public Color elementAt(Coords coords) {
      Integer col = coords.getCol();
      Integer row = Math.abs(coords.getRow());
      if (col < 0 || row > 4 || row + col > 8) {
         return Color.INVALID;
      }
      Ball ball = new Ball(Color.WHITE, coords);
      if (this.balls.contains(ball)) {
         return Color.WHITE;
      }
      ball.setColor(Color.BLACK);
      if (this.balls.contains(ball)) {
         return Color.BLACK;
      }
      return Color.NONE;
   }