正確實施List Iterator方法

Question

爲了更好地瞭解迭代器，我將自己編寫它們以嘗試獲取它們的正確功能。我有一個問題從ListIterator以前的方法中獲得正確的行爲。

例如，JavaDoc中指出：

交替調用next和以前將重複返回相同的元素。

我的Iterator

class Node<Item> { 
    public Item data; 
    public Node<Item> next; 
    public Node<Item> previous; 

    public Node() { 
     data = null; 
     next = null; 
     previous = null; 
    } 

    public Node(Item i, Node<Item> n, Node<Item> p) { 
     data = i; 
     next = n; 
     previous = p; 
    } 
} 

public ListIterator<Item> listIterator() { 

    return new ListIterator<Item>() { 

     private Node<Item> n = first; 

     public boolean hasNext() { 
      return n.next != last; 
     } 

     public Item next() { 
      n = n.next; 
      return n.data; 
     } 

     //TODO 
     public void remove() { 
     } 

     public boolean hasPrevious() { 
      return n.previous != first; 
     } 

     public Item previous() { 
      n = n.previous; 
      return n.data; 
     } 
    }; 
} 

現在，當我測試一下，我在與previous()方法不正確的行爲。

TEST

LinkedList<String> lst2 = new LinkedList<String>(); 

    for (int i = 0; i < 4; i++) 
     lst2.add("" + "data".substring(i, i + 1)); 

    ListIterator<String> it2 = lst2.listIterator(); 
    System.out.println("\nTest the list iterator.\nThe test list is " + lst2 + "\n"); 

    while (it2.hasNext()) { 
     System.out.println("next is " + it2.next()); 
     System.out.println("previous is " + it2.previous()); 
     if (removeImplemented) { 
      it2.remove(); 
      System.out.println("After remove: " + lst2); 
     } 
     System.out.println("next is " + it2.next()); 
    } 

    System.out.println("\nHere is how the built-in Java ArrayList class works\n"); 
    ArrayList<String> lst3 = new ArrayList<String>(); 

    for (int i = 0; i < 4; i++) 
     lst3.add("" + "data".substring(i, i + 1)); 

    ListIterator<String> it3 = lst3.listIterator(); 
    System.out.println("Test list iterator.\nThe test list is " + lst3 + "\n"); 

    boolean remove = false; 

    while (it3.hasNext()) { 
     System.out.println("next is " + it3.next()); 
     System.out.println("previous is " + it3.previous()); 
     if (remove) { 
      it3.remove(); 
      System.out.println("After remove: " + lst3); 
     } 
     System.out.println("next is " + it3.next()); 
    } 

我的結果

The test list is [d, a, t, a] 

next is d 
previous is null //incorrect 
next is d 
next is a 
previous is d //incorrect 
next is a 
next is t 
previous is a //incorrect 
next is t 
next is a 
previous is t //incorrect 
next is a 

正確的結果

The test list is [d, a, t, a] 

next is d 
previous is d 
next is d 
next is a 
previous is a 
next is a 
next is t 
previous is t 
next is t 
next is a 
previous is a 
next is a 

現在，我understandi ng，第二組結果是ListIterator的正確行爲。那麼，我能做些什麼來實現這種行爲？從我讀過的內容來看，它與光標被移動到元素之前有關，而不是元素本身。我很難考慮實現這一點的方式。

Answer 1

您已正確實施next()的行爲，前進到下一個節點並返回新值。

但是，在更改爲上一節點之前，previous()的行爲需要返回現有值。在更新n之前，您必須將n.data存儲在臨時變量中，然後返回存儲的臨時值。

例如：

public Item previous() { 
    Item temp = n.data; 
    n = n.previous; 
    return temp; 
}