15.4 Custom Events

Beans can use the standard event types defined in the java.awt.event and javax.swing.event packages, but they don't have to. Our YesNoPanel class defines its own event type, AnswerEvent. Defining a new event class is really quite simple; AnswerEvent is shown in Example 15-4.

Example 15-4. AnswerEvent.java

package je3.beans;

/**
 * The YesNoPanel class fires an event of this type when the user clicks one
 * of its buttons.  The id field specifies which button the user pressed.
 **/
public class AnswerEvent extends java.util.EventObject {
    public static final int YES = 0, NO = 1, CANCEL = 2;  // Button constants
    protected int id;                             // Which button was pressed?
    public AnswerEvent(Object source, int id) {
        super(source);
        this.id = id;
    }
    public int getID( ) { return id; }             // Return the button
}

Along with the AnswerEvent class, YesNoPanel also defines a new type of event listener interface, AnswerListener, that defines the methods that must be implemented by any object that wants to receive notification from a YesNoPanel. The definition of AnswerListener is shown in Example 15-5.

Example 15-5. AnswerListener.java

package je3.beans;

/**
 * Classes that want to be notified when the user clicks a button in a
 * YesNoPanel should implement this interface.  The method invoked depends
 * on which button the user clicked.
 **/
public interface AnswerListener extends java.util.EventListener {
    public void yes(AnswerEvent e);
    public void no(AnswerEvent e);
    public void cancel(AnswerEvent e);
}