怎么写一个简单的java加减法gui：报错

给个思路吧，在bt上添加动作监听，获取t1和t2的值，然后计算出结果后，设置在t3里。

这个确实很基础，建议多用一下搜索引擎。

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

class AddDemo extends JFrame {

    JLabel b1, b2;
    JTextField t1, t2, t3;
    JButton bt;

    public AddDemo() {
        b1 = new JLabel("被减数", JLabel.CENTER);
        b2 = new JLabel("减数", JLabel.CENTER);
        b1.setBorder(BorderFactory.createEtchedBorder());
        b2.setBorder(BorderFactory.createEtchedBorder());
        t1 = new JTextField(6);
        t2 = new JTextField(6);
        t3 = new JTextField(6);
        t3.setEditable(false);
        bt = new JButton("求差");
        setLayout(new GridLayout(3, 2));
        add(b1);
        add(t1);
        add(b2);
        add(t2);
        add(bt);
        add(t3);
        bt.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                int value1 = Integer.parseInt(t1.getText());
                int value2 = Integer.parseInt(t2.getText());
                t3.setText(Integer.toString(value1-value2));
            }
        });
        setSize(200, 160);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public static void main(String arg[]) {
        new AddDemo();
    }
}