3.2 Graphical Interface Events

Whenever you interact with your application's user interface, the application receives an event from the windowing system to let it know that something happened. Some events come from the mouse, such as mouse clicks, mouse movements, and mouse drags. Other events come from the keyboard, such as key presses and key releases. Every component generates events. Different components generate different events as dictated by their purpose (and their L&F). For example, pressing a JButton generates an ActionEvent (which is really just a converted mouse event). The ActionEvent class bundles up interesting stuff like which button the event came from, when the button was pressed, whether any modifier keys (such as Shift or Ctrl) were pressed at the time of the event, and so on.

While the event-dispatching and -handling mechanism is grounded in the world of AWT (and beyond the scope of this book), we do want you to know what events the various Swing components generate and when. The what of the events is discussed in conjunction with each of the components. As we introduce components like JTextField, JButton, and JTable, we show the events that they fire and the methods you use to attach listeners and catch the events.

The when of the events is a bit more difficult to describe. Rather than attempt to list every possible scenario for every component, we've built a small utility: EEL, the Every Event Listener. The EEL class implements every listener interface from the java.awt.event and javax.swing.event packages. It has a variety of logging mechanisms to show you the events coming from your components. You attach an EEL instance to a component (or to multiple components) using the component's add . . . Listener( ) method(s). You can choose to have the events sent to a file, to your console, or to an onscreen text area.

This discussion really is beyond the scope of the book. So we're posting this utility and its documentation on the web site for this book (http://www.oreilly.com/catalog/jswing2). Feel free to download it and use it to play with individual components or to debug an entire application. That's one of the beauties of delegation event handling: you can attach EEL to an existing component without breaking its normal interactions with the application.