What is the $1 in class file names?

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>

Here is the content of that Java file:

/**
   @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:[email protected]"));
      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(); 
               }
            }
         };
   }
}

Those are the .class files that hold the anonymous inner classes.

In your example WelcomeApplet.java contains a top-level class (called WelcomeApplet) and an anonymous inner class, which will be stored in WelcomeApplet$1.class.

Note that the exact name of the files holding anonymous inner classes is not standardized and might vary. But in practice I've yet to see any other scheme than the one described here.

Value-specific bodies for an enum are also anonymous inner classes:

> The optional class body of an enum constant implicitly defines an anonymous class declaration (§15.9.5) that extends the immediately enclosing enum type.

The $1 are anonymous inner classes you defined in your WelcomeApplet.java file.

e.g. compiling

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

would result in Run.class, Run$1.class and Run$innerNamed.class being generated

These are generated from the inner and static nested classes in the WelcomeApplet.java file by the java compiler.

See also this similar question and answer.

It is from this 'line' of code:

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

The way you are declaring the ActionListener you are making an instance of the anonymous inner class each time that method is called.

Even if the method is not called, the above line still gets compiled into an anonymous inner class no matter what.

The WelcomeApplet$1.class file is generated for an anonymous class in the WelcomeApplet.java source (the anonymous class is generated in the method call makeURLActionListener by calling new new ActionListener() {...})

To explain more clearly, the anonymous classes are generated at compile time any time you have an instantiation of a concrete named class that overrides some or all of the behavior of the concrete class (or interface) inline like this:

class HelloInternalClass {
  public static final void main(String[] args) {
    // print in another thread
    new Thread(new Runnable() {
      public void run() {
        System.out.println("Printed from another thread");
      }
    }).start();
  }
}

In the above sample code, the javac compiler would generate 2 class files just like in your example: HelloInternalClass.class and HelloInternalClass$1.class.

The anonymous class in this instance would be a subclass of Runnable and would be compiled into HelloInternalClass$1.class. Incidentally, if you would ask a class name from the runnable instance in the above sample (by calling getClass().getName()) you would find that it thinks of itself as "HelloInternalClass$1".

Create:

public class A {
	public static void main(String[] args) {
		X x=new X();
		X x2=new X(){	
		};
		Class<? extends Object>c2=x2.getClass();
		Class<? extends Object>s2=x2.getClass().getSuperclass();

		boolean b=false;
	}
	private static class X{		
	}
}

It is hard to see from the code (new X{}() would have been better than new X(){}), but x2 is an instance of a subclass of A$X. This subclass is A$1.