Question

I have a simple class that paints a graphic in a JPanel. This is my class:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import javax.swing.JPanel;

class Drawing_panel extends JPanel {
    public void paintComponent(Graphics g) {
    super.paintComponent(g);       
    this.setBackground(Color.white);
    g.setColor(Color.red);
    g.fillRect(150, 80, 20, 20);
}  

public Dimension getPreferredSize(){
    return new Dimension(500,500);
}

}

I have another class that instantiates this one:

Drawing_panel dp = new Drawing_panel();

There is no constructor in the Drawing_panel class and/or no explicit call to either the paintComponent() or getPreferredSize() methods. I assume the method is being called in the parent JPanel constructor, but I did not see the calls there either.

Answer 1

The paintComponent is called from a few different places. The call from JComponent.paint is probably the one you’re looking for.

Note that paintComponent is not called from any constructor. The paintComponent is called “on-demand” i.e. when the system decides that the component needs to be redrawn. (Could for instance be when the component is resized, or when the window is restored from a minimized state.) To be clear: The component is not “painted, then used”, it is “used, then painted when needed”.

This whole chain of painting-calls is nothing you should bother about, as it is taken care of entirely by the Swing and the so called Event Dispatch Thread.