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#1 evert67   User is offline

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How does repaint work in combination with paintComponent (Graphics)?

Posted 03 October 2014 - 03:43 AM

I’m teaching myself AWT/Swing using an Oracle tutorial. There’s a section a don’t fully understand.

It concerns an application that creates dynamic graphics using the Graphics class. I understand that the code that creates graphics should be put in a “paintComponent” method of a (descendant of a) JPanel class. And that when you want to change something on this panel outside “paintComponent”, you have to call the “repaint” method, that somehow causes “paintComponent” to be invoked.

However, I don’t fully understand the explanation of the example at http://docs.oracle.c...ting/step3.html . It concerns an application that moves a red square when a mouse button is clicked. The code can be found at the link, but I also repeat it below.

package painting;

import javax.swing.SwingUtilities;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.BorderFactory;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseMotionListener;
import java.awt.event.MouseMotionAdapter;

public class SwingPaintDemo3 {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {

    private static void createAndShowGUI() {
        System.out.println("Created GUI on EDT? "+
        JFrame f = new JFrame("Swing Paint Demo");
        f.add(new MyPanel());

class MyPanel extends JPanel {

    private int squareX = 50;
    private int squareY = 50;
    private int squareW = 20;
    private int squareH = 20;

    public MyPanel() {


        addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent e) {

        addMouseMotionListener(new MouseAdapter() {
            public void mouseDragged(MouseEvent e) {
    private void moveSquare(int x, int y) {
        int OFFSET = 1;
        if ((squareX!=x) || (squareY!=y)) {

    public Dimension getPreferredSize() {
        return new Dimension(250,200);
    protected void paintComponent(Graphics g) {
        g.drawString("This is my custom Panel!",10,20);

This code indeed works. However, I don’t understand why. The explanation at the Oracle site is the following. “Because we are manually setting the clip, our moveSquare method invokes the repaint method not once, but twice. The first invocation tells Swing to repaint the area of the component where the square previously was (the inherited behavior uses the UI Delegate to fill that area with the current background color.) The second invocation paints the area of the component where the square currently is.”

However, what I don’t understand: when repaint is first invoked, squareX and squareY still have their old values. If the invocation of repaint simply causes an invocation of paintComponent, then paintComponent should draw a red square at (squareX, squareY). Just like the second invocation of repaint causes paintComponent to draw a red suare at the new values of (squareX, squareY). Instead, the first invocation results in the red square being wiped out by the background color.

So apparently, the first invocation of repaint does not simply cause an invocation of paintComponent. What does it do?

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Replies To: How does repaint work in combination with paintComponent (Graphics)?

#2 Peter O   User is offline

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Re: How does repaint work in combination with paintComponent (Graphics)?

Posted 03 October 2014 - 07:17 AM

I'm not sure but the page says:


An important point worth noting is that although we have invoked repaint twice in a row in the same event handler, Swing is smart enough to take that information and repaint those sections of the screen all in one single paint operation. In other words, Swing will not repaint the component twice in a row, even if that is what the code appears to be doing.

So I guess that means that calling repaint will not call paintComponent right away but instead just remember the rectangle that needs to be redrawn and then, sometime later, it calls paintComponent once, and at that time the box coordinates have of course been updated.
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