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Lightweight Components

Starting with the JDK 1.1 Beta 3 release, the AWT supports lightweight components. Lightweight components are essential in the following situations:

You want to create a custom component that isn't rectangular.
You want to create a custom component that's partly (or entirely) transparent.
You want to use multiple components that can draw in each others' areas.
Your program contains many components and would be too inefficient if implemented with heavyweight components.

This page tells you how to create lightweight components, and then gives you links to examples -- pages with applets that use lightweight components (source code included, of course). For more details about what lightweight components are and how they work, see Java AWT: Lightweight UI Framework.

How to Create Lightweight Components

To create a lightweight component, you need to create a class that directly extends one of the following classes:

Component
Container
a class that implements a lightweight component -- for example, a non-AWT class that is a subclass of Component or Container

Your class needs to provide both the look and the feel of the component. You'll typically need to implement the following methods:

One or more constructors. If the component responds to any events, such as mouse clicks, the constructor(s) should invoke the enableEvents method.
The paint method, so that it draws the component's representation onscreen.
As appropriate, methods to let the state of the component be changed programmatically -- for example, setText. Besides changing the component's internal state, each of these methods should call the repaint method if the component's appearance should change. If the component's size should change, then call the invalidate method before calling repaint.
As appropriate, methods to let listeners be registered and unregistered for the component. For example, a button should implement the addActionListener and removeActionListener methods, which should add the specified listener using the AWTEventMulticaster add and remove methods, respectively.
The contains method if the component responds to events to only part of its entire possible drawing area.
Any appropriate processXxx methods. For example, a button would implement processMouseEvent to change the button's state and to generate actionPerformed messages as appropriate.

Lightweight components place special requirements on their containers:

Lightweight components sometimes flash noticeably unless you put them in a container that performs double buffering. See the Gauge example for an example of a double-buffered container.
Lightweight components will not appear if their container implements a paint method that doesn't call super.paint. In other words, if you implement a container that performs some drawing -- for example, one that draws a box around its display area -- make sure the container's paint method invokes super.paint!

Code Examples and Applets

This section links to four pages, each with an example that illustrates an aspect of lightweight components. Each page includes one applet and links to the applet's source code. If you're using a browser that includes support for 1.1beta3, then your browser will be able to run the applet on each page. Otherwise, you'll need to use another tool, such as the JDK Applet Viewer, to view the applet on each page.

Note: You must use a 1.1 browser such as HotJava or the JDK Applet Viewer to view the applets on the following pages. If you have trouble running the applets, go here.

Here are the examples:

Round Button Example: Demonstrates how easy it is to create a lightweight component.
Openlook Button Example: Modifies the Round Button example to show a more realistic looking lightweight component.
Spinner Example: Demonstrates how to animate a lightweight component.
Gauge Example: Demonstrates how to animate a lightweight component using double buffering to eliminate flicker.

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By Kathy Walrath