使用线性链表的队列中的 NullPointerException [重复]

Question

我在我的数据结构课程中有这个实验室植入/编码来使用线性链接列表做一个队列，但它在我的显示方法中抛出了一个NullPointerException，我不知道我的故障点在哪里

Queue链表类代码：

package lab10_moudhi;

/**
 *
 * @author Moudhi
 */
public class QueueLinkedList<E> {
    private static class Node<E>{
        private E data;
        private Node<E> next;

        public Node(E data, Node<E> next) {
            this.data = data;
            this.next = next;
        }

        public E getData() {
            return data;
        }

        public void setData(E data) {
            this.data = data;
        }

        public Node<E> getNext() {
            return next;
        }

        public void setNext(Node<E> next) {
            this.next = next;
        }

        @Override
        public String toString() {
            return  data + "";
        }

    }
    private Node<E> front=null,rear=null;
    private int size=0;

    public QueueLinkedList() {
    }
      public boolean isEmpty(){
     return size==0;
    }
       public int size(){
           return size;
       }
        public Node<E> getFront(){
            if(isEmpty()) return null;
              return front;}
         public Node<E> getRear(){
            if(isEmpty()) return null;
              return rear;}
         public void enQueue(E value){
              System.out.println(value+" - Added to Queue");
        Node newNode=new Node(value,null);
         if(isEmpty()){
             front=rear=newNode;
         }
         else{
             rear.next=newNode;
             rear=newNode;
         }
         size++;

         }
         public Node<E> deQueue(){
             if(isEmpty()){
             System.out.println("Queue underflow");
             return null;
         }
           E temp=front.data;
           front=front.next;
           rear.next=front;
           size--;
             System.out.println(temp+" -Deleted from Queue");
           if(size==0) front=rear=null;
           return (Node<E>) temp;



    }
         public void display(){
             Node current=front;
              System.out.println("----------------------Display Queue----------------------");
              if(isEmpty()){
             System.out.println("Empty Queue!!");
             return ;
         }
             do{
                 System.out.println(current.getData());//it throws the null pointer error here
                 current=current.next;
             }while(current!=front);
              System.out.println("--------------------------------------------------------");


         }

}

女仆方法::

package lab10_moudhi;

/**
 *
 * @author Moudhi
 */
public class Lab10_moudhi {
    public static void main(String[] args) {
         System.out.println("Testing a Queue using Linear Linked List:");
         System.out.println("--------------------------------------------");
         QueueLinkedList q1= new QueueLinkedList();
         q1.enQueue(10);
          q1.enQueue(20);
           q1.enQueue(30);
            q1.enQueue(40);
            q1.display();// when i call this method it throws a null pointer exception
            q1.deQueue();
            q1.deQueue(); 
            System.out.println("Front Element:"+q1.getFront());
            System.out.println("Rear Element:"+q1.getRear());
            q1.enQueue(50);
            q1.display();
    }

}

它抛出以下错误::

run:
Testing a Queue using Linear Linked List:
--------------------------------------------
10 - Added to Queue
20 - Added to Queue
30 - Added to Queue
40 - Added to Queue
Exception in thread "main" java.lang.NullPointerException
----------------------Display Queue----------------------
10
20
30
40
    at lab10_moudhi.QueueLinkedList.display(QueueLinkedList.java:93)
    at lab10_moudhi.Lab10_moudhi.main(Lab10_moudhi.java:16)
C:\Users\Moudhi\AppData\Local\NetBeans\Cache\8.2\executor-snippets\run.xml:53: Java returned: 1
BUILD FAILED (total time: 0 seconds)

Answer 1

更改您的 do-while 循环条件，以便当 current 为 null 时结束循环。

public void display() {
        Node current = front;
        System.out.println("----------------------Display Queue----------------------");
        if (isEmpty()) {
            System.out.println("Empty Queue!!");
            return;
        }
        do {

                System.out.println(current.getData());// it throws the null pointer error here
                current = current.next;

        } while (current != null); //when current becomes null end the loop
        System.out.println("--------------------------------------------------------");

    }

另外，Node deQueue() 方法作为 Follow，添加了 Inline 注释。

    public Node<E> deQueue() {
        if (isEmpty()) {
            System.out.println("Queue underflow");
            return null;
        }
        Node<E> temp = front; //Change from E to Node<E> becuase you return Node<e>, as you cannot cast from Integer to Node<E>
        front = front.next; 
        // rear.next = front; // Also comment this line, this make rear to point to front which you have not done in your enqueue steps or either make this at both places
        size--;
        System.out.println(temp + " -Deleted from Queue");
        if (size == 0)
            front = rear = null;
        return (Node<E>) temp; 

    }

上述更改后的输出：-

Testing a Queue using Linear Linked List:
--------------------------------------------
10 - Added to Queue
20 - Added to Queue
30 - Added to Queue
40 - Added to Queue
----------------------Display Queue----------------------
10
20
30
40
--------------------------------------------------------
10 -Deleted from Queue
20 -Deleted from Queue
Front Element:30
Rear Element:40
50 - Added to Queue
----------------------Display Queue----------------------
30
40
50
--------------------------------------------------------