求助：Java图形界面 为什么这段代码不用继承JFrame类

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

public class Java_3 extends MouseAdapter  implements ActionListener
{
   private JPopupMenu pop;
   private JMenu subPop;
   private JMenuItem color;
   private JMenuItem exit;
   private JMenuItem red;
   private JMenuItem blue;
   private JTextArea textArea;
   private JFrame frame;
     ...
     ...
public static void main(String args[])
   {
      Java_3 example=new Java_3();
      example.initGUI();
   } 
}

以上这段代码为什么不用继承JFrame类？

private JFrame frame;直接创建JFrame对象了