6.2.1 Interfaces as Callbacks

Interfaces can be used to implement callbacks in Java. A callback is a situation where you'd like to pass a reference to some behavior and have another object invoke it later. In C or C++, this is prime territory for function pointers; in Java, we'll use interfaces instead.

Consider two classes: a TickerTape class that displays data and a TextSource class that provides an information feed. We'd like our TextSource to send any new text data. We could have TextSource store a reference to a TickerTape object, but then we could never use our TextSource to send data to any other kind of object. Instead, we'd have to proliferate subclasses of TextSource that dealt with different types. A more elegant solution is to have TextSource store a reference to an interface type, TextUpdateable:

interface TextUpdateable { 
    receiveText( String text ); 
} 
 
class TickerTape implements TextUpdateable { 
    TextSource source; 
 
    init() { 
        source = new TextSource( this ); 
        ... 
    } 
 
    public receiveText( String text ) { 
        scrollText( text ): 
    } 
    ... 
} 
 
class TextSource { 
    TextUpdateable receiver; 
 
    TextSource( TextUpdateable r ) { 
        receiver = r; 
    } 
 
    private sendText( String s ) { 
        receiver.receiveText( s ); 
    } 
    ... 
} 

The only thing TextSource really cares about is finding the right method to invoke to send text. Thus, we can list that method in an interface called TextUpdateable and have our TickerTape implement the interface. A TickerTape object can then be used anywhere we need something of the type TextUpdateable. In this case, the TextSource constructor takes a TextUpdateable object and stores the reference in an instance variable of type TextUpdateable. Our TickerTape object simply passes a reference to itself as the callback for text updates, and the source can invoke its receiveText() method as necessary.

6.2.2 Interface Variables

Although interfaces allow us to specify behavior without implementation, there's one exception. An interface can contain constant variable identifiers; these identifiers appear in any class that implements the interface. This feature allows predefined parameters that can be used with the methods:

interface Scaleable { 
    static final int BIG = 0, MEDIUM = 1, SMALL = 2; 
 
    void setScale( int size ); 
} 

The Scaleable interface defines three integers: BIG, MEDIUM, and SMALL. All variables defined in interfaces are implicitly final and static; we don't have to use the modifiers but, for clarity, we recommend you do. A class that implements Scaleable sees these variables:

class Box implements Scaleable { 
 
    void setScale( int size ) { 
        switch( size ) { 
            case BIG: 
                ... 
            case MEDIUM: 
                ... 
            case SMALL: 
                ... 
        } 
    } 
    ... 
} 

6.2.3 Subinterfaces

An interface can extend another interface, just as a class can extend another class. Such an interface is called a subinterface:

interface DynamicallyScaleable extends Scaleable { 
    void changeScale( int size ); 
} 

The interface DynamicallyScaleable extends our previous Scaleable interface and adds an additional method. A class that implements DynamicallyScaleable must implement all methods of both interfaces.

Interfaces can't specify that they implement other interfaces because interfaces never include any method implementations. However, interfaces are allowed to extend other interfaces. In fact, an interface is allowed to extend as many interfaces as it wants (multiple inheritance of interface)--unlike classes, which can only extend a single class. If you want to extend two or more interfaces, list them after the extends keyword, separated by commas:

    interface DynamicallyScaleable extends Scaleable, SomethingElseable {
        ...