Javaテクニカルサンプル集

 long a = 0;
 for(int i = 0;i < pgn.npoints;i++){
  a += (long)(pgn.xpoints[i-1<0?pgn.npoints-1:i-1]-
        pgn.xpoints[i+1>=pgn.npoints?0:i+1])*pgn.ypoints[i];
 }
 double area = -a/2.0;