The properties you can change are the size of the circle as well as the color, size, and text of the word that is displayed when you press the mouse. A BangBean also has its own addActionListener( ) and removeActionListener( ) so you can attach your own listener that will be fired when the user clicks on the BangBean. You should be able to recognize the property and event support:
Case Study: BangBean.java
The first thing you'll notice is that BangBean implements the Serializable interface. This means that the application builder tool can "pickle" all the information for the BangBean using serialization after the program designer has adjusted the values of the properties. When the Bean is created as part of the running application, these "pickled" properties are restored so that you get exactly what you designed.
You can see that all the fields are private, which is what you'll usually do with a Bean -- allow access only through methods, usually using the "property" scheme.
When you look at the signature for addActionListener( ), you'll see that it can throw a TooManyListenersException. This indicates that it is unicast, which means it notifies only one listener when the event occurs. Ordinarily, you'll use multicast events so that many listeners can be notified of an event. However, that runs into issues that you won't be ready for until the next chapter, so it will be revisited there (under the heading "Java Beans revisited"). A unicast event sidesteps the problem.
When you press the mouse, the text is put in the middle of the BangBean, and if the actionListener field is not null, its actionPerformed( ) is called, creating a new ActionEvent object in the process. Whenever the mouse is moved, its new coordinates are captured and the canvas is repainted (erasing any text that's on the canvas, as you'll see). The main( ) is added to allow you to test the program from the command line. When a Bean is in a development environment, main( ) will not be used, but it's helpful to have a main( ) in each of your Beans because it provides for rapid testing. main( ) creates a Frame and places a BangBean within it, attaching a simple ActionListener to the BangBean to print to the console whenever an ActionEvent occurs. Usually, of course, the application builder tool would create most of the code that uses the Bean.
When you run the BangBean through BeanDumper or put the BangBean inside
a Bean-enabled development environment, you'll notice that there are many
more properties and actions than are evident from the above code. That's
because BangBean is inherited from Canvas, and Canvas is a Bean, so you're
seeing its properties and events as well.