Swing JComboBox奇怪的行为

Question

I observe the following behavior (on the Windows 7 platform) :

import java.awt.*;
import javax.swing.*;
public class Main extends JFrame{   
    JPanel p;
    JComboBox<String> l;
    JLabel title;
    public static void main(String[] arg){
      Main m = new Main();
      m.setVisible(true);
      m.setSize(400,400);
      m.p = new JPanel();
      //m.l = new JComboBox<String>();
      m.title = new JLabel("HELLO"); 
      m.p.add(m.title);
      m.setContentPane(m.p);
      }
}

Displays HELLO , but if I uncomment the line that instantiates the JComboBox , it won't display anything. What could cause that? Are you able to reproduce the bug?

Answer 1

Solution from my comment:

Move m.setVisible(true); at the end.

Another comment from Jens Schauder :

Your code should also run in the EDT. Anything else is asking for trouble

May be he wants to tell something like this:

Everything dealing with Swing components, including there construction must run in the EDT . If it doesn't it is broken, although you might not notice it.

For that you may move your logic from main method to constructor of class and call constructor as follows:

public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new Main();
            }
        });
    }

You can also write logic in some other method then constructor.