setDefaultCloseOperation窗口关闭方法无法生效

Question

请问，运行第64行代码关闭不了程序，是哪里有问题呀？

import java.awt.Button;
import java.awt.FlowLayout;
import java.awt.Frame;
import java.awt.Label;
import java.awt.TextField;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;

public class TestTF_3 {
    public static void main(String[] args) {
        new TF().launch();
    }
}

class TF extends JFrame {
    public TextField tf1, tf2, tf3;

    public void launch() {
        Frame f = new Frame();
        tf1 = new TextField(30);
        tf2 = new TextField(30);
        tf3 = new TextField(30);
        Button bn = new Button("=");
        Label lb = new Label("+");

        f.setLayout(new FlowLayout());
        f.add(tf1);
        f.add(lb);
        f.add(tf2);
        f.add(bn);
        f.add(tf3);

        bn.addActionListener(new MyMonitor2());

        f.pack();
        f.setVisible(true);

        // f.addWindowListener(new WindowAdapter() { //正常关闭
        // @Override
        // public void windowClosing(WindowEvent e) {
        // System.exit(-1);
        // }
        // });

        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //error 64行
        //this.setDefaultCloseOperation(3); //error
    }

    class MyMonitor2 implements ActionListener {
        @Override
        public void actionPerformed(ActionEvent e) {
            int num1 = Integer.parseInt(tf1.getText());
            int num2 = Integer.parseInt(tf2.getText());
            int num3 = num1 + num2;
            tf3.setText(num3 + "");
        }
    }
}

Answer 1

    public void launch() {
        JFrame f = new JFrame();
        tf1 = new TextField(30);
        tf2 = new TextField(30);
        tf3 = new TextField(30);
        Button bn = new Button("=");
        Label lb = new Label("+");

        f.setLayout(new FlowLayout());
        f.add(tf1);
        f.add(lb);
        f.add(tf2);
        f.add(bn);
        f.add(tf3);

        bn.addActionListener(new MyMonitor2());

        f.pack();
        f.setVisible(true);

        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }