Drawing a rectangle that won't disappear in next paint

I trying to create a JPanel that draws rectangles. The Panel needs to draw alot of rectangles, but they dont move. One solution to my problem was to create an list with all the rectangles i already created and draw they all in every call of "Paint". But there is a lot of rectangles and it slows the computer.
I also tried to use repaint(x, y, height, width) to rapaint just the space of the new rectangle but it did not work. (JPanel keeps erasing previous rectangles.) In sort, i need to draw rectangles that wont disappear every paint. Or a paint method that wont erase previous draws, or wont paint the background. That is part of my JPanel class:

class MyPanel extends JPanel{
  private int x, y, size;
  private Color c;
  public void DrawRect(int x, int y, int size, Color c){
      this.x = x;
      this.y = y;
      this.size = size;
      this.c = c;
      repaint();
  }
  @Override
    public void
    paint(Graphics g) {

        g.setColor(c);
      g.fillRect(x, y, size, size);
    }

}