计算多边形面积（JAVA）

import java.util.*;
class point{
	public int x;
	public int y;
	point(){
		this.x=0;
		this.y=0;
	}
}
public class caculateArea {
	    public static float caculate(point vertex[],int pointNum)
	    {
	    	int i=0;
	    	float temp=0;
	    	for(;i<pointNum-1;i++)
	    	{
	    		temp+=(vertex[i].x-vertex[i+1].x)*(vertex[i].y+vertex[i+1].y);
	    	}
	    	temp+=(vertex[i].x-vertex[0].x)*(vertex[i].y+vertex[0].y);
	    	return temp/2;
	    }
        public static void main(String args[]) 
        {
        	point[] vertex=new point[100];
        	int pointNum=0;
        	Scanner in = new Scanner(System.in);
        	for(int i=0;i<100;i++)
        	{
        		vertex[i]= new point();
        	}
        	System.out.println("请输入多边形顶点数,个数不能超过100个(逆时针）：");
        	pointNum=in.nextInt();
        	for(int i=0;i<pointNum;i++)
        	{
        		System.out.println("第"+i+"个顶点的横坐标:");
        		vertex[i].x = in.nextInt();
        		System.out.println("第"+i+"个顶点的纵坐标:");
        		vertex[i].y = in.nextInt();
        	}
        	System.out.println("多边形面积是："+caculate(vertex,pointNum));
        }
}