Re: How do I paint on an existing Panel?

Lew <>
Thu, 16 Feb 2012 13:28:04 -0800 (PST)
On Thursday, February 16, 2012 1:05:25 PM UTC-8, A B wrote:

Sorry, I'm stumped, again. I really have been trying to sort it out myself,
but no luck. I've done it by adapting Knute Johnson's code (which works
fine in itself). Mine now defines a line and calls repaint() as nice as you
like, but there's no answer.

Note that you did not start the GUI on the Event Dispatch Thread (EDT). I don't
know if this causes your problem, but it's a mistake.

I've chopped the code back to just the bits directly involved with the
drawing and sprinkled debugging statements everywhere, which established
that the bit that calls paintComponent() (via repaint() - that's right isn't
it?) is firing but paintComponent() itself isn't. Here's what's left, if
you're interested. Sorry if anyone finds it hard to read, I don't know how
you like it formatted.

We like it formatted according to the well-established Java Coding Conventions
or a slight variation thereof.

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

public class Vectorine extends JFrame implements MouseListener

"Favor composition over inheritance".

private static final long serialVersionUID = 159L;

indent 2 (or 3 or 4) spaces per level, no TABs.

Why did you pick '159L'? Just use '1L'. What is up with 159?

// List to contain all the lines generated
private final java.util.List<ColoredLine> lineList = new

Why do you import java.util.* and still use FQNs?

public static void main() {Vectorine v = new Vectorine();}

Start GUIs on the EDT! Golly!

public Vectorine()
setSize(200, 200);

Use 'setPreferredSize()'.


You forgot 'pack()'.


// Set up window's initial contents
Container contentArea = getContentPane();
FlowLayout layout = new FlowLayout();
JPanel panelA = new JPanel();

System.out.println("If you click anywhere in the box, 3 random lines should
appear in it.");

Don't use 'System.out.println()' in a GUI, nor for logging.


public void paintComponent(Graphics g2d)
  Graphics2D g = (Graphics2D)g2d;

Your naming is backwards.


Don't use 'System.out.println()' in a GUI, nor for logging.

  for (ColoredLine hand : lineList)
    System.out.println("Drawing line...");
    BasicStroke pen = new BasicStroke(hand.getThickness());

Shouldn't you be painting these lines into a component contained within the
JFrame and not the JFrame itself?

public void mouseClicked(MouseEvent event)
   System.out.println("Mouse clicked");
   double xcoord = 0, ycoord = 0;
   for (int count=0; count<3; count++)
    xcoord = 100 * Math.random();
    ycoord = 100 * Math.random();
    System.out.println("xcoord="+xcoord+", ycoord="+ycoord);
    ColoredLine hand = new ColoredLine(0F, 0F, (float)xcoord, (float)ycoord,, 2);
    System.out.println("Calling repaint()...");

I'm less familiar with idioms that interfere with the repainting loops, but
shouldn't you be invalidating? Do you actually have to repaint?

It's hard to reason about this because the other errors complicate the


/** Blank methods to keep MouseListener happy. */

You could have just inherited the listener from

public void mousePressed(MouseEvent event) {}
public void mouseReleased(MouseEvent event) {}
public void mouseEntered(MouseEvent event) {}
public void mouseExited(MouseEvent event) {}

/** The actual lines drawn are instances of the ColoredLine class. */
class ColoredLine extends Line2D.Double
{ private static final long serialVersionUID = 149L;
  private final Color color;
  private final int thickness;
  public ColoredLine(double x,double y,double w,double h,Color color,int
   this.color = color;
   this.thickness = thickness;

  public Color getColor() {return color;}
  public int getThickness() {return thickness;}

I cannot be sure without putting far too much time into it, but I think the
problem stems from a combination of factors:
- not running the GUI on the EDT;
- drawing on the JFrame itself rather than a contained component, leading to
  components drawing themselves over your graphic;
- failure to pack();
- omission of 'setPreferredSize()';

I promise that I have not analyzed your program in the depth needed to properly
suss out your problem. I do know that it helps to eliminate even potentially
complicating factors, particularly the failure to use the EDT. Most of those
factors won't impinge on the immediate question, but the effort to fix bad code
is never wasted.


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