class ball
    {
        public int ballX;
        public int ballY;

        private int trajX;
        private int trajY;

        public void checkWalls()
        {
            if (ballX < 1)
            {
                trajX = trajX + 2;
            }
            if (ballX > 790)
            {
                trajX = trajX - 2;
            }
            ballX = ballX + trajX;

            if (ballY < 590)
            {
                trajY = trajY + 2;
            }
            if (ballY > 10)
            {
                trajY = trajY - 2;
            }
            ballY = ballY + trajY;
        }