CODEWITHSUNDEEP
javaswingjpanelpaintcomponent
vader
vader

Reputation: 45

Stuff drawn by paintComponent() disappear after calling repaint()

On Head First Java book we're seeing some little animation and I'm trying to draw an animation that traces out a diagonal line. I'm doing so by using the paintComponent() method and drawing an oval at x,y (values which are updated each time through the loop). Why do I lose the previously drawn ovals? According to the book I should be getting a smear on the screen where the previously drawn ovals are not lost. This should need fixing by adding to the paintComponent() method a white background every time repaint() is called but instead i'm not getting the 'mistake'. Why is this and how do I get to KEEP the previously drawn ovals on the panel?
Using JDK 13.0.2 and Mac OSX Catalina

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

public class SimpleAnimation {

    int x = 70;
    int y = 70;

    public static void main(String[] args) {
        SimpleAnimation gui = new SimpleAnimation();
        gui.go();
    }

    public void go() {
        JFrame frame = new JFrame();
        MyDrawPanel drawPanel = new MyDrawPanel();

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.getContentPane().add(drawPanel);
        frame.setSize(300,300);
        frame.setVisible(true);

        for (int i = 0; i < 130; i++) {
            x++;
            y++;
            drawPanel.repaint();
            try {
                Thread.sleep(25);
            } catch (Exception ex){};

        }
    }
    class MyDrawPanel extends JPanel {

        public void paintComponent(Graphics g) {
            g.setColor(Color.orange);
            g.fillOval(x,y,50,50);
        }
    } // close inner class
} // close outer class

Upvotes: 0

Views: 531

Answers (1)

camickr
camickr

Reputation: 324118

Is that what prevents the smear of ovals?

class MyDrawPanel extends JPanel {
    public void paintComponent(Graphics g) {
        g.setColor(Color.orange);
        g.fillOval(x,y,50,50);
    }

The code should be:

class MyDrawPanel extends JPanel {
    public void paintComponent(Graphics g) {
        super.paintComponent(g); // added
        g.setColor(Color.orange);
        g.fillOval(x,y,50,50);
    }

You need the super.paintComponent(g) to clear the background of the panel before doing the custom painting.

Yeah, great book. The output i'm getting with the code as is, is the "corrected version"

Edit:

The book is correct. You need to understand how the EDT works. When you start an application the code invoked in the main() method executes on a separate Thread. Swing events and painting is done on the Event Dispatch Thread (EDT). So invoking sleep() in the go() method should NOT affect the painting of the circle and you should see the smear. If you only see a single oval painted after the loop is finished, then this imples your IDE or platform starts the code in the main() method on the EDT, which is not normal.

You can verify my above statement by adding:

System.out.println( SwingUtilities.isEventDispatchThread() );

to your loop to see if it is executing on the EDT or not.

Upvotes: 1

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