Objects and Classes in Java

Implementing an Interface

An interface defines a protocol of behavior. A class that implements an interface adheres to the protocol defined by that interface. Let's look at the Spot class to get a better idea of what this means.

To get mouse events from the AWT, the AWT requires a class to adhere to a certain protocol. For example, for a class to be notified by the AWT when the user presses a mouse button, the class must have a mousePressed method, for a class to be notified when the user releases the mouse button it must have a mouseReleased method, and so on. This protocol is codified in the MouseListener interface, which declares these five methods, one for each type of mouse event:

mouseClicked(MouseEvent)
mouseEntered(MouseEvent)
mouseExited(MouseEvent)
mousePressed(MouseEvent)
mouseReleased(MouseEvent)

Any class interested in getting mouse events must adhere to the protocol. It does this by implementing the MouseListener interface.

To declare a class that implements an interface, include an implements clause in the class declaration. Your class can implement more than one interface (Java supports multiple interface inheritance), so the implements keyword is followed by a comma-delimited list of the interfaces implemented by the class.

Spot, which displays a spot where the user presses the mouse button, needs to know about mouse pressed events (the event when the mouse button is initially pressed down) in order to function. Therefore, Spot must implement the MouseListener interface:

class Spot extends Applet implements MouseListener {
   . . .
}

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By Convention: The implements clause follows the extends clause, if it exists.

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When a class declares that it implements an interface, it's basically signing a contract to the effect that it will provide implementations for all of the methods in the interface. So Spot must implement all five of MouseListener's methods:

public void mousePressed(MouseEvent event) {
    clickPoint = event.getPoint();
    repaint();
}
public void mouseClicked(MouseEvent event) {}
public void mouseReleased(MouseEvent event) {}
public void mouseEntered(MouseEvent event) {}
public void mouseExited(MouseEvent event) {}

Note that the last four methods have empty implementations. This is because Spot is interested only in "mouse pressed" events. When the user presses the mouse button, Spot displays a spot where the click occurred. However, because Spot is declared to implement the MouseListener interface and MouseListener declares five methods, Spot must implement them all. You can use inner classes in conjunction with several java.awt.event "adapter classes" to avoid writing empty methods. The next section shows you how.

For a complete discussion about interfaces, what they're for, and how to write them, go to Creating Interfaces in the next lesson.

Objects and Classes in Java