Building Reusable Java Widgets

An introduction to writing pluggable do-it-yourself widgets for the Java programmer.
Events and Delegation

If your widget generates events, you have to stay within the bounds of Java's event model. In the “early days”, Java used a cumbersome inheritance-based event model. With the introduction of Java 1.1, the AWT sports a completely modern, delegation-based event model. The concept is simple. When a user interacts with a widget, it generates events. Objects can register an interest in all or some of these events. These interested objects, known as listeners, receive the events and take appropriate action. This process is known as delegation. Listeners are delegated by the widget to handle the events they generate.

When you create event classes, make sure they contain appropriate information and are generated in response to appropriate user actions. It can get confusing when you start to generate your own events. First, you create the event classes. Next, you create a listener interface. This interface defines methods of the listeners that will be called when an event occurs. Finally, you create an event multicaster. Event multicasters have the job of broadcasting events to many listeners. Sounds complicated, but hopefully it will become clear after an example or two. Figure 2 is a diagram of the multicasting process.

Figure 2. Event Multicaster

Drawing

The appearance of your widget will not strictly affect its ability to be reused. However, a widget's appearance should be in visual harmony with the rest of your GUI. Here are some things to think about, which, while not hard and fast rules, are important.

Modern widgets produce a 3D appearance by shading. Shading is created by an imaginary light source that is positioned, by convention, at the upper left of the widget. When a button is in the raised state, its upper and left borders are brighter than the face of the button (see Figure 3). Its lower and right borders are darker. This shading makes it appear to be raised. Swap the light and dark areas, and the button will appear depressed (see Figure 4).

Figure 3. A Button in its Raised State

If your widget is drawn or has drawn areas on it, keep in mind the 3D effects are what make it as attractive as possible. And remember, drawing occurs in the paint method of your class. The AWT calls the paint method any time your widget needs to be drawn.

Figure 4. A Button in its Pressed State

Widgets to Call Your Own

There are generally two methods you can use to create reusable widgets: composition and specialization. Before continuing, let's take a moment to discuss each method.

Composition Widgets that are created by composition are sometimes called super-widgets or composite widgets. This type of widget is simply a collection of other widgets that work together to accomplish a specialized task. You should create a composite widget whenever you have a recurring task which requires a number of sub-components working together. Some examples include an order form, file dialog or a color chooser. When you create a composite widget, it is important to hide the events of all its sub-components and generate events at a higher level appropriate to the semantics of the composite widget itself. You will see how this works in the e-mail entry widget and the window-bar widget.

Specialization

Sometimes a widget is required that is slightly different from a standard AWT widget. Perhaps you need to add some new behavior or look. It is in these situations that you should consider creating a widget by specialization. When you create a widget by specialization, you create a subclass and add or override existing behavior. The power of inheritance gives you all of the behavior of the superclass, so all you need to do is write the new code. An example we will discuss is the VerticalSeparator, a subclass of Canvas. It overrides the paint method to achieve its own special look. The vertical-separator is a good example of this. The collapsing pane is a more subtle example of how to use specialization.

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