链接列表的自定义迭代器

Question

我创建了一个自定义链接列表。我也为它做了一个迭代器。但是，我明白，程序没有在方法size（）中循环。它出什么问题了？是关于我的不正确的循环或iterator()。每个节点都有字段：nextIndex,prevIndex,value。

public class CustomLinkedList<E> implements List<E>{ 
    private ListNode<E> header = new ListNode<>(); 

    public CustomLinkedList() { 
     this.header = new ListNode(); 
    } 

    @Override 
    public int size() { 
     int size = 0; 
     for(E it : this) { 
      size++; 
     } 
     return size; 
    } 

    @Override 
    public boolean isEmpty() { 
     if(this.header == null){ 
      return true; 
     } else {return false;} 
    } 

    @Override 
    public boolean contains(Object o) { 
     if(((Integer) header.getValue() == 42) && ((Integer) header.getNextIndex().getValue() == 38)) return true; 
     return false; 
    } 

    @Override 
    public Iterator<E> iterator() { 
     return new Iterator<E>() { 
      private ListNode it = header; 

      @Override 
      public boolean hasNext() { 
       return (it.getNextIndex() != header) && (it.getNextIndex() != null); 
      } 

      @Override 
      public E next() { 
       it = it.getNextIndex(); 
       return (E) it.getValue(); 
      } 

      @Override 
      public void remove() 
      { 
       throw new UnsupportedOperationException("Iterator is read-only"); 
      } 
     }; 
    } 


    @Override 
    public boolean add(E e) { 
     ListNode<E> temp = new ListNode<>(); 
     temp.setValue(e); 
     if (this.size() < 1) { 
      header.setValue(e); 
      header.setPrevIndex(header); 
      header.setNextIndex(header); 
     } else { 
      temp.setNextIndex(header); 
      ListNode tempLast = header.getPrevIndex(); 
      tempLast.setNextIndex(temp); 
      header.setPrevIndex(temp); 
      temp.setPrevIndex(tempLast); 
     } 
     return true; 
    } 

    @Override 
    public boolean remove(Object o) { 
     return true; 
    } 

    @Override 
    public boolean containsAll(Collection<?> c) { 
     throw new NotImplementedException(); 
    } 

    @Override 
    public void clear() { 
     throw new NotImplementedException(); 
    } 

    @Override 
    public E get(int index) { 
     throw new NotImplementedException(); 
    } 

    @Override 
    public int indexOf(Object o) { 
     throw new NotImplementedException(); 
    } 

    @Override 
    public boolean addAll(Collection<? extends E> c) { 
     throw new NotImplementedException(); 
    } 
} 

Answer 1

你开始header.getNextIndex() == null然后调用add(E e)一次。 里面add()，size()将返回0，因此设置header.setPrevIndex(header)和header.setNextIndex(header)

接下来你叫add(E e)再次. Inside的add（）,大小（）will call hasNext（）and hasNext（）will check (it.getNextIndex() != header) && (it.getNextIndex() != null)将返回假因为it.getNextIndex() == header 因此size()将始终小于一个。

而且你的代码还有很多其他的bug。一个更好的部分实现将会像

public class CustomLinkedList<E> implements List<E> { 

    private ListNode<E> header; 

    public CustomLinkedList() { 
     this.header = null; 
    } 

    @Override 
    public int size() { 
     int size = 0; 
     for (E it : this) 
      size++; 
     return size; 
    } 

    @Override 
    public boolean isEmpty() { 
     return header == null; 
    } 

    @Override 
    public boolean contains(Object obj) { 
     if (header != null) 
      for (E it : this) 
       if (it.equals(obj)) 
        return true; 
     return false; 
    } 

    @Override 
    public Iterator<E> iterator() { 
     return new Iterator<E>() { 
      private ListNode<E> it = null; 

      @Override 
      public boolean hasNext() { 
       if (it==null && header!=null) 
        return true; 
       else if (it != null) 
        return it.getNextIndex() != null; 
       else 
        return false; 
      } 

      @Override 
      public E next() { 
       if (it==null) 
        it = header; 
       else 
        it = it.getNextIndex(); 
       return it.getValue(); 
      } 

      @Override 
      public void remove() { 
       throw new UnsupportedOperationException("Iterator is read-only"); 
      } 
     }; 
    } 

    @Override 
    public boolean add(E e) { 
     ListNode<E> temp = new ListNode<>(); 
     temp.setValue(e); 
     if (header == null) { 
      header = temp; 
     } else { 
      ListNode<E> it = header; 
      while (it.getNextIndex() != null) 
       it = it.getNextIndex(); 
      temp.setPrevIndex(it); 
      it.setNextIndex(temp); 
     } 
     return true; 
    } 

} 

Answer 2

您需要通过元素使用的iteratorhasNext和next方法循环，​​并增加了尺寸，例如：

@Override 
public int size() { 
    int size = 0; 
    for(Iterator<String> it = this.iterator(); it.hasNext(); it.next()){ 
     size++; 
    } 
    return size; 
}