Java LinkedList實現（來自AbstractList）在Collections.sort上給出nullPointerException

Question

我是Java和OOP的新手，前面學過的語言是C. 我想創建一個鏈接列表來擴展AbstractList和允許使用Collections.sort（）函數。問題是當我調用Collections.sort（）函數時，我得到一個nullPointerException。我的猜測是，這個異常來自於我列表中最後一個節點是空的（所以我可以知道列表結束的位置）。

class Node 
{ 
    Object o; 
    Node next; 
    public Node(Object n) 
    { 
     o = n; 
     next = null; 
    } 
} 

class LinkList extends AbstractList 
{ 
    Comparator c; 
    public Node head, last; 
    public LinkList(Comparator c) 
    { 
     this.c = c; 
     head = null; 
     last = null; 
    } 

    @Override 
    public boolean add(Object a) 
    { 
     Node t = new Node(a); 
     if(last == null) 
     { 
      head = t; 
      last = t; 
      last.next = null; 
     } 
     else //thanks, hyde 
     if(last != null) 
     { 
      last.next = t; 
      last = t; 
      last.next = null; 

     } 
     return true; 
    } 
    @Override 
    public Object get(int a) 
    { 
     Node it = head; 
     int contor = 0; 
     while(it!=null && contor<a) 
     { 
      it = it.next; 
     } 

     if(it!=null) 
     { 
      return it; 
     } 
     else 
      return null; 
    } 

@Override 
public Object set(int i, Object a) 
{ 
    Node it = head; 
    int contor = 0; 
    Node aux; 
    while(it!=null && contor<i) 
    { 
     it = it.next; 

    } 
    if(it!=null) 
    { 
     aux = it; 
     it.o = a; 
     // Collections.sort(this,c); 
     return aux; 
    } 
    else 
     return null; 
} 

@Override 
public int size() 
{ 
    Node it = head; 
    int contor = 0; 
    while(it!=null) 
    { 
     contor++; 
     it = it.next; 
    } 
    return contor; 
} 

@Override 
public int indexOf(Object a) 
{ 
    Node it = head; 
    int contor = 0; 
    while(it!=null && it.o.equals(a)==false) 
    { 
     it = it.next; 
     contor++; 
    } 
    if(it!=null) 
    { 
     return contor; 
    } 
    else 
     return -1; 
} 

} 

public class Test 
{ 
    public static void main(String args[]) 
    { 
     LinkList lista = new LinkList(new Comparator(){ 
     @Override 
     public int compare(Object o1, Object o2) 
     { 
      int s1 = (int) o1; 
      int s2 = (int) o2; 
      return s2-s1; 
     } 
     }); 
     lista.add(2); 
     lista.add(3); 
     Collections.sort(lista); //this is line 156 
     System.out.println(lista.size()); 
    } 
} 

基本上，我添加了兩個元素，我嘗試對列表進行排序，並得到nullPointerException。這感覺非常令人沮喪，因爲我無法控制排序功能。

Exception in thread "main" java.lang.NullPointerException 
at java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:290) 
at java.util.ComparableTimSort.sort(ComparableTimSort.java:157) 
at java.util.ComparableTimSort.sort(ComparableTimSort.java:146) 
at java.util.Arrays.sort(Arrays.java:472) 
at java.util.Collections.sort(Collections.java:155) 
at Ex6.main(Ex6.java:156) 
Java Result: 1 

Answer 1

您在循環失蹤contor++的get方法內部，它總是返回null。