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Java in a Nutshell

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4.3 The New AWT Event Model

The Java 1.1 change that will probably affect Java programmers the most is the new event processing model adopted for the AWT windowing and graphical user interface (GUI) toolkit. If you created applets or GUIs with Java 1.0, you know that it was necessary to subclass GUI components in order to handle events. This model worked okay for simple programs, but proved increasingly awkward as programs became more complex. Furthermore, with the development of the JavaBeans API, the AWT package needed an event model that would allow AWT GUI components to serve as beans. For these reasons, Java 1.1 defines a new model for dispatching and handling events.

As explained in Chapter 7, Events, the new event handling model is essentially a "callback" model. When you create a GUI component, you tell it what method or methods it should invoke when a particular event occurs on it (e.g., when the user clicks on a button or selects an item from a list). This model is very easy to use in C and C++ because those languages allow you to manipulate method pointers--to specify a callback, all you need to do is pass a pointer to the appropriate function. In Java, however, methods are not data and cannot be manipulated in this way. Only objects can be passed like this in Java, so to define a Java callback, you must define a class that implements some particular interface. Then, you can pass an instance of that class to a GUI component as a way of specifying the callback. When the desired event occurs, the GUI component invokes the appropriate method of the object you have specified.

As you might imagine, this new event handling model can lead to the creation of many small helper classes. (Sometimes these helper classes are known as "adaptor classes" because they serve as the interface between the body of an application and the GUI for that application. They are the "adaptors" that allow the GUI to be "plugged in" to the application.) This proliferation of small classes could become quite a burden, were it not for the introduction of inner classes, which, as noted above, allows this kind of special-purpose class to be nested and defined exactly where it is needed within your program.

Despite the major AWT event-handling changes, Java 1.1 does retain backwards compatibility with the event-handling model of Java 1.0. It is an all-or-nothing type of backwards compatibility, however--the two models are so different from each other that it is not really possible to mix them within the same application.


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