java - 袋子 remove() 方法

Question

我得到了这样的 Bag 类

    import java.util.Iterator;
    import java.util.NoSuchElementException;

    public class Bag<Item> implements Iterable<Item> {

    private int N;               // number of elements in bag
    private Node<Item> first;    // beginning of bag

    // helper linked list class
    private class Node<Item> {
    private Item item;
    private Node<Item> next;
    }

    /**
    * Initializes an empty bag.
     */
    public Bag() {
    first = null;
    N = 0;
    }

    /**
     * Is this bag empty?
    * @return true if this bag is empty; false otherwise
     */
    public boolean isEmpty() {
    return first == null;
    }

    /**
     * Returns the number of items in this bag.
     * @return the number of items in this bag
     */
    public int size() {
    return N;
    }

    /**
     * Adds the item to this bag.
     * @param item the item to add to this bag
     */
    public void add(Item item) {
    Node<Item> oldfirst = first;
    first = new Node<Item>();
    first.item = item;
    first.next = oldfirst;
    n++;
}

    public void remove(Item item){ 
    // currentNode is the reference to the first node in the list and to the Item
    Node<Item> currentNode = first; 
    // if items equals the first node in the list, then first = currentNode.next which will make the first item 
    Node<Item> temp = currentNode;
    while(temp.next != null){
        temp = currentNode;
        if(item.equals(currentNode.item)){
            currentNode = currentNode.next;
            temp.next = currentNode;
            break;
        }else{
            currentNode = currentNode.next;
        }
    }
    N--; 
}

/**
 * Returns an iterator that iterates over the items in the bag in arbitrary order.
 * @return an iterator that iterates over the items in the bag in arbitrary order
 */
public ListIterator<Item> iterator()  {
    return new ListIterator<Item>(first);  
}

// an iterator, doesn't implement remove() since it's optional
private class ListIterator<Item> implements Iterator<Item> {
    private Node<Item> current;

    public ListIterator(Node<Item> first) {
        current = first;
    }

    public boolean hasNext()  { return current != null;                     }
    public void remove()      { throw new UnsupportedOperationException();  }

    public Item next() {
        if (!hasNext()) throw new NoSuchElementException();
        Item item = current.item;
        current = current.next; 
        return item;
    }
}

可以看出，remove() 方法是可选的，因此它不包含算法。

我是这样写remove方法的:

public void remove(Item item)      { 
        Node<Item> currentNode = (Node<Item>) first;
        Node<Item> previousNode = null;
        while(currentNode != null){
            if(item.equals(currentNode.item)){
                if(previousNode  == null) {
                  first = (Node<Item>) currentNode.next;
                }
                else {
                  previousNode.next = currentNode.next;
                }
                n--;
            }
            else {
              previousNode = currentNode;
            }
            currentNode = currentNode.next;
        }

    }

但是，

first = (Node<Item>) currentNode.next;

这一行给出了一个“类型不匹配:无法从 Bag.Node 转换为 Bag.Node”的错误，这让我很困惑。

应该怎么做才能克服这个错误，或者我的删除方法中是否缺少部分？

Answer 1

public class Bag<Item> implements Iterable<Item> {
  // ...
  private class Node<Item> {
    // ...
  }
  // ...
}

这定义了两个不同的类型变量，都具有名称 Item .

如果你想要 Node 的实例使用与 Bag 的包含实例相同的类型变量, 删除 <Item>在 Node :

private class Node {