JPanel Visible.

[Brian Lemieux]

   
package BouncingBalls;
 
import java.awt.*;
import javax.swing.*;
 
public class Main {
 
    public static void main(String[] args) {
 
            JPanel p = new JPanel(); 
            p.setPreferredSize(new Dimension(400, 400));
            p.setVisible(true);
 
    }
 
}
 

Simply, why the JPanel is not visible when running this. Nothing appears and there are no errors.

Thanks

[Campbell Ritchie]

I think I shall have to transfer this thread to Swing, since it is a GUI-related topic.

You can't display a panel on its own; it has to be added to a container, of which the most popular type is the JFrame.

You are also supposed to start a new thread; the details are in the API for any Swing class, where the link to "Swing threading policy" is.

You need to do three things to make a frame appear properly

Give it a size

Decide what to do when it closes; I think the default is "hide." You can set a default for closing; EXIT closes the entire JVM. Or you can use a Listener, but that is too advanced for you.

Set it visible. Do this last.

public class MyFrame extends JFrame
{
   private JPanel panel;
   public MyFrame(String title)
   {
       super(title);
       add(panel = new JPanel());
       // etc etc
       setSize(200, 200);
       setDefaultClosingOperation(EXIT_ON_CLOSE);
   }
}
 . . .
public static void main(String[] args)
{
   SwingUtilities.invokeLater(new Runnable()
   {
       public void run()
       {
          new MyFrame("Bounce!").setVisible(true);
       }
   });
}

That ought to work. Needless to say there are several different versions of setting up a Frame like that. You will have to check in the API for JFrame; I am pretty certain I have spelt setDefaultClosingOperation wrongly.

And good luck with it.

[pete stein]

The best recommendation for you is to start going through the Sun Swing tutorials. This will teach you how to do Swing coding the right way from the get-go. You won't regret doing this, trust me.

[Campbell Ritchie]

I agree that the Java Tutorial is usually a good place to start for almost anything.

I managed to move this thread just at the same time that a newbie was posting a possible answer (here), which he added as a new thread in beginner's. I had to close the new thread because we can't have two halves to a conversation, but it is still worth a look.
[edit]Corrected the link[/edit]
[ December 07, 2008: Message edited by: Campbell Ritchie ]

[Glen Tanner]

Sorry about the confusion....and thanks for the welcome!
I agree the Swing tutorial is a great place to start.
The Bunkhouse is another.