我有一个点类型的链接列表，我想计算一个特定点的频率
Code

     LinkedList<Point> refernce = new LinkedList<>();
      Point neworigin = new Point();
public void distancecalculator(char [][]problem ,LinkedList<Point> refernce) {

  //Somewhere in my code
         for(int i = 0; i < 4; i++)
         {

              int a = reference.x + x[i];   // x={ 0 , 0 ,1 , -1}

              int b = reference.y + y[i];    // y ={ 1, -1 , 0 ,0}

              neworigin.x = a;
             neworigin.y = b;
            reference.add(neworigin)
        if(Collections.frequency(refernce, neworigin) < 6)
        {
                   //End the that thread
        }
   else
      {
           solver s = new solver(newproblerm , refernce );

                          som =  new Thread(s);
                          som.start();
       }

}}

错误：

  at java.lang.Thread.run(Unknown Source)
Exception in thread "Thread-742" java.lang.NullPointerException
    at java.util.LinkedList$ListItr.next(Unknown Source)
        at java.util.Collections.frequency(Unknown Source)

已更新。请帮助我。