[Java]動態生成JButton 問題究竟出在哪裡?(已解決)

問題究竟出在哪裡?

import java.awt.Toolkit;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main extends JFrame {

    JFrame myFrame;
    JPanel myPanel;
    JButton btn[];
    Toolkit toolkit = Toolkit.getDefaultToolkit();

    Main() {
        myPanel = new JPanel();


        for (int i = 0; i < 5; i++) {
            btn[i] = new JButton();
            myPanel.add(btn[i]);
        }
        add(myPanel);
    }

    public static void main(String argv[]) {
        Main app = new Main();
        app.addWindowListener(new WindowAdapter() {

            public void windowClosing(WindowEvent e) {
                System.out.println("Frame closing...");
                System.exit(0);
            }
        });
        app.setVisible(true);
        app.repaint();
    }
}

原來是這裡忘了初始= =

JButton btn[];

改成

JButton btn[] = new JButton[5];