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Exploring Java

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Understand the Abstract Windowing Toolkit

10.2 Applets

If you've been waiting for a more detailed discussion of the applet class, here it is. For examples of writing applets, please see Chapter 2, A First Applet (the tutorial) and the examples in Chapter 11, Using and Creating GUI Components and throughout the book.

An Applet is something like a Panel with a mission. It is a GUI Container that has some extra structure to allow it to be used in an "alien" environment like a Web browser or appletviewer. Applets also have a life-cycle that lets them act more like an application than a static component. Although applets tend to be relatively simple, there's no inherent restriction on their complexity. There's no reason you couldn't write an air traffic control system (well, let's be less ambitious: a word processor) as an applet.

Structurally, an applet is a sort of wrapper for your Java code. In contrast to a standalone graphical Java application, which starts up from a main() method and creates a GUI, an applet is itself a Component that expects to be dropped into someone else's GUI. Thus, an applet can't run by itself; it runs in the context of a Web browser or an appletviewer. Instead of having your application create a Frame to hold your GUI, you stuff your application inside an Applet (which is itself a Container) and let someone else add the applet to their GUI.

Pragmatically, an applet is an intruder into someone else's environment, and therefore has to be treated with suspicion. The Web browsers that run applets impose restrictions on what the applet is allowed to do. The restrictions are enforced by a security manager, which the applet is not allowed to change. The browser also provides an "applet context," which is additional support that helps the applet live within its restrictions.

Aside from that top level structure and the security restrictions, there is no difference between an applet and an application. If your application can live within the restrictions imposed by a browser's security manager, you can easily structure it to function as an applet and a standalone application. (We'll show an example of an Applet that can also be run as a standalone below.) Conversely, if you can supply all of the things that an applet requires from its environment, you can use applets within your stand-alone applications and within other applets (though this requires a bit of work).

As we said a moment ago, an Applet expects to be embedded in GUI (perhaps a document) and used in a viewing environment that provides it with special resources. In all other respects, however, applets are just ordinary Panel objects. (See Figure 10.8.) Like a Panel, an Applet can contain user-interface components and implement all the basic drawing and event-handling capabilities of the Component class. We draw on an Applet by overriding its paint() method; we respond to events in the Applet's display area by providing the appropriate event-listeners. The additional structure applets have helps them interact with the viewer environment.

Figure 10.8: The java.applet package

[Graphic: Figure 10-8]

Applet Control

The Applet class contains four methods an applet can override to guide it through its life cycle. The init(), start(), stop(), and destroy() methods are called by an applet viewer, such as a Web browser, to direct the applet's behavior. init() is called once, after the applet is created. The init() method is where you perform basic setup like parsing parameters, building a user interface, and loading resources. Given what we've said about objects, you might expect the Applet's constructor would be the right place for such initialization. However, the constructor is meant to be called by the applet's environment, for simple creation of the applet. This might happen before the applet has access to certain resources, like information about its environment. Therefore, an applet doesn't normally do any work in its constructor; it relies on the default constructor for the Applet class and does its initialization in the init() method.

The start() method is called whenever the applet becomes visible; it shouldn't be a surprise then that the stop() method is called whenever the applet becomes invisible. init() is only called once in the life of an applet, but start() and stop() can be called any number of times (but always in the logical sequence). For example, start() is called when the applet is displayed, such as when it scrolls onto the screen; stop() is called if the applet scrolls off the screen or the viewer leaves the document. start() tells the applet it should be active. The applet may want to create threads, animate, or otherwise perform useful (or annoying) activity. stop() is called to let the applet know it should go dormant. Applets should cease CPU-intensive or wasteful activity when they are stopped and resume it when (and if) they are restarted. However, there's no requirement that an invisible applet stop computing; in some applications, it may be useful for the applet to continue running in the background. Just be considerate of your user, who doesn't want an invisible applet dragging down system performance. And for the users: be aware of the tools that will develop to let you monitor and squash rogue applets in your web browser.

Finally, the destroy() method is called to give the applet a last chance to clean up before it's removed--some time after the call to stop(). For example, an applet might want to close down suspended communications channels or remove graphics frames. Exactly when destroy() is called depends on the applet viewer; Netscape Navigator calls destroy() just prior to deleting the applet from its cache. This means that although an applet can cling to life after being told to stop(), how long it can go on is unpredictable. If you want to maintain your applet as the user progresses through other activities, consider putting it in an HTML frame (see "Driving the Browser" later in this chapter).

The Applet security Sandbox

Applets are quarantined within the browser by an applet SecurityManager. The SecurityManager is part of the application that runs the applet, e.g., the web browser or applet viewer. It is installed before the browser loads any applets and implements the basic restrictions that let you run untrusted applets safely. Remember that, aside from basic language robustness, there are no inherent security restrictions on a standalone Java application. It is the browser's responsibility to install a special security manager and limit what applets are allowed to do.

Most browsers impose the following restrictions on untrusted applets:

We discuss these restrictions in more detail in the relevant chapters in this book. However, the motivation for these restrictions should be fairly obvious: you clearly wouldn't want a program coming from some random Internet site to access your files, or run arbitrary programs. Although untrusted applets cannot directly read and write files on the client side or talk to arbitrary hosts on the network, applets can work with servers to store data and communicate. For example, an applet can use Java's RMI (Remote Method Invocation) facility to do processing on its server. An applet can communicate with other applets on the Net by proxy through its server.

Trusted Applets

The latest version of Java makes it possible to sign archive files that contain applets. Because a signature identifies the applet's origin unambiguously, we can now distinguish between "trusted" applets (i.e., applets that come from a site or person you trust not to do anything harmful) and run of the mill "untrusted" applets. In web browsers that support signing, trusted applets can be granted permission to "go outside" of the applet security sandbox. Trusted applets can be allowed to do most of the things that standalone Java applications can do: read and write files, open network connections to arbitrary machines, and interact with the local operating system by starting processes. Trusted applets still can't have native methods, but including native methods in an applet would make it unportable, and would therefore be a bad idea.

Chapter 3, Tools of the Trade discusses how to package your applet's class files and resources into a JAR file and sign it with your digital signature. Currently, HotJava is the only browser that supports signing, but Netscape Navigator, Internet Explorer, and others will probably catch up soon.

Getting Applet Resources

An applet needs to communicate with its applet viewer. For example, it needs to get its parameters from the HTML document in which it appears. An applet may also need to load images, audio clips, and other items. It may also want to ask the viewer about other applets on the same HTML page in order to communicate with them. To get resources from the applet viewer environment, applets use the AppletStub and AppletContext interfaces. Unless you're writing a browser or some other application that loads and runs applets, you won't have to implement these interfaces, but you do use them within your applet.

Applet Parameters

An applet gets its parameters from the parameter tags placed inside the <applet> tag in the HTML document. For example, the code below reads the "sheep" parameter from its HTML page:

String imageName = getParameter( "imageName" ); 
try  {
    int numberOfSheep = Integer.parseInt(getParameter( "sheep" )); 
} catch ( NumberFormatException e ) { // use default } 

A friendly applet will provide information about the parameters it accepts through its getParameterInfo() method. getParameterInfo() returns an array of string arrays, listing and describing the applet's parameters. For each parameter, three strings are provided: the parameter name, its possible values or value types, and a verbose description. For example:

public String [][] getParameterInfo() {
    String [][] appletInfo = 
        {"logo",   "url",  "Main logo image"}
        {"timer", "int",   "Time to wait before becoming annoying"},
        {"flashing", "constant | intermittant",  "Flag for how to flash"},
    return appletInfo;

Applet Resources

An applet can find where it lives by calling the getDocumentBase() and getCodeBase() methods. getDocumentBase() returns the base URL of the document in which the applet appears; getCodeBase() returns the base URL of the Applet's class files. An applet can use these to construct relative URLs from which to load other resources like images, sounds, and other data. The getImage() method takes a URL and asks for an image from the viewer environment. The image may be pulled from a cache or loaded asynchronously when later used. The getAudioClip() method, similarly, retrieves sound clips. See Chapter 9, Network Programming for a full discussion of how to work with URLs, and Chapter 11, Using and Creating GUI Components for examples of applets that load images.

The following example uses getCodeBase() to construct a URL and load a properties-configuration file, located at the same location as the applet's class file. (See Chapter 7, Basic Utility Classes for a discussion of properties.)

Properties props = new Properties(); 
try {
    URL url = new URL(getCodeBase(), "appletConfig.props");
    props.load( url.openStream() ); 
} catch ( IOException e ) { // failed } 

A better way to load resources is by calling the getResource() and getResourceAsStream() methods of the Class class, which search the applet's JAR files (if any) as well as its codebase. See Chapter 8, Input/Output Facilities for a discussion of resource loading. The following code loads the properties file appletConfig.props:

Properties props = new Properties(); 
try {
    props.load( getClass().getResourceAsStream("appletConfig.props") );
} catch ( IOException e ) { // failed } 

Driving the Browser

The status line is a blurb of text that usually appears somewhere in the viewer's display, indicating a current activity. An applet can request that some text be placed in the status line with the showStatus() method. (The browser isn't required to do anything in response to this call, but most browsers will oblige you.)

An applet can also ask the browser to show a new document. To do this, the applet makes a call to the showDocument( url ) method of the AppletContext. You can get a reference to the AppletContext with the Applet's getAppletContext() method. showDocument() can take an additional String argument to tell the browser where to display the new URL:

getAppletContext().showDocument( url, name ); 

The name argument can be the name of an existing labeled HTML frame; the document referenced by the URL will be displayed in that frame. You can use this method to create an applet that "drives" the browser to new locations dynamically, but stays active on the screen in a separate frame. If the named frame doesn't exist, the browser will create a new top-level window to hold it. Alternatively, name can have one of the following special values:

_self Show in the current Frame
_parent Show in the parent of our Frame
_top Show in outer-most (top level) frame.
_blank Show in a new top level browser window.

Both showStatus() and showDocument() requests may be ignored by a cold-hearted viewer or Web browser.

    *** Missing Discussion of getApplet() ***
    *** Add a blurb about the upcoming InfoBus stuff ***

Applets vs. Standalone Applications

    *** Discuss getImage() and image loading from JAR files for applications ***

The following list summarizes the methods of the applet API:

// from the AppletStub
boolean isActive();
URL getDocumentBase();
URL getCodeBase();
String getParameter(String name);
AppletContext getAppletContext();
void appletResize(int width, int height);
// from the AppletContext
AudioClip getAudioClip(URL url);
Image getImage(URL url);
Applet getApplet(String name);
Enumeration getApplets();
void showDocument(URL url);
public void showDocument(URL url, String target);
void showStatus(String status);

These are the methods that are provided by the applet viewer environment. If your applet doesn't happen to use any of them, or if you can provide alternatives to handle special cases (such as loading images from JAR files), your applet could be made able to function as a standalone application as well as an applet. For example, our HelloWeb applet from Chapter 2, A First Applet was very simple. We can easily give it a main() method to allow it to be run as a standalone application:

public class HelloWeb extends Applet {
    public void paint( java.awt.Graphics gc ) {
        gc.drawString( "Hello Web!", 125, 95 );
    public static void main( String [] args ) {
        Frame theFrame = new Frame();
        Applet helloWeb = new HelloWeb();
        theFrame.add("Center", helloWeb);

Here we get to play "appletviewer" for a change. We have created an instance of HelloWeb using its constructor - something we don't normally do$mdash;and added it to our own Frame. We call its init() method to give the applet a chance to wake up and then call its start() method. In this example, HelloWeb doesn't implement these, init() and start(), so we're calling methods inherited from the Applet class. This is the procedure that an appletviewer would use to run an applet. (If we wanted to go further, we could implement our own AppletContext and AppletStub, and set them in the Applet before startup).

Trying to make your applets into applications as well often doesn't make sense, and is not always trivial. We show this only to get you thinking about the real differences between applets and applications. It is probably best to stay within the applet API until you have a need to go outside it. Remember that trusted applets can do almost all of the things that applications can. It is probably wiser to make an applet that requires trusted permissions than an application.


We've spent a lot of time discussing the different kinds of objects in AWT--components, containers, and a few special containers like applets. But we've neglected communications between different objects. A few times, we've mentioned events, and we have even used them in the occasional program (like our applets in Chapter 2, A First Applet), but we have deferred a discussion of events until later. Now is the time to pay that debt.

AWT objects communicate by sending events. The way we talk about "firing" events and "handling" them makes it sound as if they are part of some special Java language feature. But they aren't. An event is simply an ordinary Java object that is delivered to its receiver by invoking an ordinary Java method. Everything else, however interesting, is purely convention. The entire Java event mechanism is really just a set of conventions for the kinds of descriptive objects that should be delivered; these conventions prescribe when, how, and to whom events should be delivered.

Events are sent from a single source object to one or more listeners (or "targets"). A listener implements specific event handling methods that enable it to receive a type of event. It then registers itself with a source of that kind of event. Sometimes there may be an "adapter" object interposed between the event source and the listener, but there is always a connection established before any events are delivered.

An event object is a subclass of java.util.EventObject that holds information about "something that's happened" to its source. The EventObject class serves mainly to identify event objects; the only information it contains is a reference to the event source (the object that sent the event). Components do not normally send or receive EventObjects as such; they work with subclasses that provide more specific information. AWTEvent is a subclass of EventObject that is used within AWT; further subclasses of AWTEvent provide information about specific event types.

For example, events of type ActionEvent are fired by buttons when they are pushed. ActionEvents are also sent when a menu item is selected or when a user presses ENTER in a TextField. Similarly, MouseEvents are generated when you operate your mouse within a component's area. You can gather the general meaning of these two events from their names; they are relatively self-descriptive. ActionEvents correspond to a decisive "action" that a user has taken with the component--like pressing a button, or pressing ENTER to indicate that he has filled in a text field. An ActionEvent thus carries the name of an action to be performed (the "action command") by the program. MouseEvents describe the state of the mouse, and therefore carry information like the x and y coordinates and the state of your mouse buttons at the time it was created. You might hear someone say that ActionEvent is at a "higher semantic level" than MouseEvent. This means that ActionEvent is an interpretation of something that happened and is, therefore, conceptually more powerful than the MouseEvent, which carries raw data. An ActionEvent lets us know that a component has done its job, while a MouseEvent simply confers a lot of information about the mouse at a given time. You could figure out that somebody clicked on a Button by examining mouse events, but it is simpler to work with action events. The precise meaning of an event, however, can depend on the context in which it is received. (More on that in a moment.)

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 );
// Reciever 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 there can be several methods 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 for 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 recieve 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, we should point out two simple patterns in 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).[2]

    [2] This rule is not complete. The full Java Beans 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," e.g., 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.

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 "event source" 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 only support 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.

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.

There are no guarantees about the order in which events will be delivered. Neither are there any guarantees if you register yourself more than once with an event source; you may get the event more than once, or not. 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 sent 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 11, Using and Creating GUI Components. 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 11, Using and Creating GUI Components as well.


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.

Use the event hierarchy from Java in a Nutshell or AWT Reference.

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.


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.

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-depressing behavior. But for InputEvents, the Toolkit first delivers the event to any listeners registered with the 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 treated as 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 develoment environment in Java and you want to "turn off" components while the user arranges them.

If you need to, in a trusted application you can 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 to push in new ones.

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:








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

In the list you'll notice there are three BUTTON flags. These can be used to detect if a particular mouse button was used in a mouse press on a two or three button mouse. Be warned, 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 only displays uppercase characters, regardless of what the user types.

AWT Event Reference

The following tables summarize the AWT Events, which components fire them, and the methods of the listener interfaces that receive them:

AWT Component and Container Events

Event Fired by Listener interface(s) Handler Method(s)
ComponentEvent All Components (*) ComponentListener componentResized()
FocusEvent FocusListener focusGained()
KeyEvent KeyListener keyTyped()
MouseEvent MouseListener mouseClicked()
MouseMotionListener mouseDragged()
ContainerEvent All Containers ContainerListener componentAdded()

Component specific AWT Events

Event Fired by Listener interface(s) Handler Method(s)
ActionEvent TextField ActionListener actionPerformed()
ItemEvent List ItemListener itemStateChanged()
AdjustmentEvent ScrollPane AdjustmentListener adjustmentValueChanged()
TextEvent TextArea TextListener textValueChanged()
WindowEvent Frame, Dialog WindowListener windowOpened()

Adapter Classes

It's not ideal to have your application components implement a listener interface and receive events directly. Sometimes it's not even possible. Being an event receiver forces you to modify or subclass your objects to implement the appropriate event listener interfaces and add the code necessary to handle the events. A more subtle issue is that, since we are talking about AWT events here, you are, of necessity, building GUI logic into parts of your application that shouldn't have to know anything about the GUI. Let's look at an example:

In Figure 10.10 we have drawn the plans for our Vegomatic food processor. Here we have made our Vegomatic object implement the ActionListener interface so that it can receive events directly from the three Button components: "Chop," "Puree," and "Frappe." The problem is that our Vegomatic object now has to know more than how to mangle food. It also has to be aware that it will be driven by three controls, specifically buttons that send action commands, and be aware of which methods in itself it should invoke for those commands. Our boxes labeling the GUI and Application code overlap in an unwholesome way. If the marketing people should later want to add or remove buttons, or perhaps just change the names, we have to be careful. We may have to modify the logic in our Vegomatic Object. All is not well.

An alternative is to place an "adapter" class between our event source and receiver. An adapter is a simple object whose sole purpose is to map an incoming event to an outgoing method.

Figure 10.11 shows a better design that uses three adapter classes, one for each button. The implementation of the first adapter might look like:

class VegomaticAdapter1 implements actionListener {
    Vegotmatic vegomatic;
    VegomaticAdapter1 ( Vegotmatic vegomatic ) {
        this.vegomatic = vegomatic;
    public void actionPerformed( ActionEvent e ) {

So somewhere in the code where we build our GUI, we could register our listener like so:

// building GUI for our Vegomatic
Vegomatic theVegomatic = ...;
Button chopButton = ...;
// make the hookup 
chopButton.addActionListener( new VegomaticAdapter1(theVegomatic) );

We have completely separated the messiness of our GUI from the application code. However, we have added three new classes to our application, none of which does very much. Is that good? That depends on your vantage point.

Under different circumstances our buttons may have been able to share a common adapter class that was simply instantiated with different parameters. There are various trade-off that can be made between size, efficiency, and elegance of code. Often, adapter classes will be generated automatically by development tools. The way we have named our adapter classes "VegomaticAdapter1," "VegomaticAdapter2," and "VegomaticAdapter3" hints at this. More often, when hand coding, you'll use an inner class. At the other extreme, we can forsake Java's strong typing and use the reflection API to create a completely dynamic hookup betwen an event source and listener.

AWT Dummy Adapters

Many listener interfaces contain more than one event handler method. Unfortunately, this means that to register yourself as interested in any one of those events, you must implement the whole listener interface. And to accomplish this you might find yourself typing in dummy "stubbed out" methods, simply to complete the interface. There is really nothing wrong with this, but it is a bit tedious. To save you some trouble, AWT provides some helper classes that implement these dummy methods for you. For each listener interface containing more than one method there is an adapter class containing the stubbed methods. The adapter class serves as a base class for your own adapters. So, when you need a class to patch together your event source and listener, you can simply subclass the adapter and override only the methods you want.

For example, the MouseAdapter class implements the MouseListener interface and provides the following implementation:

public void mouseClicked(MouseEvent e) {};
public void mousePressed(MouseEvent e) {};
public void mouseReleased(MouseEvent e) {};
public void mouseEntered(MouseEvent e) {};
public void mouseExited(MouseEvent e) {};

This may not look like a tremendous time saver, and you're right. It's simply a bit of sugar. The primary advantage comes into play when we use the MouseAdapter as the base for your own adapter in an anonymous inner class. For example, suppose we want to catch a mousePressed() event in some component and blow up a building. We can use the following to make the hookup:

someComponent.addMouseListener( new MouseAdapter() {
    public void MousePressed(MouseEvent e) {
} );

We've taken artistic liberties with the formatting, but I think it's very readable, and I like it. It's a common enough activity that it's nice to avoid typing those extra few lines and perhaps stave off the onset of carpal tunnel syndrome for a few more hours. Remember that any time you use an inner class, the compiler is generating a class for you, so the messiness you've saved in your source still exists in the output classes.

Old Style and New Style Event Processing

Although Java is still a youngster, it has a bit of a legacy. Versions of Java before 1.1 used a different style of event delivery. Back in the old days (a few months ago) event types were limited and events were only delivered to the Component that generated it, or one of its parent containers. The old style component event handler methods (now deprecated) returned a boolean value declaring whether or not they had "handled" the event.

boolean handleEvent( Event e ) {

If the method returns false, the event is automatically redelivered to the component's container to give it a chance. If the container does not handle it, it is passed on to its parent container and so on. In this way, events were propogated up the containment hierarchy until they were either consumed or passed over to the component peer, just as current InputEvents are ultimately interpreted used the peer if no registered event listeners have consumed them.

Although this style of event delivery was convenient for some simple applications, it is not very flexible. Events could only be handled by components, which meant that you always had to subclass a Component or Container type to handle events. This was a degenerate use of inheritance (bad design) that led to the creation of lots of unnecessary classes.

We could, alternatively, receive the events for many embedded components in a single parent container, but that would often lead to very convoluted logic in the container's event handling methods. It is also very costly to run every single AWT event through a guantlet of (often empty) tests as it traverses its way up the tree of containers. This is why Java now provides the more dynamic and general event source/listener model that we have described in this chapter. The old style events and event handler methods are, however, still with us.

Java is not allowed to simply change and break an established API. Instead, as we described in Chapter 1, Yet Another Language?, older ways of doing things are simply "deprecated" in favor of the new ones. This means that code using the old style event handler methods will still work; you may see old-style code around for a long time. The problem with relying on old-style event delivery, however, is that the old and new ways of doing things cannot be mixed.

By default, Java is obligated to perform the old behavior--offering events to the component and each of its parent containers. However, Java turns off the old style delivery whenever it thinks that we have elected to use the new style. Java determines whether a Component should recieve old style or new style events based on whether any event listeners are registered, or whether new style events have been explicitly enabled. When an AWT event listener is registered with a Component, new style events are implicitly turned on (a flag is set). Additionally, a mask is set telling the component the types of AWT events it should process. The mask allows components to be more selective about which events they process.


When new style events are enabled, all events are first routed to the dispatchEvent() method of the Component class. The dispatchEvent() method examines the component's event mask and decides whether the event should be processed or ignored. Events that have been "enabled" are sent to the processEvent() method, which simply looks at the event's type and delegates it to a "helper" processing method named for its type. The helper processing method finally dispatches the event to the set of registered listeners for its type.

This process closely parallels the way in which old style events are processed, and the way in which events are first directed to a single handleEvent() method that dispatches them to more specific handler methods in the Component class. The differences are that new style events are not delivered unless someone is listening for them, and the listener registration mechanism means that we don't have to subclass the component in order to override its event handler methods and insert our own code.

Enabling New Style Events Explicitly

Still, if you are subclassing a Component type for other reasons, or you really want to process all events in a single method, you should be aware that it is possible to emulate the old style event handling and override your component's event processing methods. You simply have to call the Component's enableEvents() method with the appropriate mask value to turn on processing for the given type of event. You can then override the corresponding method and insert your code. The mask values are found in the java.awt.AWTEvent class:

java.awt.AWTEvent mask method
COMPONENT_EVENT_MASK processComponentEvent()
FOCUS_EVENT_MASK processFocusEvent()
KEY_EVENT_MASK processKeyEvent()
MOUSE_EVENT_MASK processMouseEvent()
MOUSE_MOTION_EVENT_MASK processMouseMotionEvent()

For example:

    public void init() {
        enableEvent( AWTEvent.KEY_EVENT_MASK ):
    public void processKeyEvent(KeyEvent e) {
        if ( e.getID() == KeyEvent.KEY_TYPED )
            // do work

If you do this it is vital that you remember to make a call to super.process...Event() in order to allow normal event delegation to continue. Of course, by emulating old-style event handling, we're giving up the virtues of the new style; in particular, this code is a lot less flexible than the code we could write with the new event model. As we've seen, the user interface is hopelessly tangled with the actual work your program does. A compromise solution would be to have your subclass declare that it implements the appropriate listener interface and register itself, as we have done in the simpler examples in this book:

class MyApplet implements KeyListener ...
    public void init() {
        addKeyListener( this ):
    public void keyTyped(KeyEvent e) {
        // do work

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