android中计算多边形的面积

List<PointXY> list :保存的 是点的坐标

public float caculate(List<PointXY>list){     float temp=0;     for (int i=0;i<list.size();i++){         if (i<list.size()-1){             PointXY p1 = list.get(i);             PointXY p2 = list.get(i+1);             temp += p1.getX() * p2.getY() - p2.getX() * p1.getY();         }else {             PointXY pn = list.get(i);             PointXY p0 = list.get(0);             temp += pn.getX() * p0.getY() - p0.getX() * pn.getY();         }     }        temp=temp/2;     List<Float> area = getScreenSizeOfDevice();           temp=(temp/area.get(0))*area.get(1)*2.54f;     Log.d(TAG, "画图的面积 : "+temp);     return temp; }    private List<Float> getScreenSizeOfDevice(){        List<Float> list02 = new ArrayList<Float>();        Point point = new Point();        getWindowManager().getDefaultDisplay().getRealSize(point);        DisplayMetrics dm = getResources().getDisplayMetrics();        float area = point.x * point.y;//得到像素面积，        Log.d(TAG, "屏幕的总像素面积: "+area);        list02.clear();        list02.add(area);        double x = Math.pow(point.x / dm.xdpi, 2);        double y = Math.pow(point.y/ dm.ydpi,2);        double screenInches = Math.sqrt(x+y);//得到尺寸大小        list02.add((float)(x * y));//得到手机屏幕的物理面积        Log.d(TAG, "屏幕尺寸大小: "+screenInches);        return list02;    }}

public class PointXY {
public float X;
public float Y;
public PointXY(){
this.X=0;
this.Y=0;
}

public PointXY(float x, float y) {
X = x;
Y = y;
}

public float getX() {
return X;
}

public void setX(float x) {
X = x;
}

public float getY() {
return Y;
}

public void setY(float y) {
Y = y;
}
}