判断两条线段的相对位置，用int[] s1表示第一条线段，int[] s2表示第二条线段，即两线段两端点的坐标为：

(s1[0]，s1[1]),(s1[2],s1[3])和(s2[0]，s2[1]),(s2[2],s2[3])。如果两线段没有交点则返回"NO"，只有一个交点则返回"POINT"，多个交点则返回"SEGMENT"。

public class TestLine {
	/**
	 * @return 上1下-1左-1右1中0
	 */
	public static int relative(int x1, int y1, int x2, int y2, int px, int py) {
		x2 -= x1;   //把x1,y1看成原点，转换成相对原点的坐标
		y2 -= y1;
		px -= x1;
		py -= y1;
		int flag = px * y2 - py * x2;  //判断P点在线段所在直线的上方还是下方还是线段上
		if (flag == 0) {
			flag = px * x2 + py * y2; //向量相乘，x1*x2+y1*y2=|a|*|b|*cosα，正负可推出相对原点(x1,y1)的位置
			if (flag > 0) {
				px -= x2;
				py -= y2;  //转换px,py相对x2,y2的坐标，即把x2,y2看成原点
				flag = px * x2 + py * y2;  //向量相乘
				if (flag < 0) {   //点位于线段上
					flag = 0;
				}
			}
		}
		return (flag < 0) ? -1 : ((flag > 0) ? 1 : 0);
	}

	public static String testState(int[] s1, int[] s2) {
		int[] state = new int[4];
		state[0] = relative(s2[0], s2[1], s2[2], s2[3], s1[0], s1[1]);
		state[1] = relative(s2[0], s2[1], s2[2], s2[3], s1[2], s1[3]);
		state[2] = relative(s1[0], s1[1], s1[2], s1[3], s2[0], s2[1]);
		state[3] = relative(s1[0], s1[1], s1[2], s1[3], s2[2], s2[3]);

		int zeroCount = 0;
		for (int i = 0; i < state.length; i++) {
			if (state[i] == 0)
				zeroCount++;
		}
		if ((state[0] * state[1] > 0) || (state[2] * state[3] > 0)) {
			return "NO";
		} else if (zeroCount > 1) {
			if (zeroCount > 2)
				return "SEGMENT";
			int index1 = 0;
			int index2 = 0;
			if (state[0] != 0)
				index1 = 2;
			if (state[2] != 0)
				index2 = 2;
			if (s1[index1] == s2[index2] && s1[++index1] == s2[++index2]) {
				return "POINT";
			}
			return "SEGMENT";
		} else
			return "POINT";
	}

	public static void main(String[] args) {
		int[] s1 = new int[] { 0, 0, 0, 1 };
		int[] s2 = new int[] { 0, 1, 0, 2 };
		System.out.println(testState(s1, s2));
	}

}

 输出结果：POINT