找出Java中关注的应用程序(窗口)

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

public class MyApp {
    public static void main(String[] args) {
        JFrame frame = new JFrame("My Application");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 300);

        JButton button = new JButton("Click me!");
        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JOptionPane.showMessageDialog(frame, "Hello, world!");
            }
        });

        frame.getContentPane().add(button);
        frame.setVisible(true);
    }
}