一个看不懂的问题

我写的一个程序是说有3个控件一个清空一个转换和一个jTextField1。当我在jTextField1里面输入1234时就输出4321，在我运行着个程序时没有错误！但是当单击转换是就出现的下面的错误
看不懂请指教！我是用Swing编写的
C:\JBUILDERX\JDK1.4\bin\javaw -classpath "C:\WINDOWS\jbproject\untitled41\classes;C:\JBuilderX\lib\jbcl.jar;C:\JBuilderX\lib\dx.jar;C:\JBuilderX\lib\beandt.jar;C:\JBUILDERX\JDK1.4\demo\jfc\Java2D\Java2Demo.jar;C:\JBUILDERX\JDK1.4\demo\plugin\jfc\Java2D\Java2Demo.jar;C:\JBUILDERX\JDK1.4\jre\javaws\javaws.jar;C:\JBUILDERX\JDK1.4\jre\lib\charsets.jar;C:\JBUILDERX\JDK1.4\jre\lib\ext\dnsns.jar;C:\JBUILDERX\JDK1.4\jre\lib\ext\ldapsec.jar;C:\JBUILDERX\JDK1.4\jre\lib\ext\localedata.jar;C:\JBUILDERX\JDK1.4\jre\lib\ext\sunjce_provider.jar;C:\JBUILDERX\JDK1.4\jre\lib\im\indicim.jar;C:\JBUILDERX\JDK1.4\jre\lib\im\thaiim.jar;C:\JBUILDERX\JDK1.4\jre\lib\jce.jar;C:\JBUILDERX\JDK1.4\jre\lib\jsse.jar;C:\JBUILDERX\JDK1.4\jre\lib\plugin.jar;C:\JBUILDERX\JDK1.4\jre\lib\rt.jar;C:\JBUILDERX\JDK1.4\jre\lib\sunrsasign.jar;C:\JBUILDERX\JDK1.4\lib\dt.jar;C:\JBUILDERX\JDK1.4\lib\htmlconverter.jar;C:\JBUILDERX\JDK1.4\lib\tools.jar"  untitled41.li 
java.lang.ClassCastException  at untitled41.Frame1.jButton1_actionPerformed(Frame1.java:65)  at untitled41.Frame1_jButton1_actionAdapter.actionPerformed(Frame1.java:90)  at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1786)  at javax.swing.AbstractButton$ForwardActionEvents.actionPerformed(AbstractButton.java:1839)  at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:420)  at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:258)  at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:245)  at java.awt.Component.processMouseEvent(Component.java:5100)  at java.awt.Component.processEvent(Component.java:4897)  at java.awt.Container.processEvent(Container.java:1569)  at java.awt.Component.dispatchEventImpl(Component.java:3615)  at java.awt.Container.dispatchEventImpl(Container.java:1627)  at java.awt.Component.dispatchEvent(Component.java:3477)  at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:3483)  at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3198)  at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3128)  at java.awt.Container.dispatchEventImpl(Container.java:1613)  at java.awt.Window.dispatchEventImpl(Window.java:1606)  at java.awt.Component.dispatchEvent(Component.java:3477)  at java.awt.EventQueue.dispatchEvent(EventQueue.java:456)  at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:201)  at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)  at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:145)  at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:137)  at java.awt.EventDispatchThread.run(EventDispatchThread.java:100) 
代码：
package untitled41;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import com.borland.jbcl.layout.*;
import java.io.*;
/**
 * <p>Title: </p>
 * <p>Description: </p>
 * <p>Copyright: Copyright (c) 2005</p>
 * <p>Company: </p>
 * @author not attributable
 * @version 1.0
 */public class Frame1 extends JFrame {
  private int num1;
  private int j;
  JPanel contentPane;
  XYLayout xYLayout1 = new XYLayout();
  JButton jButton1 = new JButton();
  JTextField jTextField1 = new JTextField();
  JButton jButton2 = new JButton();  //Construct the frame
  public Frame1() {
    enableEvents(AWTEvent.WINDOW_EVENT_MASK);
    try {
      jbInit();
    }
    catch(Exception e) {
      e.printStackTrace();
    }
  }  //Component initialization
  private void jbInit() throws Exception  {
    contentPane = (JPanel) this.getContentPane();
    jButton1.setText("转换");
    jButton1.addActionListener(new Frame1_jButton1_actionAdapter(this));
    contentPane.setLayout(xYLayout1);
    this.setSize(new Dimension(400, 300));
    this.setTitle("Frame Title");
    jTextField1.setText("");
    jButton2.setText("清空");
    jButton2.addActionListener(new Frame1_jButton2_actionAdapter(this));
    contentPane.add(jButton1,  new XYConstraints(51, 60, 65, 31));
    contentPane.add(jTextField1,   new XYConstraints(158, 80, 193, 30));
    contentPane.add(jButton2,    new XYConstraints(51, 107, 66, 31));
  }  //Overridden so we can exit when window is closed
  protected void processWindowEvent(WindowEvent e) {
    super.processWindowEvent(e);
    if (e.getID() == WindowEvent.WINDOW_CLOSING) {
      System.exit(0);
    }
  }  void jButton1_actionPerformed(ActionEvent e) {
    String result = " ";if(e.getActionCommand()=="转换"){
num1=Integer.parseInt(((Choice)e.getSource()).getSelectedItem().trim());
      do{
        j=num1%10;
        num1/=10;
      result += j;
    jTextField1.setText(result);
      }while(num1!=0);     }
}  void jButton2_actionPerformed(ActionEvent e) {
jTextField1.setText("");  }
}class Frame1_jButton1_actionAdapter implements java.awt.event.ActionListener {
  Frame1 adaptee;  Frame1_jButton1_actionAdapter(Frame1 adaptee) {
    this.adaptee = adaptee;
  }
  public void actionPerformed(ActionEvent e) {
    adaptee.jButton1_actionPerformed(e);
  }
}class Frame1_jButton2_actionAdapter implements java.awt.event.ActionListener {
  Frame1 adaptee;  Frame1_jButton2_actionAdapter(Frame1 adaptee) {
    this.adaptee = adaptee;
  }
  public void actionPerformed(ActionEvent e) {
    adaptee.jButton2_actionPerformed(e);
  }
}