codice:
public class Test extends JFrame {
  public Test() {
    this.add(new MyPanel());
    setSize(100, 100);
    setVisible(true);
  }
  
  class MyPanel extends JPanel {
    public void paint(Graphics g) {
      g.drawRect(1, 1, 10, 10);
    }
  }
  public static void main(String[] args) {
    new Test();
  }
}