Java repaint method does not work when called from another class

Question

I have been coding up Java with Netbeans for about a year now, and have written a lot of data manipulation code which plots graphs on-screen. I generally plant a JPanel object in my main window, write custom painting code, and call the repaint() method as needed.

But today, for the first time, I tried to invoke a repaint on a panel from a class (object) other than the one that contained the panel. Although the compiler found nothing wrong with this, and in debugging mode, it single-stepped properly to the exterior call to the repaint, no repaint actually occurred, and the code did not actually step into the repaint method.

I wrote a minimalist program to demonstrate the problem, as shown below (Main is ommitted, since it only contains code to set up the two on-screen panels.)

--- Description of classes, first contains the drawing surface, other the repaint call ---

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Panel1 extends JComponent
{
   GraphPnl graphPnl;
   boolean colorFlag;

   public Panel1()
   {
     setLayout(null);
     colorFlag = true;

     graphPnl = new GraphPnl();
     graphPnl.setBounds(10, 10, 110, 110);
     graphPnl.setBackground(Color.black);
     add(graphPnl);

}//Panel1()

public class GraphPnl extends JPanel
{
  //just draws a line segment, toggling color

  @Override
  public void paint(Graphics g)
  {
      super.paint(g);
      Graphics2D g2 = (Graphics2D) g;
  g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
               RenderingHints.VALUE_ANTIALIAS_ON);

      if (colorFlag) {g2.setColor(Color.red);} else {g2.setColor(Color.green);}
      g2.drawLine(10, 10, 50, 50);
   }//paint
 }//GraphPnl
}//Panel1

import javax.swing.*;
import java.awt.event.*;

public class Panel2 extends JComponent
{
   JButton testBtn;
   TestAction testAction;
   Panel1 p1;

   public Panel2()
   {
      p1 = new Panel1();
      testBtn = new JButton("Click");
      testBtn.setBounds(10, 10, 80, 30);
      add(testBtn);
      testAction = new TestAction();
      testBtn.addActionListener(testAction);
   }//Panel2()


   public class TestAction implements ActionListener
   {
     public void actionPerformed(ActionEvent evt)
     {
       p1.colorFlag = ! p1.colorFlag;
       p1.graphPnl.repaint();
     }
   }//TestAction
}//Panel2

If anyone has any insights into this, or knows of a workaround, I'd be very happy to hear from you.

Thanks in advance for any insights.

John Doner

Answer 1

I believe that when you are painting a JComponent, the clip region is set to that JComponent. So if other components try to paint (or if you call their repaint), they won't, because of the clipping.

Answer 2

Main is ommitted, since it only contains code to set up the two on-screen panels.)

Well, by definition when you have a problem you don't know what code is or isn't relative until the problem is solved. So a complete SSCCE should be posted.

As a wild guess I would say your component has a size of 0 so there is nothing to paint.

I generally plant a JPanel object in my main window, write custom painting code, and call the repaint() method as needed

You probably got lucky because you added the panel to the center of a BorderLayout which automatically gives the panel all the space available to the frame.

trashgod's example shows one way to set the preferred size of a custom component. Another way is to override the getPreferredSize() method to return the proper value.

You really should learn how to use layout manager rather than using null layouts and you will avoid problems like this in the future. There is no need to use a null layout unless you have a drag/drop type of application.

Answer 3

"Swing programs should override paintComponent() instead of overriding paint()."—Painting in AWT and Swing: The Paint Methods.

main is ommitted, since it only contains code to set up the two on-screen panels.

Verify that you construct your GUI on the EDT, as shown in the article Initial Threads.

Addendum: Here's an example showing both principles:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import javax.swing.AbstractAction;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

/** @see http://stackoverflow.com/questions/4282159 */
public class GraphPanel extends JPanel {

    private boolean colorFlag;

    public GraphPanel() {
        this.setPreferredSize(new Dimension(640, 480));
    }

    public void toggle() {
        colorFlag = !colorFlag;
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
        if (colorFlag) {
            g2.setColor(Color.red);
        } else {
            g2.setColor(Color.blue);
        }
        g2.drawLine(0, 0, getWidth(), getHeight());
    }

    private void display() {
        JFrame f = new JFrame("GraphPanel");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(this, BorderLayout.CENTER);
        f.add(new ControlPanel(this), BorderLayout.SOUTH);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new GraphPanel().display();
            }
        });
    }
}

class ControlPanel extends JPanel {

    public ControlPanel(final GraphPanel gp) {
        this.add(new JButton(new AbstractAction("Click") {

            @Override
            public void actionPerformed(ActionEvent e) {
                gp.toggle();
                gp.repaint();
            }
        }));
    }
}

Addendum: As noted in @camickr's comment, A Visual Guide to Layout Managers may help guide your layout selection.