13.2 Event and listener types

You can see that each type of component supports only certain types of events. It's helpful to see the events supported by each component, as shown in the following table:
 

Component type	Events supported by this component
Adjustable	AdjustmentEvent
Applet	ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Button	ActionEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Canvas	FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Checkbox	ItemEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
CheckboxMenuItem	ActionEvent, ItemEvent
Choice	ItemEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Component	FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Container	ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Dialog	ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
FileDialog	ContainerEvent, WindowEvent,FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Frame	ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Label	FocusEvent, KeyEvent, MouseEvent, ComponentEvent
List	ActionEvent, FocusEvent, KeyEvent, MouseEvent, ItemEvent, ComponentEvent
Menu	ActionEvent
MenuItem	ActionEvent
Panel	ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
PopupMenu	ActionEvent
Scrollbar	AdjustmentEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
ScrollPane	ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
TextArea	TextEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
TextComponent	TextEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
TextField	ActionEvent, TextEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent
Window	ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent

 
Once you know which events a particular component supports, you don't need to look anything up to react to that event. You simply:

1. Take the name of the event class and remove the word "Event." Add the word "Listener" to what remains. This is the listener interface you need to implement in your inner class.
2. Implement the interface above and write out the methods for the events you want to capture. For example, you might be looking for mouse movements, so you write code for the mouseMoved( ) method of the MouseMotionListener interface. (You must implement the other methods, of course, but there's a shortcut for that which you'll see soon.)
3. Create an object of the listener class in step 2. Register it with your component with the method produced by prefixing "add" to your listener name. For example, addMouseMotionListener( ).

In the table above, you can see that some listener interfaces have only one method. These are trivial to implement since you'll implement them only when you want to write that particular method. However, the listener interfaces that have multiple methods could be less pleasant to use. For example, something you must always do when creating an application is provide a WindowListener to the Frame so that when you get the windowClosing( ) event you can call System.exit(0) to exit the application. But since WindowListener is an interface, you must implement all of the other methods even if they don't do anything. This can be annoying.

To solve the problem, each of the listener interfaces that have more than one method are provided with adapters, the names of which you can see in the table above. Each adapter provides default methods for each of the interface methods. (Alas, WindowAdapter does not have a default windowClosing( ) that calls System.exit(0).) Then all you need to do is inherit from the adapter and override only the methods you need to change. For example, the typical WindowListener you'll use looks like this:

class MyWindowListener extends WindowAdapter {

    public void windowClosing(WindowEvent e) {

        System.exit(0);

    }

}

There is a downside to adapters, however, in the form of a pitfall. Suppose you write a WindowAdapter like the one above:

class MyWindowListener extends WindowAdapter {

    public void WindowClosing(WindowEvent e) {

        System.exit(0);

    }

}