How does this class repaint automatically after the mouse is pressed?

Question

I just don't see how the paint() or paintComponent() is being called here. This is from an old youtube video back in 2011; the comment section of that video didn't help. I was expecting this code to call repaint() in the mousePressed() method, but it didn't, and it just works.

This is the youtube video: https://www.youtube.com/watch?v=PrPwCKr6WNI

import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;

public class JavaApplication17 extends JFrame {

  int GWIDTH = 800;
  int GHEIGHT = 600;
  int x, y;
  private Image dbImage;
  private Graphics dbg;

  public JavaApplication17() {
    setSize(GWIDTH, GHEIGHT);
    setTitle("Game");
    setResizable(false);
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    addMouseListener(new Mouse());
    x = 15;
    y = 15;
  }

  public class Mouse extends MouseAdapter {

    @Override
    public void mousePressed(MouseEvent e) {
      int xCoord = e.getX();
      int yCoord = e.getY();
      x = xCoord + 7;
      y = yCoord + 7;
    }
  }

  public void paint(Graphics g) {
    dbImage = createImage(getWidth(), getHeight());
    dbg = dbImage.getGraphics();
    paintComponent(dbg);
    g.drawImage(dbImage, 0, 0, this);
  }

  public void paintComponent(Graphics g) {
    g.fillOval(x, y, 15, 15);
    repaint();
  }

  public static void main(String[] args) {
    JavaApplication17 main = new JavaApplication17();
  }

}

Answer 1

how the paint() or paintComponent() is being called here?

paintComponent() is called by your code in paint() method.

paint() method is called by Swing framework every time when frame update is needed. By invoking repaint() method you're informing the Swing framework that your frame needs to update its content.

In IntelliJ IDEA this code looks like:

Note the letter O in blue circle. It means that the method paint() overrides some method in a parent class. In Java, it's recommended to add @Override modifier to mark overridden methods.

Your class JavaApplication17 extends class JFrame which means it inherits all methods of JFrame and its ancestors. JFrame class in its turn extends Frame and inherits all methods of Frame and its ancestors. And so on. See the class hierarchy diagram:

Window and Container classes both implement their public void paint(Graphics g) methods. And in the abstract Component class paint() is called by update() method. You don't need to call the paint() method on your own.

If you want to learn more and understand how it works, read about OOP concepts in Java (abstraction, encapsulation, inheritance, and polymorphism).