13.5.1 Event Receivers and Listener Interfaces

An event is delivered by passing it as an argument to an event-handler method in the receiving object. ActionEvents, for example, are always delivered to a method called actionPerformed() in the receiver:

// Receiver
public void actionPerformed( ActionEvent e ) {
	...
}

For each type of event, there is a corresponding listener interface that describes the methods it must provide to receive those events. In this case, any object that receives ActionEvents must implement the ActionListener interface:

public interface ActionListener extends java.util.EventListener {
	public void actionPerformed( ActionEvent e );
}
// Receiver implements ActionListener

All listener interfaces are subinterfaces of java.util.EventListener, but EventListener is simply an empty interface. It exists only to help the compiler identify listener interfaces.

Listener interfaces are required for a number of reasons. First, they help to identify objects that are capable of receiving a given type of event. This way we can give the event-handler methods friendly, descriptive names and still make it easy for documentation, tools, and humans to recognize them in a class. Next, listener interfaces are useful because several methods can be specified for an event receiver. For example, the FocusListener interface contains two methods:

abstract void focusGained( FocusEvent e );
abstract void focusLost( FocusEvent e );

Athough these methods both take a FocusEvent as an argument, they correspond to different meanings as to why the event was fired; in this case, whether the FocusEvent means that focus was received or lost. You could figure out what happened by inspecting the event; all AWTEvents contain a constant specifying the event's subtype. By requiring two methods, the FocusListener interface saves you the effort: if focusGained() is called, you know the event type was FOCUS_GAINED. Similarly, the MouseListener interface defines five methods for receiving mouse events (and MouseMotionListener defines two more), each of which gives you some additional information about why the event occurred. In general, the listener interfaces group sets of related event-handler methods; the method called in any given situation provides a context for the information in the event object.

There can be more than one listener interface for dealing with a particular kind of event. For example, the MouseListener interface describes methods for receiving MouseEvents when the mouse enters or exits an area, or a mouse button is pressed or released. MouseMotionListener is an entirely separate interface that describes methods to get mouse events when the mouse is moved (no buttons pressed) or dragged (buttons pressed). By separating mouse events into these two categories, Java lets you be a little more selective about the circumstances under which you want to receive MouseEvents. You can register as a listener for mouse events without receiving mouse motion events; since mouse motion events are extremely common, you don't want to handle them if you don't need to.

Finally, two simple patterns govern the naming of AWT event listener interfaces and handler methods:

• Event-handler methods are public methods that return type void and take a single event object (a subclass of java.util.EventObject as an argument).[1]

  [1] This rule is not complete. JavaBeans allows event-handler methods to take additional arguments when absolutely necessary and also to throw checked exceptions.

• Listener interfaces are subclasses of java.util.EventListener that are named with the suffix "Listener"--for example, FooListener.

These may seem pretty obvious, but they are important because they are our first hint of a design pattern governing how to build components that work with events.

13.5.2 Event Sources

The previous section described the machinery that an event receiver uses to accept events. In this section we'll describe how the receiver tells an event source to start sending it events as they occur.

To receive events, an eligible listener must register itself with an event source. It does this by calling an "add listener" method in the event source and passing a reference (a callback) to itself. For example, the AWT Button class is a source of ActionEvents. In order to receive these events, our code might do something like the following:

// source of ActionEvents
Button theButton = new Button("Belly");
  
// receiver of ActionEvents
class TheReceiver implements ActionListener {
   setupReceiver() {
      ...
      theButton.addActionListener( this );
   }
   public void actionPerformed( ActionEvent e ) {
      // Belly Button pushed...
   }

The receiver makes a call to addActionListener() to complete its setup and become eligible to receive ActionEvents from the button when they occur. It passes the reference this, to add itself as the ActionListener.

To manage its listeners, an ActionEvent source (like the Button) always implements two methods:

// ActionEvent source
public void addActionListener(ActionListener listener) {
   ...
}
public void removeActionListener(ActionListener listener) {
   ...
}

The removeActionListener() method complements addActionListener() and does what you'd expect: it removes the listener from the list so that it will not receive future events from that source.

Now, you may be expecting an EventSource interface listing these two methods, but there isn't one. There are no event source interfaces in the current conventions. If you are analyzing a class and trying to determine what events it generates, you have to look for the add and remove methods. For example, the presence of the addActionListener() and removeActionListener() methods define the object as a source of ActionEvents. If you happen to be a human being, you can simply look at the documentation; but if the documentation isn't available, or if you're writing a program that needs to analyze a class (a process called "reflection"), you can look for this design pattern:

• A source of events for the FooListener interface must implement a pair of add/remove methods:

  • addFooListener(FooListener listener)

  • removeFooListener(FooListener listener)

• If an event source can support only one event listener (unicast delivery), the add listener method can throw the checked exception java.util.TooManyListenersException.

So what do all the naming patterns up to this point accomplish? Well, for one thing they make it possible for automated tools and integrated development environments to divine what are sources and what are sinks of particular events. Tools that work with Java Beans will use the Java reflection and introspection APIs to search for these kinds of design patterns and identify the events that can be fired and received by a component.

It also means that event hookups are strongly typed, just like the rest of Java. So it's not easy to accidentally hook up the wrong kind of components; for example, you can't register to receive ItemEvents from a Button, because a button doesn't have an addItemListener() method. Java knows at compile-time what types of events can be delivered to whom.

13.5.3 Event Delivery

AWT events are multicast; every event is associated with a single source but can be delivered to any number of receivers. Events are registered and distributed using an observer/observable model. When an event listener registers itself with an event source, the event source adds the listener to a list. When an event is fired, it is delivered individually to each listener on the list (Figure 13.9).

Figure 13.9: Event delivery

There are no guarantees about the order in which events will be delivered. Neither are there any guarantees about what happens if you register yourself more than once with an event source; you may or may not get the event more than once. Similarly, you should assume that every listener receives the same event data. Events are immutable; they can't be changed by their listeners. There's one important exception to this rule, which we'll discuss later.

To be complete we could say that event delivery is synchronous with respect to the event source, but that follows from the fact that the event delivery is really just the invocation of a normal Java method. The source of the event calls the handler method of each listener. However, listeners shouldn't assume that all of the events will be sent in the same thread. An event source could decide to send out events to all of the listeners in parallel.

How exactly an event source maintains its set of listeners, constructs, and fires the events is up to it. Often it is sufficient to use a Vector to hold the list. We'll show the code for a component that uses a custom event in Chapter 14. For efficiency, AWT components all use the java.awt.AWTEventMulticaster object, which maintains a linked tree of the listeners for the component. You can use that too, if you are firing standard AWT events. We'll describe the event multicaster in Chapter 14 as well.

13.5.4 AWTEvents

All of the events used by AWT GUI components are subclasses of java.awt.AWTEvent. AWTEvent holds some common information that is used by AWT to identify and process events. You can use or subclass any of the AWTEvent types for use in your own components. Figure 13.11 shows the event hierarchy.

Figure 13.10: AWT event hierarchy

ComponentEvent is the base class for events that can be fired by any AWT component. This includes events that provide notification when a component changes its dimensions or visibility, as well as the other event types for mouse operation and key presses. ContainerEvents are fired by AWT containers when components are added or removed.

13.5.5 java.awt.event.InputEvent

MouseEvents, which track the state of the mouse, and KeyEvents, which are fired when the user uses the keyboard, are types of java.awt.event.InputEvent. Input events from the mouse and keyboard are a little bit special. They are normally produced by the native Java machinery associated with the peers. When the user touches a key or moves the mouse within a component's area, the events are generated with that component as the source.

Input events and some other AWT events are placed on a special event queue that is managed by the AWT toolkit. This gives the toolkit control over how the events are delivered. First, under some circumstances, the toolkit can decide to compress a sequence of the same type of event into a single event. This is done to make some event types more efficient--in particular, mouse events and some special internal events used to control repainting. Perhaps more important to us, input events are delivered with a special arrangement that lets listeners decide if the component itself should act on the event.

13.5.6 Consuming Events

Normally, the native peer of a standard AWT component operates by receiving InputEvents telling it about the mouse and keyboard. When you push a Button, the native ButtonPeer object receives a MouseEvent and does its job in native land to accomplish the button-pressing behavior. But for InputEvents, the Toolkit first delivers the event to any listeners registered with the component and gives those listeners a chance to mark the event as "consumed," effectively telling the peer to ignore it. An InputEvent is marked "consumed" by calling the consume() method. (Yes, this is a case where an event is not immutable.)

So we could stop our Button from working by registering a listener with it that catches "mouse button depressed" events. When it got one, we could call its consume() method to tell the ButtonPeer to ignore that event. This is particularly useful if you happen to be building a development environment in Java, and you want to "turn off" components while the user arranges them.

In a trusted application you can, if necessary, get access to the AWT event queue. The toolkit uses an instance of java.awt.EventQueue. With it you can peek at pending AWT events or even push in new ones.

13.5.7 Mouse and Key Modifiers on InputEvents

InputEvents come with a set of flags for special modifiers. These let you detect if the Shift or Alt key was held down during a mouse button or key press, or if the second or third mouse buttons were pressed. The following are the flag values contained in java.awt.event.InputEvent:

• SHIFT_MASK

• CTRL_MASK

• META_MASK

• ALT_MASK

• BUTTON1_MASK

• BUTTON2_MASK

• BUTTON3_MASK

To check for these masks, you can simply do a boolean AND of the modifiers, returned by the InputEvent's getModifiers() method and the flag or flags you're interested in:

public void mousePressed (MouseEvent e) {
    int mods = e.getModifiers();
    if ((mods & InputEvent.SHIFT_MASK) != 0)
        
      { // Shifted Mouse Button press }
}

The three BUTTON flags can be used to detect which mouse button was pressed on a two- or three-button mouse. If you use these, you run the risk that your program won't work on platforms without multibutton mice. Currently, BUTTON2_MASK is equivalent to ALT_MASK, and BUTTON3_MASK is equivalent to META_MASK. This means that pushing the second mouse button is equivalent to pressing the first (or only) button with the Alt key depressed, and the third button is equivalent to the first with the Meta key depressed. However, if you really want to guarantee portability, you should limit yourself to a single button, possibly in combination with keyboard modifiers, rather than relying on the button masks.

Key events provide one other situation in which events aren't immutable. You can change the character that the user typed by calling setKeyChar(), setKeyCode(), or setKeyModifiers(). A user's keystroke isn't displayed until the KeyEvent is delivered to the peer. Therefore, by changing the character in the KeyEvent, you can change the character displayed on the screen. This is a good way to implement a field that displays only uppercase characters, regardless of what the user types.