abstract. The use of these modifiers on a method declaration in an interface is discouraged as a matter of style.

An interface can contain constant declarations in addition to method declarations. The Sleeper interface declares two constants that are useful arguments to letMeSleepFor. All constant values defined in an interface are implicitly public, static, and final. The use of these modifiers on a constant declaration in an interface is discouraged as a matter of style.

Any class can use an interface's constants from the name of the interface, like this:

Sleeper.ONE_SECOND

Classes that implement an interface can treat the constants as though they were inherited. This is why GUIClock can use ONE_MINUTE directly when calling letMeSleepFor:

public class GUIClock extends Applet implements Sleeper {
    . . .
    public void wakeUp() {
        repaint();
        clock.letMeSleepFor(this, ONE_MINUTE);
    }
}

Member declarations in an interface disallow the use of some declaration modifiers; you may not use transient, volatile, or synchronized in a member declaration in an interface. Also, you may not use the private and protected specifiers when declaring members of an interface.

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Note: Previous releases of the Java platform allowed you to use the abstract modifier on interface declarations and on method declarations within interfaces. However this is unnecessary, since interfaces and their methods are implicitly abstract. You should no longer be using abstract in your interface declarations or in your method declarations within interfaces.

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More Features of the Java Language