我已经使用双向链表在java中实现了Deque，当我调用iterator.next时，我得到一个空指针异常

Question

当我运行我在这里提供的main((时，我总是收到此错误。

java.lang.NullPointerException
    at Deque$Node.access$100(Deque.java:98)
    at Deque$DequeIterator.hasNext(Deque.java:83)
    at Deque.main(Deque.java:116)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:622)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

为什么迭代器即使对于 hasNext(( 也不返回任何内容？我可以在drjava中使用交互面板，并且可以获取迭代器的指针，但是如果我尝试使用与之关联的任何函数，我会得到NullPointerExceptions

。

import java.util.Iterator;

public class Deque<Item> implements Iterable<Item> {
 private int size;// number of elements
 private Node before;//sentinel marking the beginning of a list
 private Node after;//sentinel marking the end of a list

    public Deque()                           // construct an empty deque
    {
     before= new Node();
     after = new Node();
     before.next=after;
     after.previous=before;
    }
    public boolean isEmpty()                 // is the deque empty?
    {
     return size>0;
    }
    public int size()                        // return the number of items on the deque
    {
     return size;
    }
    public void addFirst(Item item)          // insert the item at the front
    {
     size++;
     Node node = new Node();
     node.item=item;//sets new node equal to the item
     node.previous=before;//sets new node's previous to before which is the beginning sentinel
     node.next=before.next;//sets new node's next to the old first node which was befores next.
     before.next.previous=node;//sets old first item's previous to the new Node making the old first second in the list.
     before.next=node;//sets before's next to the new Node making the new Node first in the list.
    }
    public void addLast(Item item)           // insert the item at the end
    {
     size++;
     Node node = new Node();
     node.item=item;//sets node item to item
     node.next=after;//sets new node's next to after 
     after.previous.next=node;//sets old last node's next to the new node 
     node.previous=after.previous;//sets new node's previous to old last node
     after.previous=node;//now after's previous is the new node making it last in the list.

    }
    public Item removeFirst()                // delete and return the item at the front
    {
     size--;
     Node first= before.next;//gets the first node.
     Node second=first.next;//gets the second element in the list
     second.previous=before;//sets second's previous to before
     before.next=second;//sets before's next to second
     return first.item;

    }
    public Item removeLast()                 // delete and return the item at the end
    {
     size--;
     Node last = after.previous;//sets last node
     Node second2Last= after.previous.previous;//sets second2Last
     second2Last.next=after;//sets second2Last's next to after
     after.previous=second2Last;//sets after's previous to second2last
     return last.item;
    }
    public Iterator<Item> iterator()         // return an iterator over items in order from front to end
    {
     return new DequeIterator();
    }
    private class DequeIterator implements Iterator<Item>{
     Node current=after.next;
     public boolean hasNext(){
      return current.next!=after;
     }
     public void remove(){
     }
     public Item next(){
      Item item=current.item;
      current=current.next;
      return item;   
     }
    }//end class DequeIterator
    private class Node{
     private Item item;
     private Node next;
     private Node previous;
    }
 /**
  * @param args
  */
 public static void main(String[] args) {
  // TODO Auto-generated method stub
  Deque d = new Deque();
  d.addFirst(1);
  d.addLast(2);
  d.addFirst(3);
  Iterator i = d.iterator();
  System.out.println(i.hasNext());
  while(i.hasNext()){
   System.out.println(i.next());
  }

 }
}

Answer 1

你有这个类的定义：

private class DequeIterator implements Iterator<Item>{
    Node current=after.next;
    //rest of code
}

您正在定义current节点是after的下一个节点，它具有null值，而after是双链表中的最后一个元素，它将null，因此使用current执行任何操作都将导致NullPointerException。

解决方案：将current定义为before。