Can you help me understand this basic GUI concept?

[EscSequenceAlpha]

Ok so this is the basic skelington for a GUI

package chapter_iv;

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

public class GUIGraphics extends JPanel
{
   public void paintComponent(Graphics g)
   {
      super.paintComponent( g );
      
      int width = getWidth();   
      int height = getHeight();
      
      g.drawLine(0, 0, width, height);
   }
}

and

package chapter_iv;

import javax.swing.JFrame;

public class GUIGraphicsTest
{
public static void main( String[] args )
{
   GUIGraphics panel = new GUIGraphics();
   
   JFrame application = new JFrame();
   
   application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

   application.add( panel );     
   application.setSize(250, 250 );
   application.setVisible( true ); 

}
}

I was hoping you guys could make this clear to me.

From what I understand we have a JFrame which is a window right? so this is the main structure... you then want to give the JFrame a Jpanel object.
Does that mean Jpanel is like a whiteboard you stick onto the window so that you can draw on it?

I know that I have created a GUI which has inherited from Jpanal, so I override its paintComponent method which renders the window.

what confuses me is how am I actually using it? all I do in my main method is instantiate my new Jpanel object and add it to the window, I dont even give GUI any arguments... how does it know to run paintcomponent()? I dont tell application to do it either

[JosAH]

The Swing framework runs its own Thread, the EDT (Event Dispatch Thread); it calls the paint( ... ) method for each component that needs to be repainted. Swing components split up the responsibility for painting their borders, children and themselves; the latter is done by the paintComponent( ... ) method; you have overridden it in your class so whenever your panel needs to be painted its paintComponent( ... ) method is called. You did it correctly.

kind regards,

Jos