Java实现判断点是否在多边形内部的算法

package cn.com.maxtech.assess.service;

import java.util.ArrayList;
import java.util.List;

/**
 * 
 * 
 * @author zhouhaigang
 * 
 */
public class SystemTask  {






 double INFINITY = 1e10;

 double ESP = 1e-5;

 int MAX_N = 1000;



List<Point> Polygon;

// 计算叉乘 |P0P1| × |P0P2|

double Multiply(Point p1, Point p2, Point p0){
    return ( (p1.x - p0.x) * (p2.y - p0.y) - (p2.x - p0.x) * (p1.y - p0.y) );
}


// 判断线段是否包含点point

private boolean IsOnline(Point point, LineSegment line)
{
    return( ( Math.abs(Multiply(line.pt1, line.pt2, point)) < ESP ) &&

    ( ( point.x - line.pt1.x ) * ( point.x - line.pt2.x ) <= 0 ) &&

    ( ( point.y - line.pt1.y ) * ( point.y - line.pt2.y ) <= 0 ) );
}

// 判断线段相交
private boolean Intersect(LineSegment L1, LineSegment L2)
{
    return( (Math.max(L1.pt1.x, L1.pt2.x) >= Math.min(L2.pt1.x, L2.pt2.x)) &&

    (Math.max(L2.pt1.x, L2.pt2.x) >= Math.min(L1.pt1.x, L1.pt2.x)) &&

    (Math.max(L1.pt1.y, L1.pt2.y) >= Math.min(L2.pt1.y, L2.pt2.y)) &&

    (Math.max(L2.pt1.y, L2.pt2.y) >= Math.min(L1.pt1.y, L1.pt2.y)) &&

    (Multiply(L2.pt1, L1.pt2, L1.pt1) * Multiply(L1.pt2, L2.pt2, L1.pt1) >= 0) &&

    (Multiply(L1.pt1, L2.pt2, L2.pt1) * Multiply(L2.pt2, L1.pt2, L2.pt1) >= 0)