BorderLayout - child component remains visible aft

问题:

I try to do the following thing:

1. Add a component to a JFrame with a BorderLayout
2. Add another component to the JFrame

I would expect the new component to 'overwrite' the old component, since I'm using a BorderLayout. This works if I overwrite the old component before I call pack(). Now if I add the 2nd component after I call pack(), both components remain visible. Example:

public class Test extends JFrame{
    public Test(){
        setLayout(new BorderLayout());
        add(new JLabel("Quite a long text"));
        setVisible(true);
        pack();
        add(new JLabel("Another text"));
    }
}

Result:

public class Test extends JFrame{
    public Test(){
        setLayout(new BorderLayout());
        add(new JLabel("Quite a long text"));
        setVisible(true);
        add(new JLabel("Another text"));
        pack();
    }
}

Result:

I tried adding validate(); and repaint();, but that wouldn't help. What's going wrong here?

回答1:

You are calling things that affect the UI after setViisble() is called -- a Swing no-no. Put things that affect the UI into an invokeLater call; look it up.

While you're at it, I think you're still supposed to get the content pane from the frame for adding components... And I don't think creating things in the constructor is a good idea.

import java.awt.BorderLayout;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingUtilities;

public class SwingTest extends JFrame
{
    public void createUI()
    {
        setDefaultCloseOperation(DISPOSE_ON_CLOSE);
        SwingUtilities.invokeLater(
                new Runnable()
                {
                    public void run()
                    {
                        getContentPane().setLayout(new BorderLayout());
                        getContentPane().add(new JLabel("Quite a long text"), BorderLayout.CENTER);
                        getContentPane().add(new JLabel("Another text"), BorderLayout.CENTER);
                        pack();
                        setVisible(true);
                    }
                }
                );
    }

    public static void main(String ... args) 
    { 
        SwingTest st = new SwingTest();
        st.createUI();
    }
}