扩展JPanel的类不会更新UI

Question

I have two classes: one which extends JFrame ( MyFrame ) and one which extends JPanel ( MyPanel ). The idea is to be able to add an instance of the MyPanel class to an existing, visible instance of MyFrame .

public class MyFrame extends JFrame {
    public MyFrame () {
        setVisible(true);
        JPanel panel = new MyPanel();
        add(panel);
    }
}

public class MyPanel extends JPanel {
    public MyPanel () {
        add(new JButton("Test"));
    }
}

public class Program {
    public static void main (String[] args) {
        new MyFrame();
    } 
}

The code runs and doesn't error, but the UI isn't updated when MyPanel is added to MyFrame.

Answer 1

Like all the other questions relating to similar issues, move setVisible(true); to the end of the constructor.

Something like...

public class MyFrame extends JFrame {

    public MyFrame() {
        JPanel panel = new MyPanel();
        add(panel);
        pack();
        setVisible(true);
    }
}

Swing layouts are lazy, they won't update until you tell them to (or some other action, like the top level container been resized/reshown occurs)

You should also use pack to give your window a initial size, base on the size requirements of its content.

Having said all that, you really should avoid extending from top level containers like JFrame , lets face it, you're not really adding any new functionality to the class and instead, simply create a new instance when you need it...

public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            try {
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                ex.printStackTrace();
            }

            JFrame frame = new JFrame("Testing");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(new MyPanel());
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        }
    });
}