初始化鏈表

Question

當我運行我的驅動程序類時，我的列表不會初始化。這是啓動清單所必需的。

public class SortedListReferenceBased implements ListInterface { 
    private Node head; 
    private int numItems; // number of items in list 

    public SortedListReferenceBased() 
    // creates an empty list 
    { 
     head = new Node(null); 
     numItems = 0; 

    } // end default constructor 

    public boolean isEmpty() 
    // Determines whether a list is empty 
    { 
     return true; 
    } // end isEmpty 

    public int size() 
    // Returns the number of items that are in a list 
    { 
     return numItems; 
    } // end size 

    public void removeAll() 
    // Removes all the items in the list 
    { 
     head = null; 
     numItems = 0; 

    } // end removeAll 

    public Object get(int index) throws ListIndexOutOfBoundsException 
    // Retrieves the item at position index of a sorted list, if 0 <= index < 
    // size(). 
    // The list is left unchanged by this operation. 
    // Throws an exception when index is out of range. 
    { 
     if (index < 0 || index >= numItems) { 
      throw new ListIndexOutOfBoundsException(index + " is an invalid index"); 
     } 
     return new Object(); 
    } 

    public void add(Object item) throws ListException 
    // Inserts item into its proper position in a sorted list 
    // Throws an exception if the item connot be placed on the list 
    { 
     try { 
      Node newNode = new Node(item); 
      if (head != null) { 
       int i = locateIndexToAdd(item); 
       if (i >= -1 && i <= numItems) { 
        add(i, item); 
       } 
      } else { 
       head = new Node(newNode); 
       Node curr = head.getNext(); 
       curr.setNext(null); 
       numItems++; 
      } 
     } catch (Exception e) { 
      throw new ListException("Add to List failed: " + e.toString()); 
     } 
    } 
} 

Answer 1

這個問題似乎是你add方法的這一部分：

head = new Node(newNode); 
Node curr = head.getNext(); 
curr.setNext(null); 
numItems++; 

我承擔Node的next變量被null所以head.getNext()回報null爲curr初始化。當你打電話給curr.setNext(null)時，你會得到一個NullPointerException。由於這添加了列表中的第一個條目，因此要將第一個元素的next設置爲null。 此外，您不必將newNode換成Node，因爲它已經是一個節點。

head = newNode; 
head.setNext(null); 
numItems++; 

但既然你null初始化反正你沒有做任何事情。

head = newNode; 
numItems++;