example10_1.java

很好

import java.applet.*;import java.awt.*;import java.awt.event.*;
public class Example10_1 extends Applet implements ActionListener
{  TextField text;
   Button buttonEnter,buttonQuit;
   public void init()
   {  text=new TextField("0",10); add(text);
      buttonEnter=new Button("确定"); buttonQuit =new Button("清除");
      add(buttonEnter); add(buttonQuit);
      buttonEnter.addActionListener(this);
      buttonQuit.addActionListener(this);
      text.addActionListener(this);
   }
 public void paint(Graphics g)
   {  g.drawString("在文本框输入数字字符回车或单击按钮",10,100);
      g.drawString("第文本框显示该数的平方根",10,120);
   }
   public void actionPerformed(ActionEvent e)
   {  if(e.getSource()==buttonEnter||e.getSource()==text)
        {  double number=0;
           try {  number=Double.valueOf(text.getText()).doubleValue();
                  text.setText(""+Math.sqrt(number));
               }
           catch(NumberFormatException event)
               {  text.setText("请输入数字字符");
               }
        }
      else if(e.getSource()==buttonQuit)
       {  text.setText("0");
       }
   }
}