为什么我的图标不重新粉刷？

Question

Here is my main class:

package fast;

import java.awt.BorderLayout;
import java.awt.Color;
import java.io.IOException;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Fast {

    public JFrame frame = new JFrame("Fast");
    public JPanel panel = new JPanel();
    public Screen screen = new Screen();

    public Fast() throws IOException {
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(1000, 500);
        frame.setVisible(true);
        frame.setLocationRelativeTo(null);
        frame.setLayout(new BorderLayout());
        frame.setBackground(Color.YELLOW);
        frame.add(screen);
        screen.setBackground(Color.WHITE);
    }
    public static void main(String[] args) throws IOException {
        Fast f = new Fast();
        Thread thread = new Thread(new Screen());
        thread.start();
    }
}

Here is my screen class:

package fast;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JPanel;

public class Screen extends JPanel implements Runnable {
    BufferedImage img = ImageIO.read(new File("bg.png"));
    ImageIcon car = new ImageIcon("sCar.png");
    public int x = 50;

    public Screen() throws IOException {

    }
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.GREEN);
        car.paintIcon(this, g, x, 50); // I want this to move
        drawBackground(g);
    }
    private void drawBackground(Graphics g) { // testing
        g.setColor(Color.YELLOW);
        g.fillRect(x, 100, 50, 50);
    }
    @Override
    public void run() {
        System.out.println("Hello");
        x = 300;
        repaint();
    }
}

When my program reaches "Hello", I want it to repaint the car at x = 300, but it does not. What should I do to make this work? I have it in a run method because I plan on having it run as a thread later, but for now, I just want it to move.

Answer 1

The instance of Screen which is displayed on the screen and the instance of Screen you're trying to update are not the same

public class Fast {

    // screen #1    
    public Screen screen = new Screen();

    public Fast() throws IOException {
        //...
        // Screen #1 on the screen
        frame.add(screen);
        //...
    }
    public static void main(String[] args) throws IOException {
        //...
        // Screen #2, which is not on the screen
        Thread thread = new Thread(new Screen());
        thread.start();
    }
}

I'd also be careful about using Thread s to update the state of the UI as this could cause issues and generate unexpected results, instead, I'd encourage you to use a Swing Timer for something like this.

Have a look at Concurrency in Swing and How to use Swing Timers for more details