Question

Can somebody please help me understand why my custom JComponent ‘Bar’, only displays when added directly to the JFrame, and not when added to a JPanel (which is then added to the JFrame)?

Thanks!

package main;

import java.awt.BorderLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class Board {

    public void start(){
        JFrame frame = new JFrame();
        JButton button1 = new JButton("Button 1");
        Bar statusbar = new Bar();
        JLabel status = new JLabel("Status: ");

        JPanel topPanel = new JPanel();
        topPanel.add(status);
        topPanel.add(statusbar);

        JPanel mainPanel = new JPanel();
        mainPanel.add(button1);
        mainPanel.add(statusbar);

        frame.getContentPane().add(BorderLayout.NORTH, topPanel);
        frame.getContentPane().add(BorderLayout.SOUTH, mainPanel);
        frame.getContentPane().add(BorderLayout.CENTER, statusbar);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(200,100);
        frame.setVisible(true);
    }
}

Here is my Bar Class…

package main;

import java.awt.Graphics;

import javax.swing.JComponent;

public class Bar extends JComponent{
    public void paint(Graphics g){
        g.fillRect(0, 0, 100, 10);
    }
}

Answer 1

You’re adding statusbar to several different places in the component tree, Swing doesn’t deal with that well (or at all).

Create a separate Bar instances each time you use it, if you want their display to be synchronized, they should share the same model.

Edit

Ah, on a second glance, the problem here is that you never set a size (or preferred size) for the Bar components, so they get squished to 0 by the layout manager.

Try:

public static class Bar extends JComponent {

    private Bar() {
        setPreferredSize(new Dimension(25, 5));
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.fillRect(0, 0, 100, 10);
    }
}

You should also add a frame.pack() before display.

(the multiple references to the same component thing is still true, too)