这是一个家庭作业问题。我有一个Double Linked Node类，实现Iterable的Circular Double Linked List类，以及一个实现Iterator的Iterator类。我理解迭代器的概念，其中隐式游标位于两个节点之间，并且在调用next()时它返回它刚刚跳过的节点。我创建了一个带有新列表对象的测试类。然后我创建了一个迭代器并调用了next，我收到了正确的No Such Element exception但是当我向列表中添加了一些节点时，我收到了Null Pointer Exception而不是返回最后返回的节点。我怎么能纠正这个？
我的节点类：
public class DoubleLinkedNode<E>{
private E data;
private DoubleLinkedNode<E> next;
private DoubleLinkedNode<E> prev;
public DoubleLinkedNode(E data, DoubleLinkedNode<E> next, DoubleLinkedNode<E> prev){
this.data = data;
this.next = next;
this.prev = prev;
}
/**
* Used to construct a node that points to null for its prev and next.
* @param data Data is a generic variable to be stored in the node.
*/
public DoubleLinkedNode(E data){
this(data, null, null);
}
//getters
public E getData(){
return data;
}
public DoubleLinkedNode<E> getNext(){
return next;
}
public DoubleLinkedNode<E> getPrev(){
return prev;
}
//setters
public void setData(E data){
this.data = data;
}
public void setNext(DoubleLinkedNode<E> next){
this.next = next;
}
public void setPrev(DoubleLinkedNode<E> prev){
this.prev = prev;
}
@Override public String toString(){
if(data == null){
return null;
}
else{
return data.toString();
}
}
}
具有私有内部Iterator类的List类：
import java.util.Iterator;
import java.util.ListIterator;
import java.util.NoSuchElementException;
public class CircularDoubleLinkedList<E> implements Iterable<E>{
private int size;
private DoubleLinkedNode<E> head;
public CircularDoubleLinkedList(){
this.head = null;
this.size = 0;
}
/**
* Adds an item to the end of the list.
*
* @param data The Data that is to be stored in the node.
*/
public void add(E data){
DoubleLinkedNode<E> newNode = new DoubleLinkedNode<E>(data);
if(this.head == null){
newNode.setNext(newNode);
newNode.setPrev(newNode);
this.head = newNode;
this.size++;
//if list is empty create a new node and insert
}
else{
DoubleLinkedNode<E> temp = this.head.getPrev();
this.head.getPrev().setNext(newNode);
this.head.setPrev(newNode);
newNode.setPrev(temp);
newNode.setNext(this.head);
this.size++;
}
}
/**
* Which adds an item at the specified index.
*
* @param index The index in which the new Node is added.
* @param data The data which is to be stored in the node.
*/
public void add(int index, E data){
int currIndex = 0;
DoubleLinkedNode<E> currNode = this.head;
DoubleLinkedNode<E> nextNode = this.head.getNext();
DoubleLinkedNode<E> prevNode = this.head.getPrev();
DoubleLinkedNode<E> newNode = new DoubleLinkedNode<E>(data);
if(index == 0){
prevNode.setNext(newNode);
currNode.setPrev(newNode);
newNode.setPrev(prevNode);
newNode.setNext(currNode);
this.head = newNode;
this.size++;
}
else if (index > 0){
while(currIndex != this.size){
if(currIndex != index%size){
currIndex++;
currNode = currNode.getNext();
nextNode = nextNode.getNext();
prevNode = prevNode.getNext();
}else{
newNode.setPrev(prevNode);
newNode.setNext(currNode);
prevNode.setNext(newNode);
currNode.setPrev(newNode);
currNode = newNode;
this.size++;
break;
}
}
}
else if (index < 0){
while(currIndex != -this.size){
if(currIndex != index%size){
currIndex--;
currNode = currNode.getPrev();
prevNode = prevNode.getPrev();
nextNode = nextNode.getPrev();
}else{
newNode.setNext(nextNode);
newNode.setPrev(currNode);
currNode.setNext(newNode);
nextNode.setPrev(newNode);
currNode = newNode;
this.size++;
break;
}
}
}
}
/**
* Returns the data stored at the specified index.
*
* @param index The index determines the node whose data is returned.
* @return Returns the data of the node at the index.
*/
public E get(int index){//returns the data stored at the specified index
int currIndex = 0;
DoubleLinkedNode<E> currNode = this.head;
E temp = null;
if(index == 0){//zero case
temp = currNode.getData();
}
else if(index > 0){//positive
while(currIndex != this.size){
if(currIndex != index%size){
currIndex++;
currNode = currNode.getNext();
}else{
temp = currNode.getData();
break;
}
}
}
else if(index < 0){//negative
while(currIndex != -this.size){
if(currIndex != index%size){
currIndex--;
currNode = currNode.getPrev();
}else{
temp = currNode.getData();
break;
}
}
}
return temp;
}
/**
* Which removes and returns an item from the list.
*
* @param index Removes the node at the current index.
* @return Returns the data of the removed node.
*/
public E remove(int index){//which removes and returns an item from the list
int currIndex = 0;
DoubleLinkedNode<E> currNode = this.head;
DoubleLinkedNode<E> nextNode = this.head.getNext();
DoubleLinkedNode<E> prevNode = this.head.getPrev();
E temp = null;
if(index == 0){
temp = currNode.getData();
prevNode.setNext(currNode.getNext());
nextNode.setPrev(currNode.getPrev());
this.head = nextNode;
size--;
}
else if(index > 0){//positive
while(currIndex != this.size){
if(currIndex != index%size){
currIndex++;
currNode = currNode.getNext();
nextNode = nextNode.getNext();
prevNode = prevNode.getNext();
}else{
temp = currNode.getData();
prevNode.setNext(currNode.getNext());
nextNode.setPrev(currNode.getPrev());
currNode = nextNode;
size--;
break;
}
}
}
else if(index < 0){//negative
while(currIndex != -this.size){
if(currIndex != index%size){
currIndex--;
currNode = currNode.getPrev();
prevNode = prevNode.getPrev();
nextNode = nextNode.getPrev();
}else{
temp = currNode.getData();
prevNode.setNext(currNode.getNext());
nextNode.setPrev(currNode.getPrev());
currNode = prevNode;
size--;
break;
}
}
}
return temp;
}
/**
* Returns the size.
*
* @return
*/
public int size(){
return size;
}
@Override public String toString(){
String str = "[";
int index = 0;
DoubleLinkedNode<E> curr = head;
if(size == 0){
return "There is no one here to kill.";
}else{
while (index <this.size) {
str += curr.getData();
curr = curr.getNext();
index++;
if (index<this.size) {
str += ", ";
}
}
str += "]";
}
return str;
}
@Override
public Iterator<E> iterator() {
return new CircularDoubleIterator();
}
// Iterator inner class begins
private class CircularDoubleIterator implements ListIterator<E> {
private DoubleLinkedNode<E> nextItem;//reference to next item
private int index = 0;
private DoubleLinkedNode<E> lastReturned;// the last node to be returned by prev() or next()
// reset to null after a remove() or add()
@Override
public E next() {
if(!hasNext()){
throw new NoSuchElementException("No such element.");
}
else{
nextItem = head; //edited 11Sept13
lastReturned = nextItem.getNext();
nextItem = nextItem.getNext();
head = nextItem; //edited 11Sept13
index++;
return lastReturned.getData();
}
}
@Override
public E previous() {
if(!hasPrevious()){
throw new NoSuchElementException("No such element.");
}
else{
index--;
return ;
}
}
@Override
public int nextIndex() {
return index;
}
@Override
public int previousIndex() {
return index-1;
}
@Override
public boolean hasNext() {
return size != 0;
}
@Override
public boolean hasPrevious() {
return size!= 0;
}
@Override
public void remove() {
}
@Override
public void set(E theData) {
if(lastReturned == null){
throw new IllegalStateException();
}
else{
}
}
@Override
public void add(E theData) {
if(size == 0){
}
else if(size != 0){
}
}
}
//Iterator inner class ends
}
答案 0 :(得分：0)
在创建迭代器时，我没有看到您为private DoubleLinkedNode<E> nextItem;赋值的位置。
我看不到整个代码。所以我假设nextItem为空或其next字段为空。