The JComponent Class

The JComponent class provides the framework for building all kinds of user interface components. Particular components—such as buttons, labels, and list boxes—are implemented as subclasses of JComponent. We override methods in such a subclass to implement the behavior of our particular component. This may sound restrictive, as if we are limited to some predefined set of routines, but that is not the case at all. Keep in mind that the methods we are talking about are ways to interact with the windowing system. We don’t have to squeeze our whole application in there. A realistic application might involve hundreds or thousands of classes, with legions of methods and variables and many threads of execution. The vast majority of these are related to the particulars of our job (these are called domain objects). The JComponent class and other predefined classes serve only as a framework on which to base code that handles certain types of user interface events and displays information to the user. The paintComponent() method is an important method of the JComponent class; we override it to implement the way our particular component displays itself on the screen. The default behavior of paintComponent() doesn’t do any drawing at all. If we hadn’t overridden it in our subclass, our component would simply have been invisible. Here, we’re overriding paintComponent() to do something only slightly more interesting. We don’t override any of the other inherited members of JComponent because they provide basic functionality and reasonable defaults for this (trivial) example. As HelloJava grows, we’ll delve deeper into the inherited members and use additional methods. We will also add some application-specific methods and variables specifically for the needs of HelloComponent. JComponent is really the tip of another iceberg called Swing. Swing is Java’s user interface toolkit, represented in our example by the import statement at the top; we’ll discuss about it in some detail.