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

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 Creating the Demo Application (Step 3) (The Java™ Tutorials > Creating a GUI With JFC/Swing > Performing Custom Painting) . 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() {
                createAndShowGUI(); 
            }
        });
    }
 
    private static void createAndShowGUI() {
        System.out.println("Created GUI on EDT? "+
        SwingUtilities.isEventDispatchThread());
        JFrame f = new JFrame("Swing Paint Demo");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
        f.add(new MyPanel());
        f.pack();
        f.setVisible(true);
    } 
}
 
class MyPanel extends JPanel {
 
    private int squareX = 50;
    private int squareY = 50;
    private int squareW = 20;
    private int squareH = 20;
 
    public MyPanel() {
 
        setBorder(BorderFactory.createLineBorder(Color.black));
 
        addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                moveSquare(e.getX(),e.getY());
            }
        });
 
        addMouseMotionListener(new MouseAdapter() {
            public void mouseDragged(MouseEvent e) {
                moveSquare(e.getX(),e.getY());
            }
        });
 
    }
 
    private void moveSquare(int x, int y) {
        int OFFSET = 1;
        if ((squareX!=x) || (squareY!=y)) {
            repaint(squareX,squareY,squareW+OFFSET,squareH+OFFSET);
            squareX=x;
            squareY=y;
            repaint(squareX,squareY,squareW+OFFSET,squareH+OFFSET);
        } 
    }
 
 
    public Dimension getPreferredSize() {
        return new Dimension(250,200);
    }
 
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);       
        g.drawString("This is my custom Panel!",10,20);
        g.setColor(Color.RED);
        g.fillRect(squareX,squareY,squareW,squareH);
        g.setColor(Color.BLACK);
        g.drawRect(squareX,squareY,squareW,squareH);
    }  
}

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?

Also posted here.

Yup. Are these forums somehow connected and, if so, is there a preferred place?

repaint() does NOT paint the component. The only thing it does is mark your component to indicate that it needs to be repainted at some point in the future. When this will be is hard to say, its up to the Swing rendering scheduler, but it will happen soon enough. If you call repaint 10 times in your method your component will not be painted 10 times, it will probably only get painted once after all 10 repaint requests have been made.

Thanks, this clarifies it a bit. But what I still don't understand is how the first invocation of repaint causes the red box to be wiped out.

Try messing with the code. Take out the if() statement, remove the first repaint(), change either or both of the calls to repaint( x, y, z, q ) to simply repaint(). I don't know why most of the Oracle and legacy Sun example code is largely uncommented, but that's the way it is.

I already messed with the code. What I found out is that the code actually works as if the first repaint removes the red box: if you remove that line, the red box remains displayed also at its old location. However, I don't understand how this line can remove the red box.

The call to "super.paintComponent(g);" within your JPanel will invoke the default painting of panels. The default is to clear everything with the background color.
In this case your panel will remove any leftovers from the previous paintComponent.

This thread has been cross posted here:

http://www.coderanch.com/t/640432/GUI/java/repaint-work-combination-paintComponent-Graphics

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