18.4. Event Classes

Your program may need to respond to many different kinds of events—from menus, from buttons, from the mouse, from the keyboard, and from a number of other components. To have a structured approach to handling events, the events are broken down into subsets. At the topmost level, there are two broad categories of events in Java:

  • Low-Level Events—These are system-level events that arise from the keyboard or from the mouse, or events associated with operations on a window, such as reducing it to an icon or closing it. The meaning of a low-level event is something like "the mouse was moved," "this window has been closed," or "this key was pressed."

  • Semantic Events—These are specific component-related events such as pressing a button by clicking it to cause some program action or adjusting a scrollbar. They originate, and you interpret them, in the context of the GUI you have created for your program. The meaning of a semantic event is typically along the lines of "the OK button was pressed," or "the Save menu item was selected." Each kind of component, a button or a menu item, for example, can generate a particular kind of semantic event.

These two categories can seem to be a bit confusing as they overlap in a way. If you click a button, you create a semantic event as well as a low level event. The click produces a low-level event object in the form of 'the mouse was clicked' as well as a semantic event 'the button was pushed'. In fact it produces more than one ...

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