Question about this particular method: paintComponent(Graphics g)

[Kagiso]

	private static class DisplayPanel extends JPanel
	{
		public void paintComponent(Graphics g)
		{
			super.paintComponent(g);   // why is it that when I remove this line the background color doesn't change
			setBackground(Color.GRAY);
			g.drawString("Hello World",50,50);
		}
	}

why can't I just call setBackground() ? what am I missing here?

thanks in advance

[jim829]

The setBackground method you are calling is inherited by JPanel. Even though you override paintComponent in your subclass, JPanel and its inheritance hierarchy still perform some graphics functions. One of those functions is to paint the background.

Regards,
Jim

[Kagiso]

Please correct me if I got it wrong

super.paintComponent(g) creates the background(a wall where you can paint on )
then setBackground() paints a color on that wall

[jim829]

paintComponent (at whatever level it is invoked) just does painting. The "wall" so to speak, is created by the JPanel. JFrame is similar in that it has a variety of panes on which to paint.

Regards,
Jim

[gimbal2]

The code makes little sense anyway; what is happening here is

- paint panel
- set the color property of the panel

But that needs to be the other way around; set the color first, then paint. You don't want to set the color WHILE painting, you want to set that once when you create the JPanel, before it is painted for the first time.

The reason why this works at all is because you'll get the proper color on screen when the panel repaints itself a second time. But the first time it still has the default background color. You may see a little glitch when starting the application perhaps.

[Kagiso]

Jimmy, please bear with me if I'm being difficult... but I really need to understand
super.paintComponent()... what does it actually do?
Why can't I simply call setBackground() to change the color of DisplayPanel ?

[Kagiso]

The code makes little sense anyway; what is happening here is

- paint panel
- set the color property of the panel

But that needs to be the other way around; set the color first, then paint. You don't want to set the color WHILE painting, you want to set that once when you create the JPanel, before it is painted for the first time.

The reason why this works at all is because you'll get the proper color on screen when the panel repaints itself a second time. But the first time it still has the default background color. You may see a little glitch when starting the application perhaps.

What are you talking about? this code is fine(It's taken from a textBook... for your information)... there's no glitch... perhaps you also need to learn a bit more about javax.swing

[jim829]

First, Gimbal is correct! The example is wrong. setBackground is pertinent to the JPanel and has nothing to do with the paintComponent at this level. The only way to correct it would be to put the setBackground before the super.paintComponent() invocation. That is because the paintComponent method that paints the background uses the value that you are setting. But if you set it after the fact, it won't work first time around. The setBackground call should be done higher up in your program, probably in the constructor of the DisplayPanel class.

One more clarification which I didn't mention. By wrong, I didn't mean wouldn't work. It is inefficient. When you do the setBackground in your paintComponent method it will determine if the background needs to be repainted and do so resulting in two paintings upon startup.

And of course, you have to add this to a JFrame or some other top level container for it to display at all.

Regards,
Jim

[jim829]

Not to belabor the point but try this code.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.JFrame;
import javax.swing.JPanel;


public class ColorCheck extends JPanel {
   JFrame        frame;
   int           width         = 600;
   int           height        = 600;
   public ColorCheck () {
      frame = new JFrame();
      frame.add(this);
      setPreferredSize(new Dimension(width, height));
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }
   public static void main(String[] args) {
      new ColorCheck();
   }
   public void paintComponent(Graphics g) {
      System.out.println("painting");
      setBackground(Color.red);
      setBackground(Color.blue);
      super.paintComponent(g);
   }
}

Then move one of the setBackgrounds to your constructor and see what happens when you run it. And finally, move the last one to your constructor and see what happens.

Regards,
Jim

[DarrylBurke]

What are you talking about? this code is fine(It's taken from a textBook... for your information)... there's no glitch... perhaps you also need to learn a bit more about javax.swing

Burn the book. It's a crime against trees to print crap like that.

The only way to correct it would be to put the setBackground before the super.paintComponent() invocation.

No. The state of a Component should never be changed inside a painting method override. The only way to correct this is to setBackground(...) elsewhere. Keep painting methods for painting and painting alone.

Oh, and setBackground(...) (like so many methods that change the state of a Component) internally calls repaint().

    public void setBackground(Color bg) {
	Color oldBg = getBackground();
	super.setBackground(bg);
	if ((oldBg != null) ? !oldBg.equals(bg) : ((bg != null) && !bg.equals(oldBg))) {
	    // background already bound in AWT1.2
	    repaint();
	}
    }

The only reason this defective code doesn't result in a CPU-toasting infinite loop is the check that the background really has been changed, and that pending painting events are coalesced.

db

[jim829]

No. The state of a Component should never be changed inside a painting method override. The only way to correct this is to setBackground(...) elsewhere. Keep painting methods for painting and painting alone.

Yes, I know. I forgot to remove that statement (which should have been somewhat apparent from my other comments).

Regards,
Jim

[Kagiso]

OK now I get it

private static class DisplayPanel extends JPanel 
{
	private DisplayPanel()
	{
		setBackground(Color.GRAY); // it's supposed to be here
	}
	
	public void paintComponent(Graphics g)
	{
		super.paintComponent(g);
	//	setBackground(Color.GRAY);       this line isn't supposed to be in this method
		g.drawString("Hello World",50,50);
	}
}

To Gimbal... I appologise, you were right...
To Darryl .... to be honest, I'm the one that added the setBackground()
To Jim.... I've copied the code... I'll play around with it

Thanx to all of you ...

[gimbal2]

To Gimbal... I appologise, you were right...

No worries, high five!

But to answer your earlier question: what I'm talking about? Something drawn from experience and knowledge. Next time you go to a forum to ask for help, give the people answering the benefit of the doubt and you won't have to apologize. See something that doesn't mesh with what you think is right? Look it up, don't flat out reject it.