java做链表队列增删节点的时候报错，只列出了链表部分，和主程序部分，刚刚入门java，很多地方不懂，求教

Question

import java.util.NoSuchElementException;

public class ListQueue implements MyQueue {

private class Node {

    T item; 
    Node next; 


    public Node(T item) {
        this.item=item;

    }
}

private Node head = null;
private Node tail = null; // root node
private int size = 0;


public boolean isEmpty() {


    return size == 0;
}


public int size() {


    return size;
}


public void enqueue(T item) {

    Node temptail = new Node(item);
    if (isEmpty()) {
        tail = temptail;
    } else {
        tail.next = temptail;

    }


    size++;

}

/**
 * dequeue: remove and return the head of the queue
 * 
 * @return the deleted value
 * @throws NoSuchElementException if queue is empty
 */
public T dequeue() {

    if (isEmpty()) {
        throw new NoSuchElementException("the list is empty, cannot be taken data");
    }
    else if (size()==1) { // when list just only has one elements
        head = null;
    } else {
        head = head.next;
    }
    size--;


    return null;
}

/**
 * peek: view the head of queue without removing it.
 * 
 * @return Null if queue is empty
 */
public T peek() {
    // if array is empty, cannot view its head
    if (isEmpty()) {
        return null;
    }
    return head.item;
}

}

public class Driver{

public static void main(String[] args) {

    MyQueue<Integer> q;

    boolean useList = true;  // make it false if you want to array implementation

    if (useList)
        q = new ListQueue<Integer>();
    else
        q = new ArrayQueue<Integer>();

    for(int i = 0; i < 1000; i++)  // add a large number of items onto the queue
    {
        q.enqueue(i);  
    }

    System.out.println("now, dequeue items!");
    while(!q.isEmpty())  
    {
        System.out.print(q.dequeue() + " ");  
    }
    System.out.println("end of dequeueing");    



}

}

//运行程序以后错误是now, dequeue items!
Exception in thread "main" java.lang.NullPointerException
at yao.as1.ListQueue.dequeue(ListQueue.java:73)
at yao.as1.Driver.main(Driver.java:24)

还请给点思路

Answer 1

public T dequeue() {

    if (isEmpty()) {
        throw new NoSuchElementException("the list is empty, cannot be taken data");
    }
    else if (size()==1) { // when list just only has one elements
        T temp = head; //加上
        head = null;
                return temp; //加上
    } else {
        T temp = head; //加上
        head = head.next;
                return temp; //加上
    }
    size--;


    return null;
}

这个方法，无论队列是否为空都返回了null，看我的更改

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