Building Reusable Java Widgets
There is no getting around it, if you are programming with Java, eventually you will need a specialized widget. I know, Sun Microsystems provides the Abstract Windowing Toolkit (AWT) with every download of Java; however, the AWT's small set of widgets and fairly low-level graphics will eventually leave you wanting. When it's time to wow the boss, the customer or the dog, it's time to roll up your sleeves and do it yourself.
This article examines some good techniques for extending the AWT. You will not simply learn how to create interesting widgets. You will also (and perhaps more importantly) learn how to make them reusable by employing good design practices and adhering to the AWT's architecture. To do this I will introduce some of the important concepts of the AWT's event model, hierarchy and graphics. [Perhaps the most significant change from version 1.0x to 1.1x was the shift from an inheritance-based event model to a more powerful delegation-based model.] This article will deal solely with Java version 1.1x and above, including the new event model.
I have written this article with the expectation that you have some basic experience with Java. You will need to know how to compile and run Java code to try out the examples. It would be a bonus if you have created the obligatory “Hello World” applet and are familiar with layout managers.
There is nothing Linux-specific in this article. One of Java's primary strengths is that it will run on any platform that has the Java virtual machine ported to it. Not surprisingly, Linux is one. In fact, chances are good that your version of Linux has Java compiled into the kernel. I have run these examples on various versions of Linux and Solaris 2.5. If you wish to experiment with building widgets on Linux, you will need to get a Linux port of the Java Development Kit (JDK) from Blackdown or sunsite. Of course, you will be able to run the example applets in any Java-enabled browser that runs on Linux—that would be Netscape.
If you write your GUI relying solely on the AWT, chances are pretty good that it will be ugly. While the AWT provides an excellent framework for GUI development, it was never meant to be the last word in GUI toolkits. The AWT provides a set of “least common denominator” widgets, so if you are interested in simple push buttons, sliders or text fields, the AWT has you covered. Unfortunately, modern user interfaces often call for GUI controls beyond these old standbys.
When your design requirements or creative wishes exceed the capabilities of these standard widgets, you need to create your own. When the time comes that you are ready to create your own widget, you will have choices. You could create a very uncooperative widget which is tightly coupled with your application. Or, you could choose to leverage the power of object-oriented programming, and the existing architecture of the AWT. It should be no surprise that I advocate the latter. If you agree, you will create open, reusable widgets that can be used by anyone in the Java community, regardless of his choice of operating system.
Linux users are a generally individualistic lot. Individuality has its place, but when it comes to the architecture of your widget I advocate fitting in with the crowd. When I go to the trouble of creating a widget I want to make sure that it can be used over and over painlessly. I achieve this by adhering to the design of the AWT. Although the AWT is not the most visually appealing widget toolkit, it does provide an excellent framework for widget creation and interaction. By working within the bounds of this framework we adhere to the basic architecture of the AWT and get reusable widgets.
When you design your widgets to cooperate with the existing architecture, you ensure that they are robust, maintainable and easily reusable by any Java developer (including yourself). Your widgets will be “pluggable”. Any Java user will be able to simply plug in the widget and use it like any other standard GUI componet provided in the AWT.
There are several important aspects to keep in mind when designing and creating widgets. In this section I will discuss some of these and evolve a template for widget creation.
In order to cooperate with the AWT, your widget must have Component as its ancestor. I don't mean to imply that you must subclass Component directly; here in fact, you can often inherit some useful behavior by subclassng further down the hierarchy (see Figure 1 for a portion of the Component hierarchy).
Your widget might be drawn on the fly or created from images stored on a local or remote machine (GIFs for example). The Canvas class is a good choice for this type of component. The Canvas class provides a blank area for drawing. It will take some experience to be able to choose an appropriate superclass for your widget. It is wise to take some time to study the Component hierarchy of the AWT before choosing a superclass.
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