java 内部类

内部类

我觉得这个源代码对理解匿名内部类有很好的帮助，
原来的代码标号是001，后面的都是我自己改的，我都试过了都可以运行，
我已经试着尽量描述的清楚些，可是涉及的方面太多，
我只能捡重要的说，如果有看不懂的话可以再问


可以在自己机器上先运行一下，注意如果放在包里，前面要加包的名称
整个的代码是

//001
// ExceptTest

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;

public class ExceptTest
{ 
   public static void main(String[] args)
   { 
      ExceptTestFrame frame = new ExceptTestFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.show(); 
   }
}

/**
   A frame with a panel for testing various exceptions
*/
class ExceptTestFrame extends JFrame
{
   public ExceptTestFrame()
   {
      setTitle("ExceptTest");
      Container contentPane = getContentPane();
      ExceptTestPanel panel = new ExceptTestPanel();
      contentPane.add(panel);
      pack();
   }
}

/**
   A panel with radio buttons for running code snippets
   and studying their exception behavior
*/
class ExceptTestPanel extends Box
{
   public ExceptTestPanel()
   { 
      super(BoxLayout.Y_AXIS);
      group = new ButtonGroup();

      // add radio buttons for code snippets

      addRadioButton("Integer divide by zero", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               a[1] = 1 / (a.length - a.length);
            }
         });

      addRadioButton("Floating point divide by zero", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               a[1] = a[2] / (a[3] - a[3]);
            }
         });

      addRadioButton("Array bounds", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               a[1] = a[10];
            }
         });

      addRadioButton("Bad cast", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               a = (double[])event.getSource();
            }
         });

      addRadioButton("Null pointer", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               event = null;
               System.out.println(event.getSource());
            }
         });

      addRadioButton("sqrt(-1)", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               a[1] = Math.sqrt(-1);
            }
         });

      addRadioButton("Overflow", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               a[1] = 1000 * 1000 * 1000 * 1000;
               int n = (int)a[1];
            }
         });

      addRadioButton("No such file", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               try
               {
                  FileInputStream is
                     = new FileInputStream("No such file");
               }
               catch (IOException exception)
               {
                  textField.setText(exception.toString());
               }
            }
         });

      addRadioButton("Throw unknown", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               throw new UnknownError();
            }
         });

      // add the text field for exception display
      textField = new JTextField(30);
      add(textField);
   }

   /**
      Adds a radio button with a given listener to the
      panel. Traps any exceptions in the actionPerformed
      method of the listener.
      @param s the label of the radio button
      @param listener the action listener for the radio button
   */
   private void addRadioButton(String s, ActionListener listener)
   { 
      JRadioButton button = new JRadioButton(s, false)
         {
            // the button calls this method to fire an
            // action event. We override it to trap exceptions
            protected void fireActionPerformed(ActionEvent event)
            {
               try
               {
                  super.fireActionPerformed(event);
                  textField.setText("No exception");
               }
               catch (Exception exception)
               {
                  textField.setText(exception.toString());
               }
            }
         };

      button.addActionListener(listener);
      add(button);
      group.add(button);
   }

   private ButtonGroup group;
   private JTextField textField;
   private double[] a = new double[10];
}





我简化了一下

//002
//ExceptTest

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;

public class ExceptTest
{ 
   public static void main(String[] args)
   { 
      ExceptTestFrame frame = new ExceptTestFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.show(); 
   }
}

class ExceptTestFrame extends JFrame
{
   public ExceptTestFrame()
   {
      setTitle("ExceptTest");
      Container contentPane = getContentPane();
      ExceptTestPanel panel = new ExceptTestPanel();

      contentPane.add(panel);
      pack();
   }
}


class ExceptTestPanel extends Box
{
   public ExceptTestPanel()
   { 
      super(BoxLayout.Y_AXIS);
      group = new ButtonGroup();

     addRadioButton("Integer divide by zero", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               a[1] = 1 / (a.length - a.length);
            }
         }
      );


      // add the text field for exception display
      textField = new JTextField(30);
     
    
      add(textField);
   }


   private void addRadioButton(String s, ActionListener listener)
   { 
      JRadioButton button = new JRadioButton(s, false)
         {

            protected void fireActionPerformed(ActionEvent event)
            {
               try
               {
                  super.fireActionPerformed(event);
                  textField.setText("No exception");
               }
               catch (Exception exception)
               {
                  textField.setText(exception.toString());
               }
            }
         };

      button.addActionListener(listener);
      add(button);
     group.add(button);
   }

   private ButtonGroup group;
   private JTextField textField;
   private double[] a = new double[10];
}

前面的几个都是gui的惯例，只要不出错，就不用去管它，
先看这个ExceptTestPanel 类里的私有函数
private void addRadioButton(String s, ActionListener listener)，
其实如果只要加一个圆形的互斥按钮的话，
就完全不需要这个私有函数，你可以简单的这莫写这个ExceptTestPanel类，
可以用这个代替原来的，然后运行一下，结果是一样的，

class ExceptTestPanel extends Box implements ActionListener {

    public ExceptTestPanel() {

        super(BoxLayout.Y_AXIS);
        group = new ButtonGroup();

        JRadioButton button = new JRadioButton("Integer divide by zero", false) {

            protected void fireActionPerformed(ActionEvent event) {
                try {
                    super.fireActionPerformed(event);
                    textField.setText("No exception");
                } catch (Exception exception) {
                    textField.setText(exception.toString());
                }
            }
        };

        button.addActionListener(this);
        add(button);
        group.add(button);

        textField = new JTextField(30);
        add(textField);
    }

    private ButtonGroup group;

    private JTextField textField;

    private double[] a = new double[10];

    public void actionPerformed(ActionEvent arg0) {
        a[1] = 1 / (a.length - a.length);

    }
}


但是你既然用了ButtonGroup就肯定不止一个按钮了，所以如果再这样加上第二个按钮这个类就变成了下面这样了，
你可以先替换一下原来的类，再运行一下，

class ExceptTestPanel extends Box implements ActionListener {

    public ExceptTestPanel() {

        super(BoxLayout.Y_AXIS);
        group = new ButtonGroup();

        JRadioButton button = new JRadioButton("Integer divide by zero", false) {

            protected void fireActionPerformed(ActionEvent event) {
                try {
                    super.fireActionPerformed(event);
                    textField.setText("No exception");
                } catch (Exception exception) {
                    textField.setText(exception.toString());
                }
            }
        };

        button.addActionListener(this);
        add(button);
        group.add(button);

        JRadioButton button2 = new JRadioButton("Array bounds", false) {

            protected void fireActionPerformed(ActionEvent event) {
                try {
                    super.fireActionPerformed(event);
                    textField.setText("No exception");
                } catch (Exception exception) {
                    textField.setText(exception.toString());
                }
            }
        };

        button2.addActionListener(this);
        add(button2);
        group.add(button2);

        textField = new JTextField(30);
        add(textField);
    }

    private ButtonGroup group;

    private JTextField textField;

    private double[] a = new double[10];

    public void actionPerformed(ActionEvent e) {
        String s;
        s = e.getActionCommand();

        if (s == "Integer divide by zero") {
            a[1] = 1 / (a.length - a.length);
        } else if (s == "Array bounds") {
            a[1] = a[10];
        }

    }
}


因为，ExceptTestPanel 这个类实现了事件监听 ActionListener 这个接口，
但是如果你加上2个以上的按钮的话，必须在 actionPerformed 里区分这个送过来的事件对象，
因为你在按钮上注册的都是this, 如果我们用一个类C实现了ActionListener 这个接口，
你就可以在按钮上加上这个C类的实例，但是这莫做会使代码变得重复而且复杂不容易发现错误，
而且要定义 JRadioButton 的实例名称，如果用button2,button3,这莫命名，不但不好，
而且容易出错，并且我们基本上用不到JRadioButton 的实例名称

然后现在再回来看简化的版本002， 它把重复用到的这些方法放在一个私有函数里面，把里面的方法需要的变量用
参数给传进去，这个就是匿名内部类，

      JRadioButton button = new JRadioButton(s, false)
         {

            protected void fireActionPerformed(ActionEvent event)
            {
               try
               {
                  super.fireActionPerformed(event);
                  textField.setText("No exception");
               }
               catch (Exception exception)
               {
                  textField.setText(exception.toString());
               }
            }
         };


然后，调用这个函数的时候，用的也是一个匿名内部类，因为这个私有函数的参数是一个ActionListener 事件监听接口，
接口如果没有被一个类实现，是不能有实例的，也就是说接口不能被实例化，也就是不能用new ActionListener() 这种写法，
但是匿名内部类是唯一可以这莫用的，因为匿名内部类也是没有名字的，new ActionListener() 这莫写，
意思就是 内部类实现该接口，


     addRadioButton("Integer divide by zero", new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               a[1] = 1 / (a.length - a.length);
            }
         }
      );



按钮是一个主题， ActionListener 是观察者，即使是代码的结构变了，这个还是不变的，
fireActionPerformed 其实就是 主题里的 Notify， 就是通知ActionListener的实现者，
按钮被按下了，然后把事件对象给送过去，ActionListener的实现者接到这个事件对象然后
事件被执行，如果在事件执行时有异常，就捕获异常然后把异常的描述放在textField里，
如果事件执行时没有异常，就把继续执行下个语句 把No exception 放在textField里。