空指針異常與隊列

Question

引起我相信錯誤是由不正確添加到隊列引起的，可能有其他錯誤，但我認爲這是這裏的東西

public void add(E data) 
    { 
     if(size == 0) 
     { 
         size++; 
      front = new ListNode(data,null); 
     } 
     else 
     { 
        size++; 
      ListNode <E> temp = end; 
      temp.setNext(null); 
      temp = temp.getNext(); 
      temp.setData(data); 
      end = temp; 
     } 
    } 

，如果你需要的代碼的其他部分尋找這裏的錯誤是滿級

import java.util.*; 

public class Queue<E> 
{ 
     private ListNode <E> front; 
     private ListNode <E> end; 
     private int size; 

    public Queue() 
    { 
     front = null; 
     end = null; 
     size = 0; 
    } 

    public E peek() 
    { 
     return front.getData(); 
    } 

    public E remove() 
    { 
     if(size == 0){return null;} 
     else 
     { 
     ListNode <E> temp = front; 
     front = temp.getNext(); 
     size--; 
     return temp.getData(); 
     } 

    } 

    public void add(E data) 
    { 
     if(size == 0) 
     { 
        size++; 
      front = new ListNode(data,null); 
     } 
     else 
     { 
        size++; 
      ListNode <E> temp = end; 
      temp.setNext(null); 
      temp = temp.getNext(); 
      temp.setData(data); 
      end = temp; 
     } 
    } 

    public boolean isEmpty() 
    { 
     if (size == 0) 
      return true; 
     else 
      return false; 

    } 

    // [data, data, data, data] 
    public String toString() 
    { 
     String s =""; 
     ListNode <E> temp = front; 
     while(temp.getNext()!= null) 
     { 
      s+=temp.getData() + ", "; 
     } 

     return s; 
    } 

    public int size() 
    { 
     return size; 
    } 
} 

，這裏是我使用

public class ListNode<E> 
{ 
    private E data; 
    private ListNode<E> next; 

    /** 
    * Constructs a ListNode with a specified data and next 
    * @param d the data for the node 
    * @param n the next reference 
    */ 
    public ListNode(E d, ListNode<E> n) 
    { 
     data = d; 
     n = next; 
    } 

    /** 
    * returns the data from the node 
    * @return the data field 
    */ 
    public E getData() {return data;} 

    /** 
    * sets the data for the node 
    * @param d the new data field 
    */ 
    public void setData(E d) {data = d;} 

    /** 
    * gets the next reference of the node 
    * @return the next reference 
    */ 
    public ListNode<E> getNext() { return next; } 

    /** 
    * sets the next reference for the node 
    * @param n the new next reference 
    */ 
    public void setNext(ListNode<E> n) { next = n;} 
} 

一個節點第二，這是我在用測試它

public class QueueTester 
{ 
    public static void main (String args[]) 
    { 
     Queue<Integer> queue = new Queue<Integer>(); 

     for (int k = 1; k <= 100; k++) // add 1 to 100 
      queue.add(k); 

     System.out.println ("Size: " + queue.size()); 
     System.out.println(queue + "\n"); 

     for (int k = 1; k <= 50; k++) // remove 1 to 50, contents 51 to 100 
     { 
      int number = queue.remove(); 
     } 

     System.out.println ("Size: " + queue.size()); 
     System.out.println(queue + "\n"); 

     for (int k = 200; k <= 500; k+=10) // add tens 200 to 500 (after 51 to 100) 
      queue.add(k); 

     System.out.println ("Size: " + queue.size()); 
     System.out.println(queue + "\n"); 

     while (!queue.isEmpty()) // prints contents (should be 51 to 100, then 200 to 500 tens) 
     { 
      System.out.print(queue.remove() + " "); 
     } 
     System.out.println("\n"); 

     System.out.println ("Size: " + queue.size()); 
     System.out.println(queue); // empty 
     System.out.println ("Remove from empty queue: " + queue.remove() + "\n") ; 

    }   
} 

Answer 1

試試這個..

if(size == 0) 
    { 
     front = new ListNode(data,null); 
     end = first; 
     size++; 
    } 
    else 
    { 
     ListNode <E> temp = new ListNode(data); 
     end.setNext(temp); 
     end = end.getNext(); 
     size++; 
    } 

Answer 2

你add方法不正確實現，並且將永遠只是改變front。

當你調用最終調用remove()

public E remove() 
{ 
    if(size == 0){return null;} 
    else 
    { 
    ListNode <E> temp = front; 
    front = temp.getNext(); 
    size--; 
    return temp.getData(); 
    } 

} 

因爲 size == 0

，您將返回null。然後嘗試取消引用null爲int

int number = queue.remove(); 

這將引發NullPointerException。

檢查你的邏輯，如果你添加一些東西到列表中，你應該增加大小。

另外，還要注意在你add()方法

ListNode <E> temp = end; 
temp.setNext(null); 

也將失敗，因爲end原本null。