调用 repaint() 后，paintComponent() 绘制的东西消失了

Question

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

Answer 1

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.

Answer 2

Hey I see you've read the Head First Java book too, good book isn't it?

I've had the same problem when I was learning about chapter 12 in the same book.

After learning about threads in chapter 15, I figured why the ovals don't "smear" across the screen.

So first of all, about the repaint() method.

Calling the repaint() method doesn't instantly call paintComponent(Graphics g) in the JPanel. Instead, it schedules the call to the Event Dispatch Thread for later use.

I'm not sure what output you are getting since you didn't tell us about it.

I'm assuming that there is no animation at all. There's probably just one oval in the end screen when you run it.

If that's the case, this is how the program is running. You run the loop 150 times so you called repaint() 150 times. However, like I said before, repaint() only calls a schedule in the Event Dispatching Thread, instead of running paintComponent(Graphics g) instantly.

That means the for loop is finished before the paintComponent(Graphics g) actually got called. That's most likely why you are not getting "The Mistake" in the book.

How do you get "The Mistake"?

Try making the thread sleep for a longer time.