JComboBox向上弹出

package com.JComB;

import javax.swing.*;

import javax.swing.plaf.basic.*;

public class JComboBoxEx extends JComboBox 
{
       public JComboBoxEx() 
       {
        super();
    }

       public void firePopupMenuWillBecomeVisible() 
       {
              int maxHight = 0;
              
        BasicComboPopup comboPopup = (BasicComboPopup)getUI().getAccessibleChild(this, 0);
        //此为 ComboPopup 接口的基本实现。 此类表示组合框的弹出部分的 UI
        maxHight = comboPopup.getHeight();
        System.out.println("11"+maxHight);
        if (maxHight == 0) 
        	//封装了一个构件的高度和宽度，这个类与一个构件的许多属性具有相关性，因此在Component类中定义多个与之有关的方法，
        	//LayoutManager接口也与一个Dimension对象有关联。Dimension类的高度和宽度值是一个整数，表明有多少个像素点
        {	//getPreferredSize获得相应组件的大小
               maxHight = comboPopup.getPreferredSize().height;
        }
        comboPopup.setLocation(getLocationOnScreen().x, getLocationOnScreen().y - maxHight);

        super.firePopupMenuWillBecomeVisible();
    }
}
package com.JComB;
/*
 <applet code="MainFrame" width="300" height="100"></applet>
*/
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class MainFrame extends JApplet 

{
  JComboBoxEx box;
  public MainFrame(){     

  }

  public void init() 

  {

     box = new JComboBoxEx();      
     Container c=getContentPane();   
     c.setLayout(null);   
     Rectangle rc = c.getBounds();
     box.setBounds(new Rectangle(30,30,100, 30));
     box.addItem("1");
     box.addItem("2");
     box.addItem("3");
     box.addItem("4");
     add(box);   
  }

}