無法擺脫NullPointerException異常爲比較

Question

我一直在努力擺脫調用的代碼行時出現NullPointerException異常的：

if (priorityComparator.compare(temp.next.value, newNode.value) >= 0) 

完整的代碼是：

public class HeaderLinkedPriorityQueue<E> extends 
     AbstractPriorityQueue<E> implements PriorityQueue<E> { 

    //Some other methods, constructors etc. 

public boolean add (E e) { 

    ListNode<E> temp = highest; 


    ListNode<E> newNode = new ListNode<E>(e, null); 

    if (temp.next == null){ 
     //first node in a list. 
     temp.next = newNode; 
     objectCount++; 
     return true; 
    } 

    //if the value of the first element following the header node is greater than the newNode add to back. 
    if (priorityComparator.compare(temp.next.value, newNode.value) >= 0) { 
     temp.next.next = newNode; 
     objectCount++; 
    } 
    else { 
     //add before the first node in the list. have temp.next point to newNode and have newNode point to the old temp.next. 
     newNode.next = temp.next; 
     temp.next = newNode; 
     objectCount++; 
    } 
    return true; 
} 

//class variables. 
private ListNode<E>   highest  = new ListNode(null, null); 
private int     objectCount = 0; 
private Comparator<? super E> priorityComparator; 

我沒有看到任何錯誤的參數，所以我真的難倒了。我怎樣才能解決這個問題？

Answer 1

看起來你沒有初始化你的PriorityComparator。

private Comparator<? super E> priorityComparator; 

應該像

private Comparator<? super E> priorityComparator = new PriorityComparator();