【发布时间】：2021-11-26 03:42:53
【问题描述】：
我正在尝试使用我创建的 MyLinkedListIterator 类迭代 MylinkedList。我希望它能够打印出 drop、goofy、Donald、Duck、Mouse 等……但是我编写的 while 循环无法正常工作。我尝试通过 system.out.print(iter.next()) 进行调试，期望第一个元素打印在链表“drop”中，但返回 null。
public class xxx_xxx_xxxx{
public static void main(String[] args) throws Exception {
MyLinkedList<String> list = new MyLinkedList<>();
list.add("drop");
list.add("goofy");
list.add("Donald");
list.add("Duck");
list.add("Mouse");
list.add("Kangaroo");
list.add("Koala");
// initialized list of MyLinkedListIterator
MyLinkedListIterator<String> iter = new MyLinkedListIterator<String>();
while (iter.hasNext()) {
System.out.print(iter.next());
}
}
}
interface MyList<E> {
public void insert(int index, E object) throws Exception;
public void add(E object);
public E get(int index) throws Exception;
public int indexOf(E object);
public int lastIndexOf(E object);
public E remove(int index) throws Exception;
public E set(int index, E object) throws Exception;
public int size();
}
class Node<E> {
E element;
Node<E> next;
public Node(E element) {
this.element = element;
}
}
class MyLinkedList<E> implements MyList<E>, Iterator<E> {
Node<E> head = null;
Node<E> tail = null;
int size = 0;
@Override
public void insert(int index, E object) throws Exception {
if (index < 0 || index > size - 1)
throw new Exception("Invalid index.");
Node<E> newNode = new Node<>(object);
Node<E> current = head;
int counter = 0;
Node<E> previous = null;
while (counter < index) {
previous = current;
current = current.next;
counter++;
}
if (previous != null)
previous.next = newNode;
newNode.next = current;
size++;
if (index == 0)
head = newNode;
if (index == size - 1)
tail = newNode;
}
@Override
public void add(E object) {
Node<E> newNode = new Node<E>(object);
size++;
if (head == null)
head = newNode;
else
tail.next = newNode;
tail = newNode;
}
@Override
public E get(int index) throws Exception {
if (index < 0 || index > size - 1)
throw new Exception("Invalid index.");
Node<E> current = head;
int counter = 0;
while (counter < index) {
current = current.next;
counter++;
}
return current.element;
}
@Override
public int indexOf(E object) {
Node<E> current = head;
int index = 0;
while (current != null) {
if (object.equals(current.element))
return index;
current = current.next;
index++;
}
return -1;
}
@Override
public int lastIndexOf(E object) {
int result = -1;
Node<E> current = head;
int index = 0;
while (current != null) {
if (object.equals(current.element))
result = index;
current = current.next;
index++;
}
return result;
}
@Override
public E remove(int index) throws Exception {
if (index < 0 || index > size - 1)
throw new Exception("Invalid index.");
Node<E> current = head;
int counter = 0;
Node<E> previous = null;
while (counter < index) {
previous = current;
current = current.next;
counter++;
}
if (previous != null)
previous.next = current.next;
E result = current.element;
size--;
if (index == 0)
head = current.next;
if (index == size - 1)
tail = previous;
return result;
}
@Override
public E set(int index, E object) throws Exception {
if (index < 0 || index > size - 1)
throw new Exception("Invalid index.");
Node<E> current = head;
int counter = 0;
while (counter < index) {
current = current.next;
counter++;
}
E result = current.element;
current.element = object;
return result;
}
@Override
public int size() {
// TODO Auto-generated method stub
return size;
}
public Iterator<E> iterator() {
// TODO Auto-generated method stub
return new MyLinkedListIterator();
}
@Override
public String toString() {
String result = "[";
Node<E> current = head;
while (current != null) {
result += current.element;
if (current.next != null)
result += ", ";
current = current.next;
}
return result + "]";
}
@Override
public boolean hasNext() {
// TODO Auto-generated method stub
return false;
}
@Override
public E next() {
// TODO Auto-generated method stub
return null;
}
}
//Create the class MyLinkedListIterator<E>, which implements the Iterator<E> interface.
class MyLinkedListIterator<E> implements Iterator<E> {
// MyLinkedListIterator should contain the following data items,
// "list" of type MyLinkedList
MyLinkedList<E> list = new MyLinkedList<>();
// "currentNode" of type Node<E>, initially set to list.head
Node<E> currentNode = list.head;
// The hasNext() method should return true as long as currentNode is not null
public boolean hasNext() {
return currentNode != null;
}
// The next() method should return the list's data item at currentNode, and
// advance currentNode
public E next() {
Node<E> res = list.head;
currentNode = list.tail;
return (E) res;
}
}
【问题讨论】：
-
您的代码中发生的事情太多，根本无法正常工作。您要实现什么，您是否只是在自己实现链表？如果是这样，那么除了对下面的答案评论的更正之外，
MyLinkedList不应该实现Iterator，而是应该实现Iterable？只有MyLinkedListIterator<E>应该实现Iterator<E>。在此处查看实现迭代器如何工作的示例：gist.github.com/jnwhiteh/68d095c630dfcaddffd1
标签： java linked-list iterator