nest(inner) class

嵌套类，摘自：
        http://www.ntu.edu.sg/home/ehchua/programmin/java/J4a_GUI.html

A nested class has these properties:

1. A nested class is a proper class. That is, it could contain constructors, member variables and member methods. You can create an instance of a nested class via the new operator and constructor.
2. A nested class is a member of the outer class, just like any member variables and methods defined inside a class.
3. Most importantly, a nested class can access the private members (variables/methods) of the enclosing outer class, as it is at the same level as these private members. This is the property that makes inner class useful.
4. A nested class can have private, public, protected, or the default access, just like any member variables and methods defined inside a class. A private inner class is only accessible by the enclosing outer class, and is not accessible by any other classes. [An top-level outer class cannot be declared private, as no one can use a private outer class.]
5. A nested class can also be declared static, final or abstract, just like any ordinary class.
6. A nested class is NOT a subclass of the outer class. That is, the nested class does not inherit the variables and methods of the outer class. It is an ordinary self-contained class. [Nonetheless, you could declare it as a subclass of the outer class, via keyword "extends OuterClassName", in the nested class's definition.]

The usages of nested class are:

1. To control visibilities (of the member variables and methods) between inner/outer class. The nested class, being defined inside an outer class, can access private members of the outer class.
2. To place a piece of class definition codes closer to where it is going to be used, to make the program clearer and easier to understand.
3. For namespace management.

1.
import java.awt.*;
import java.awt.event.*;
 
// An AWT GUI program inherits from the top-level container java.awt.Frame
public class AWTCounterNamedInnerClass extends Frame {
   // This class is NOT a ActionListener, hence, it does not implement ActionListener
 
   // The event-handler actionPerformed() needs to access these "private" variables
   private TextField tfCount;
   private int count = 0;
 
   /** Constructor to setup the GUI */
   public AWTCounterNamedInnerClass () {
      setLayout(new FlowLayout());  // "super" Frame sets to FlowLayout
      add(new Label("Counter"));    // anonymous instance of Label
      tfCount = new TextField("0", 10);
      tfCount.setEditable(false);   // read-only
      add(tfCount);                 // "super" Frame adds tfCount
 
      Button btnCount = new Button("Count");
      add(btnCount);                // "super" Frame adds btnCount
 
      // Construct an anonymous instance of BtnCountListener (a named inner class).
      // btnCount adds this instance as a ActionListener.
      btnCount.addActionListener(new BtnCountListener());
 
      setTitle("AWT Counter");
      setSize(250, 100);
      setVisible(true);
   }
 
   /** The entry main method */
   public static void main(String[] args) {
      new AWTCounterNamedInnerClass(); // Let the constructor do the job
   }
 
   /**
    * BtnCountListener is a "named inner class" used as ActionListener.
    * This inner class can access private variables of the outer class.
    */
   private class BtnCountListener implements ActionListener {
      @Override
      public void actionPerformed(ActionEvent e) {
         ++count;
         tfCount.setText(count + "");
      }
   }
}

2.

import java.awt.*;
import java.awt.event.*;
 
// An AWT GUI program inherits from the top-level container java.awt.Frame
public class AWTCounterAnonymousInnerClass extends Frame {
   // This class is NOT a ActionListener, hence, it does not implement ActionListener
 
   // The event-handler actionPerformed() needs to access these private variables
   private TextField tfCount;
   private int count = 0;
 
   /** Constructor to setup the GUI */
   public AWTCounterAnonymousInnerClass () {
      setLayout(new FlowLayout());  // "super" Frame sets to FlowLayout
      add(new Label("Counter"));    // an anonymous instance of Label
      tfCount = new TextField("0", 10);
      tfCount.setEditable(false);   // read-only
      add(tfCount);                 // "super" Frame adds tfCount
 
      Button btnCount = new Button("Count");
      add(btnCount);                // "super" Frame adds btnCount
 
      // Construct an anonymous instance of an anonymous class.
      // btnCount adds this instance as a ActionListener.
      btnCount.addActionListener(new ActionListener() {
         @Override
         public void actionPerformed(ActionEvent e) {
            ++count;
            tfCount.setText(count + "");
         }
      });
 
      setTitle("AWT Counter");
      setSize(250, 100);
      setVisible(true);
   }
 
   /** The entry main method */
   public static void main(String[] args) {
      new AWTCounterAnonymousInnerClass(); // Let the constructor do the job
   }
}