import javax.swing.*;
import java.awt.event.*;
public class TextFieldExample{
  public static void main(String []args){
    JFrame f=new JFrame();
    f.setResizable(false);
    JPanel p=new JPanel();
    JLabel labPrice=new JLabel("商品單價");
    p.add(labPrice);		
    final JTextField txtPrice=new JTextField(20);
    p.add(txtPrice);		
    JLabel labAmount=new JLabel("商品數量");
    p.add(labAmount);		
    final JTextField txtAmount=new JTextField(20);
    p.add(txtAmount);		
    JLabel labTotal=new JLabel("商品總額");
    p.add(labTotal);		
    final JTextField txtTotal=new JTextField(20);
    txtTotal.setEditable(false);
    p.add(txtTotal);		
    txtPrice.addActionListener(new ActionListener(){
      public void actionPerformed(ActionEvent e){			
        //txtAmount.requestFocus(); //獲取焦點
        txtAmount.selectAll(); //選中全部內容				
      }
    });		
    txtAmount.addActionListener(new ActionListener(){
      public void actionPerformed(ActionEvent e){
	try{
	  double price=Double.parseDouble(txtPrice.getText());
	  int amount=Integer.parseInt(txtAmount.getText());
	  txtTotal.setText(""+price*amount);
	  txtPrice.requestFocus();
	  txtPrice.selectAll();
	}catch(Exception ex){
	  ex.printStackTrace();
	  txtTotal.setText("不能計算");
	  txtPrice.requestFocus();
	  txtPrice.selectAll();
	}
      }
    });		
    f.getContentPane().add(p);
    f.setSize(300,110);
    f.show();
  }	
}