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public class Point3D extends Point3f implements Comparable<Point3f>    {        /**         * Comment for <code>serialVersionUID</code>         */        private static final long serialVersionUID = -1615396748786955162L;        /**         * @param i         * @param j         * @param k         */        public Point3D(int i, int j, int k)        {            super(i,j,k);        }        /* (non-Javadoc)         * @see java.lang.Comparable#compareTo(T)         */        public int compareTo(Point3f point)        {            if (point == null)                throw new NullPointerException();            if (point.x > this.x)            {                return 1;            }            else if (point.x == this.x)            {                if (point.y > this.y)                {                    return 1;                }                else if (point.y == this.y)                {                    return 0;                }                return -1;            }            return -1;        }            }