Event Listener's Adapter Classes

摘自：
    http://www.ntu.edu.sg/home/ehchua/programming/java/J4a_GUI.html

Refer to the WindowEventDemo, a WindowEvent listener is required to implement the WindowListener interface,
which declares 7 abstract methods. Although we are only interested in windowClosing(), we need to provide 
an empty body to the other 6 methods in order to compile the program. This is tedious.
　　当实现例如WindowListener接口的时候，虽然只需要用到其中的一个函数，但是却需要将其他的函数都写出，这样非常多余。
An adapter class called WindowAdapter is therefore provided, which implements the WindowListener interface 
and provides default implementations to all the 7 abstract methods. You can then derive a subclass from 
WindowAdapter and override only methods of interest and leave the rest to their default implementation.
　　有了adapter类之后，就只需要写出需要实现的方法就行，其他不需要使用的函数就不用写出。
import java.awt.*;
import java.awt.event.*;

// An AWT GUI program inherits the top-level container java.awt.Frame
public class WindowEventDemoAdapter extends Frame {
   private TextField tfCount;
   private int count = 0;

   /** Constructor to setup the GUI */
   public WindowEventDemoAdapter () {
      setLayout(new FlowLayout());
      add(new Label("Counter"));
      tfCount = new TextField("0", 10);
      tfCount.setEditable(false);
      add(tfCount);

      Button btnCount = new Button("Count");
      add(btnCount);
      btnCount.addActionListener(new ActionListener() {
         @Override
         public void actionPerformed(ActionEvent evt) {
            ++count;
            tfCount.setText(count + "");
         }
      });

      // Allocate an anonymous instance of an anonymous inner class
      // that extends WindowAdapter.
      // "this" Frame adds the instance as WindowEvent listener.
      addWindowListener(new WindowAdapter() {
         @Override
         public void windowClosing(WindowEvent e) {
            System.exit(0);  // Terminate the program
         }
      });

      setTitle("WindowEvent Demo");
      setSize(250, 100);
      setVisible(true);
   }

   /** The entry main method */
   public static void main(String[] args) {
      new WindowEventDemoAdapter();   // Let the constructor do the job
   }
}

Similarly, adapter classes such as MouseAdapter, MouseMotionAdapter, KeyAdapter, FocusAdapter are available for 
MouseListener, MouseMotionListener, KeyListener, and FocusListener, respectively.

There is no ActionAdapter for ActionListener, because there is only one abstract method (i.e. actionPerformed()) 
declared in the ActionListener interface. This method has to be overridden and there is no need for an adapter.