How to use paintComponent

Question

I'm pretty new to graphics programing in java. I was wondering how you would implement paintComponent, so that my graphics will remain onscreen.
Here is my code.
import javax.swing.*;
import java.awt.*;
class main extends JFrame
     public JFrame mainFrame = new JFrame("Test");
     public Container mainPane = mainFrame.getContentPane();
     public GraphicsEnvironment ev = GraphicsEnvironment.getLocalGraphicsEnvironment();
     public Point DEFAULT_WINDOW_POSITION = new Point(ev.getCenterPoint());
     public Graphics g;
     public static void main(String[] args)
          main m = new main();
     public main()
          mainFrame.setSize(600,500);
          mainFrame.setResizable(false);
          mainFrame.setLocation((int)DEFAULT_WINDOW_POSITION.getX()-((int)mainFrame.getWidth()/2),(int)DEFAULT_WINDOW_POSITION.getY()-((int)mainFrame.getHeight()/2));
          mainPane.setBackground(Color.white);
          mainPane.setLayout(null);
          mainFrame.setDefaultCloseOperation(EXIT_ON_CLOSE);
          mainFrame.setVisible(true);
          g = mainPane.getGraphics();
          drawSomething(g);
     public void drawSomething(Graphics g)
          g.fillRect(100,100,100,100);  //shows for 1/10 of a second.  why?
}

Answer 1

You misunderstood me.
class MyPanel extends JPanel
   public void paintComponent(Graphics g)
class MyFrame extends JFrame
   public MyFrame()
      setContentPane(new MyPanel());
}Get it?