没有调用paintComponent()

Question

Here is a little program that should (in theory) draw an image of a ball on screen. The problem is that paintComponent seems to not get called. The program consists of two classes.

import java.awt.*;
import java.io.*;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import javax.swing.*;

public class ScreenSaver extends JPanel {
    private static final long serialVersionUID = 001;

    public static void main(String[] args) {
        new ScreenSaver();
    }
    public ScreenSaver() {
        new Window(1600, 900, "ScreenSaver", this);
    }
    //----------------------------------------------------------------------------------
    private static BufferedImage ball;
    public static BufferedImage getBallSprite() {
            try {
                 File pathToBall = new File("ball.png");
                 ball = ImageIO.read(pathToBall);
            } catch (IOException ex) {
                    ex.printStackTrace();
            }
            return ball;
    }
}

import java.awt.*;
import java.io.*;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import javax.swing.*;

public class Window extends Canvas {
private static final long serialVersionUID = 002;

    public Window(int width, int height, String title, ScreenSaver ScreenSaver) {
        JFrame frame = new JFrame(title);
        frame.setPreferredSize(new Dimension(width, height));
        frame.setMaximumSize(new Dimension(width, height));
        frame.setMinimumSize(new Dimension(width, height));
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        repaint();
    }

    public void paintComponent(Graphics g) {
        System.out.println("Painting...");
        BufferedImage ball = ScreenSaver.getBallSprite();
        g.drawImage(ball, 0, 0, 100, 100, this);
  }
}

As you can see, I tested if paintComponent was called using a console message. Sadly this was not the case. Can someone explain?

Answer 1

java.awt.Canvas does not inherit from JComponent so paintComponent won't be called automatically

public class MyWindow extends JComponent {

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
      ...
}

Answer 2

It's no wonder that paintComponent is not called, because Canvas has no implementation of paintComponent which you can override. With a canvas you have to overwrite paint for your purposes. In your code you use both a JPanel and a Canvas , which is not necessary at all. Use either of the two.

The following is an example with a Canvas :

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

public class ScreenSaver extends Canvas{
    public static void main(String[] args) {
        JFrame window = new JFrame("Screensaver");
        window.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        window.setResizable(false);

        ScreenSaver canvas = new ScreenSaver();
        canvas.setPreferredSize(new Dimension(1600, 900));
        canvas.setBackground(Color.BLACK);

        window.add(canvas);
        window.pack();
        window.setLocationRelativeTo(null);
        window.setVisible(true);
    }

    @Override
    public void paint(Graphics g){
        g.setColor(Color.GREEN);
        g.fillOval(100, 100, 100, 100);
    }
}

The annotation Override above the method to be overwritten ensures that the compiler can issue a warning message if the overwritten method does not exist or there is a typo. I hope this helps you further.