java - Dequeue 对象类的 NullPointerException

Question

我正在尝试创建一个链接列表出队类，它接受头部和尾部的节点。一切都编译得很好，但是当我运行它时，我得到一个 NullPointerException。它指向我的内部 QueueNode 类，但我不确定如何修复它。有什么想法吗？谢谢!

public class LinkedDequeue 
{
   private QueueNode front;      //index of current front element
   private QueueNode rear;       //index of current rear element
   private int count;            //current # of elements

   class QueueNode 
   {
      private Object data;
      private QueueNode link;
   }

   public LinkedDequeue ()
   {
      front = rear = null;
      count = 0;      
   } 

   public void headAdd (Object o)
   {
      if (isEmpty()) 
      {
         front.data = o;
         rear.data = o;
         front.link = rear;
         rear.link = null;
      }
      else
      {
         QueueNode temp = new QueueNode();
         temp.data = o;    
         front.data = temp;
         front.link = front;
      }

      count++;
   }

   public boolean isEmpty()
   {
      return (count == 0);   
   }

   public static void main (String [] args)
   {
      LinkedDequeue list = new LinkedDequeue ();

      list.headAdd ("test?");

      System.out.println (list.toString());
   }

}

Answer 1

我认为问题在于，当您创建列表时，您的 front 和 back == null ，然后尝试编辑 front.data (null 没有数据字段)。因此，您应该在代码中添加这些行:

  if (isEmpty()) 
  {
     front = new QueueNode();
     rear = new QueueNode();
     front.data = o;
     rear.data = o;
     front.link = rear;
     rear.link = null;
  }