袋子删除()方法

Question

>我被给了这样的包类

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，因此它不包括算法。

我像这样编写了删除方法：

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;
}
}

然而

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

这一行给出了"类型不匹配：无法从 Bag.Node 转换为 Bag.Node"错误，这让我感到困惑。

应该怎么做才能克服此错误，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> {
// ...
}
// ...
}

这是定义两个不同的类型变量，两者都具有名称Item。

如果希望Node的实例使用与包含Bag实例相同的类型变量，请删除Node上的<Item>：

private class Node {

Answer 2

我以这种方式克服了这个问题。

public void removeAllOccurences(Item item){ 
Node<Item> currentNode = first;
Bag<Item> b = new Bag<Item>();
Bag<Item> reverseB = new Bag<Item>();
Node<Item> previous = new Node<Item>();
if(item.equals(first.item)){
first = currentNode.next;
N--;
currentNode = currentNode.next;
}
previous.item = currentNode.item;
b.add(previous.item);
currentNode = currentNode.next;
while(currentNode != null){
if(item.equals(currentNode.item))
{
previous.item = previous.item;
N--;
} else{
previous.item = currentNode.item;
b.add(previous.item);
}

currentNode = currentNode.next;

}
for(Item i: b)
reverseB.add(i);
this.first = reverseB.first;
}

如果您想测试...

public static void main(String[] args) {
Bag<Integer> b = new Bag<Integer>();
Random rm = new Random();
for(int i =0; i < 10; i++)
b.add(rm.nextInt(3));
System.out.println("Bag before removing op: ");
for(Integer i: b)
System.out.print(" "+ i);
System.out.println("nBag size BEFORE: "+b.size());
b.removeAllOccurences(2);
System.out.println("nBag after removing op: ");
for(Integer i: b)
System.out.print(" "+ i);
System.out.println("nBag size AFTER: "+b.size());
}

输出删除所有出现的 2：

0 2 2 0 1 1 0 0 0 2
Bag size BEFORE: 10
Bag after removing op: 
0 0 1 1 0 0 0
Bag size AFTER: 7