Developing Java Beans

A java.awt Example

The components provided by the java.awt package expose properties using the techniques just described. These user interface elements can be manipulated in the same way as any other object that conforms to the JavaBeans architecture. So let’s create an applet that illustrates the concepts of bound and constrained properties.

Our applet will have a data member named primaryLabel which is an instance of class NumberLabel. A NumberLabel is a subclass of java.awt.Label that has a bound and constrained property named Value. The Value property is of type int, and the label always displays the contents of this property. We also have another instance of the NumberLabel class, named mirrorLabel, that will bind itself to the Value property of primaryLabel, and will keep its display consistent with it. There is one button to decrement the Value property of primaryLabel, and one to increment it; these button instances will be named decButton and incButton, respectively. Lastly, we will create an instance of class Constrainer, named cnstr, that constrains the Value property of primaryLabel to be between 10 and 20. A diagram of the interactions that will take place between the objects is shown in Figure 4.3.

Figure 4-3. Constraining a property with a VetoableChangeEvent

Let’s take a look at the code for the NumberLabel class first:

import java.applet.*; import java.awt.*; import ...