我有一个完成DoublyLinkList任务的任务，我在使用其中两个方法时遇到了一些困难。我获得了MyList和MyAbstractList的代码，并被告知使用我的DoublyLinkedList扩展MyAbstractList。我不允许更改MyList或MyAbstractList。
以下是我提供的MyList代码：
public interface MyList<E extends Comparable<E>> {
/** Return true if this list contains no elements */
public boolean isEmpty();
/** Return the number of elements in this list */
public int size();
/** Add a new element at the proper point */
public void add(E e);
/** Clear the list */
public void clear();
/** Return true if this list contains the element */
public boolean contains(E e);
/** Return the element from this list at the specified index */
public E get(int index);
/** Return the first element in the list */
public E getFirst();
/** Return the last element in the list */
public E getLast();
/** Return the index of the first matching element in this list.
* Return -1 if no match. */
public int indexOf(E e);
/** Return the index of the last matching element in this list
* Return -1 if no match. */
public int lastIndexOf(E e);
/** Remove the first occurrence of the element o from this list.
* Shift any subsequent elements to the left.
* Return true if the element is removed. */
public boolean remove(E e);
/** Remove the element at the specified position in this list
* Shift any subsequent elements to the left.
* Return the element that was removed from the list. */
public boolean remove(int index);
/** Remove the first element in the list, return true if done, false if list is empty */
public boolean removeFirst();
/** Remove the Last element in the list, return true if done, false if list is empty */
public boolean removeLast();
/** Replace the element at the specified position in this list
* with the specified element and return true if done, false if index out of range. */
public boolean set(int index, E e);
}
这是我为MyAbstractList提供的代码：
public abstract class MyAbstractList<E extends Comparable<E>> implements MyList<E> {
protected int size = 0; // The size of the list
/** Create a default list */
protected MyAbstractList() {
}
/** Create a list from an array of objects */
public MyAbstractList(E[] objects) {
for (int i = 0; i < objects.length; i++)
add(objects[i]);
}
@Override /** Return true if this list contains no elements */
public boolean isEmpty() {
return size == 0;
}
@Override /** Return the number of elements in this list */
public int size() {
return size;
}
}
最后这是我的MyDoublyLinkedList代码：
import java.util.ArrayList;
public class MyDoublyLinkedList<E extends Comparable<E>> extends MyAbstractList<E>
{
int size =0;
Node<E> head = null;
Node<E> current = null;
Node<E> tail = null;
public MyDoublyLinkedList() {
}
/** Create a list from an array of objects */
public MyDoublyLinkedList(E[] objects) {
super(objects);
}
@Override
public boolean isEmpty()
{
return size == 0;
}
@Override
public int size()
{
return size;
}
/** Add a new element at the proper point */
@Override
public void add(E e)
{
Node<E> newNode = new Node<E>(e);
if (tail == null) {
head = tail = newNode;
}
else //if (head != null)
{
tail.next = newNode;
tail = newNode;
}
size++;
}
@Override
public void clear()
{
size = 0;
head = tail = null;
}
@Override
public boolean contains(E e)
{
Node<E> current = head;
for (int i = 0; i < size; i++) {
if (current.element.equals(e))
return true;
current = current.next;
}
return false;
}
@Override
public E get(int index)
{
if (index < 0 || index > size - 1)
return null;
Node<E> current = head;
for (int i = 0; i < index; i++)
current = current.next;
return current.element;
}
@Override
public E getFirst()
{
if (size == 0) {
return null;
}
else {
return head.element;
}
}
@Override
public E getLast()
{
if (size == 0) {
return null;
}
else {
return tail.element;
}
}
@Override
public int indexOf(E e)
{
Node<E> current = head;
for (int i = 0; i < size; i++) {
if (current.element.equals(e))
return i;
current = current.next;
}
return -1;
}
@Override
public int lastIndexOf(E e)
{
int lastIndex = -1;
Node<E> current = head;
for (int i = 0; i < size; i++) {
if (current.element.equals(e))
lastIndex = i;
current = current.next;
}
return lastIndex;
}
/** Remove the first occurrence of the element o from this list.
* Shift any subsequent elements to the left.
* Return true if the element is removed. */
@Override
public boolean remove(E e)
{
int index = indexOf(e);
if (index != -1) {
return remove(index);
} else {
return false;
}
}
/** Remove the element at the specified position in this list
* Shift any subsequent elements to the left.
* Return the element that was removed from the list. */
@Override
public boolean remove(int index)
{
if (index < 0 || index >= size) {
return false;
}
else if (index == 0) {
return removeFirst();
}
else if (index == size - 1) {
return removeLast();
}
else {
Node<E> previous = head;
for (int i = 1; i < index; i++) {
previous = previous.next;
}
Node<E> current = previous.next;
previous.next = current.next;
size--;
return true;
}
}
@Override
public boolean removeFirst()
{
if (size == 0) {
return false;
}
else {
Node<E> temp = head;
head = head.next;
size--;
if (head == null) {
tail = null;
}
return true;
}
}
@Override
public boolean removeLast()
{
if (size == 0) {
return false;
}
else if (size == 1) {
Node<E> temp = head;
head = tail = null;
size = 0;
return true;
}
else {
Node<E> current = head;
for (int i = 0; i < size - 2; i++) {
current = current.next;
}
Node<E> temp = tail;
tail = current;
tail.next = null;
size--;
return true;
}
}
@Override
public boolean set(int index, E e)
{
if (index < 0 || index > size - 1)
return false;
Node<E> current = head;
for (int i = 0; i < index; i++)
current = current.next;
E temp = current.element;
current.element = e;
return true;
}
@Override
public String toString()
{
StringBuilder result = new StringBuilder("[");
Node<E> current = head;
for (int i = 0; i < size; i++) {
result.append(current.element);
current = current.next;
if (current != null) {
result.append(", "); // Separate two elements with a comma
}
else {
result.append("]"); // Insert the closing ] in the string
}
}
return result.toString();
}
public String toStringBack()
{
StringBuilder result = new StringBuilder("[");
Node<E> current = tail;
for (int i = 0; i < size; i++) {
result.append(current.element);
current = current.previous;
if (current != null) {
result.append(", "); // Separate two elements with a comma
}
else {
result.append("]"); // Insert the closing ] in the string
}
}
return result.toString();
}
public void add(int index, E e)
{
if (index ==0) {
addFirst(e);// The new node is the only node in list
}
else if(index > size)
{
addLast(e);//The index location was bigger than size
}
else
{
Node<E> current = getAtIndex(index-1);
Node<E> temp = current.next;
current.next = new Node<E>(e);
(current.next).next = temp;
size++;
}
}
public void addFirst(E e)
{
Node<E> newNode = new Node<E>(e); // Create a new node
newNode.next = head; // link the new node with the head
head = newNode; // head points to the new node
size++; // Increase list size
if (tail == null) // the new node is the only node in list
tail = head;
}
public void addLast(E e)
{
Node<E> newNode = new Node<E>(e); // Create a new for element e
if (tail == null) {
head = tail = newNode; // The new node is the only node in list
}
else {
tail.next = newNode; // Link the new with the last node
tail = tail.next; // tail now points to the last node
}
size++;
}
protected Node<E> getAtIndex(int index)
{
int count;
if (index < 0 || index > size - 1)
return null;
Node<E> temp = null;
if(index <= (size/2))
{
count = -1;
temp = head;
while(++count <= index){
temp = temp.next;
}
}
else if (index > (size/2))
{
count = size;
temp = tail;
while(--count >= index){
temp = temp.previous; //--> this is Where the null pointer exception occurs
}
}
return temp;
}
private static class Node<E>{
E element;
Node<E> next;
Node<E> previous;
public Node(E element){
this.element = element;
next = null;
previous = null;
}
}
}
当我运行add（int index，E e）方法时，我在getAtIndext（）方法中收到一个空指针异常。当我更改当前位置时，我也遇到了使添加（E e）方法正确添加的问题。这些方法都是因为我被要求使用它们。不允许使用迭代器，但它们会很好。我使用Bluj作为编译器。请告诉我你有什么问题。
谢谢
编辑：我确实知道Null Pointer Exception是什么，我无法弄清楚为什么这会返回null。节点“当前”应指向结尾，然后返回列表，直到它到达索引位置。这只发生在我尝试在列表的后半部分添加节点时。错误发生在temp = temp.previous;
堆栈跟踪是：
myDoublyLinkedList.getAtIndex（MyDoublyLinkedList.java:345）的MyDoublyLinkedList.add（MyDoublyLinkedList.java：289）中的java.lang.NullPointerException，位于TestMyDoublyLinkedList.main（TestMyDoublyLinkedList.java:50）
编辑：我弄明白了这个问题。 add（）和getAtIndex函数正常运行。当我运行测试时，抛出Null Pointer Exception的函数将它抛出到位置8.它在前几个节点中循环很好，但在节点8处死亡
答案 0 :(得分：0)
在add()方法中插入时，您永远不会分配previous，因此所有节点都为空。