Learning Java, 4th Edition

Events

We’ve spent a lot of time discussing the different kinds of objects in Swing—components, containers, and special containers such as frames and windows. Now it’s time to discuss interobject communication in detail.

Swing objects communicate by sending events. The way we talk about events—“firing” them 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. A listener implements prescribed 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 an adapter object may be interposed between the event source and the listener, but in any case, registration of a listener is always established before any events are delivered.

An event object is an instance of a subclass of java.util.EventObject; it holds information about something that’s happened to its source. The EventObject parent class itself serves mainly to identify event objects; the only information it contains is a reference to the event source ...