什么是1美元的类文件名？

Question

C:\Program Files\Java\jdk1.6.0_05\CoreJava\v1\v1ch2\WelcomeApplet>dir
 Volume in drive C has no label.
 Volume Serial Number is 2041-64E7

 Directory of C:\Program Files\Java\jdk1.6.0_05\CoreJava\v1\v1ch2\WelcomeApplet

2009-07-02  23:54              .
2009-07-02  23:54              ..
2004-09-06  14:57               582 WelcomeApplet.html
2004-09-06  15:04             1,402 WelcomeApplet.java
               2 File(s)          1,984 bytes
               2 Dir(s)   2,557,210,624 bytes free

C:\Program Files\Java\jdk1.6.0_05\CoreJava\v1\v1ch2\WelcomeApplet>javac WelcomeApplet.java

C:\Program Files\Java\jdk1.6.0_05\CoreJava\v1\v1ch2\WelcomeApplet>dir
 Volume in drive C has no label.
 Volume Serial Number is 2041-64E7

 Directory of C:\Program Files\Java\jdk1.6.0_05\CoreJava\v1\v1ch2\WelcomeApplet

2009-07-02  23:54              .
2009-07-02  23:54              ..
2009-07-02  23:54               975 WelcomeApplet$1.class
2009-07-02  23:54             1,379 WelcomeApplet.class
2004-09-06  14:57               582 WelcomeApplet.html
2004-09-06  15:04             1,402 WelcomeApplet.java
               4 File(s)          4,338 bytes
               2 Dir(s)   2,557,202,432 bytes free

C:\Program Files\Java\jdk1.6.0_05\CoreJava\v1\v1ch2\WelcomeApplet>

以下是该Java文件的内容:

/**
   @version 1.21 2002-06-19
   @author Cay Horstmann
*/

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

public class WelcomeApplet extends JApplet
{
   public void init()
   {
      setLayout(new BorderLayout());

      JLabel label = new JLabel(getParameter("greeting"), SwingConstants.CENTER);
      label.setFont(new Font("Serif", Font.BOLD, 18));
      add(label, BorderLayout.CENTER);

      JPanel panel = new JPanel();

      JButton cayButton = new JButton("Cay Horstmann");
      cayButton.addActionListener(makeURLActionListener(
         "http://www.horstmann.com"));
      panel.add(cayButton);

      JButton garyButton = new JButton("Gary Cornell");
      garyButton.addActionListener(makeURLActionListener(
         "mailto:gary@thecornells.com"));
      panel.add(garyButton);

      add(panel, BorderLayout.SOUTH);
   }

   private ActionListener makeURLActionListener(final String u)
   {
      return new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               try
               {
                  getAppletContext().showDocument(new URL(u));
               }
               catch(MalformedURLException e) 
               { 
                  e.printStackTrace(); 
               }
            }
         };
   }
}

Answer 1

这些是包含匿名内部类的.class文件.

在您的示例中WelcomeApplet.java包含一个顶级类(称为WelcomeApplet)和一个匿名内部类,它将存储在其中WelcomeApplet$1.class.

请注意,包含匿名内部类的文件的确切名称不是标准化的,可能会有所不同.但在实践中,我还没有看到任何其他方案,而不是这里描述的方案.

a的值特定主体enum 也是匿名内部类:

枚举常量的可选类体隐式定义了一个匿名类声明(第15.9.5节),该声明扩展了直接封闭的枚举类型.

Answer 2

$ 1是您在WelcomeApplet.java文件中定义的匿名内部类.

例如编译

public class Run {
    public static void main(String[] args) {
        System.out.println(new Object() {
            public String toString() {
                return "77";
            }
        });
    }
    private class innerNamed {
    }
}

将导致Run.class,Run$1.class与Run$innerNamed.class正在生成

Answer 3

这些是由Java编译器从WelcomeApplet.java文件中的内部和静态嵌套类生成的。

另请参阅此类似的问答。

Answer 4

它来自这“行”代码：

return new
    ActionListener()
    {
        public void actionPerformed(ActionEvent event)
        {
            try
            {
                getAppletContext().showDocument(new URL(u));
            }
            catch(MalformedURLException e) 
            { 
                e.printStackTrace(); 
            }
        }
    };

您声明的方式ActionListener每次调用该方法时，

即使没有调用该方法，上面的行仍然会被编译为匿名内部类。