import java.util.*;
public class MyTwoWayLinkedList<E> extends java.util.AbstractSequentialList<E> {
private Node<E> head, tail;
private int size = 0;
private List<E> list;
/** Create a default list */
public MyTwoWayLinkedList() {
list = new LinkedList<E>();
}
public MyTwoWayLinkedList(E[] objects) {
list = new LinkedList<E>();
for (int i = 0; i < objects.length; i++)
add(objects[i]);
}
/** Return the head element in the list */
public E getFirst() {
if (size == 0) {
return null;
}
else {
return head.element;
}
}
/** Return the last element in the list */
public E getLast() {
if (size == 0) {
return null;
}
else {
return tail.element;
}
}
/** Add an element to the beginning of the list */
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.previous = newNode; //link the old node with new 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;
}
/** Add an element to the end of the list */
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
newNode.previous = tail;
tail = tail.next; // tail now points to the last node
}
size++; // Increase size
}
@Override /** Add a new element at the specified index
* in this list. The index of the head element is 0 */
public void add(int index, E e) {
if (index == 0) {
addFirst(e);
}
else if (index >= size) {
addLast(e);
}
else {
Node<E> current = tail;
for (int i = size - 1; i > index; i--) {
current = current.previous;
}
Node<E> temp = current.next;
current.next = new Node<E>(e);
(current.next).previous = current;
(current.next).next = temp;
size++;
}
}
/** Remove the head node and
* return the object that is contained in the removed node. */
public E removeFirst() {
if (size == 0) {
return null;
}
else {
Node<E> temp = head;
head = head.next;
head.previous = null;
size--;
if (head == null) {
tail = null;
}
return temp.element;
}
}
/** Remove the last node and
* return the object that is contained in the removed node. */
public E removeLast() {
if (size == 0) {
return null;
}
else if (size == 1) {
Node<E> temp = head;
head = tail = null;
size = 0;
return temp.element;
}
else {
Node<E> temp = tail;
tail = tail.previous;
tail.next = null;
size--;
return temp.element;
}
}
@Override /** Remove the element at the specified position in this
* list. Return the element that was removed from the list. */
public E remove(int index) {
if (index < 0 || index >= size) {
return null;
}
else if (index == 0) {
return removeFirst();
}
else if (index == size - 1) {
return removeLast();
}
else {
Node<E> previous = tail;
for (int i = size - 1; i > index; i--) {
previous = previous.previous;
}
Node<E> current = previous.next;
(current.next).previous = previous;
previous.next = current.next;
size--;
return current.element;
}
}
@Override /** Override toString() to return elements in the list */
public String toString() {
StringBuilder result = new StringBuilder("[");
Node<E> current = tail;
for (int i = size - 1; i > 0; 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();
}
@Override /** Clear the list */
public void clear() {
size = 0;
head = tail = null;
}
@Override /** Override iterator() defined in Iterable */
public ListIterator<E> listIterator() {
Node<E> current = head; // Current index
return list.listIterator();
}
@Override /** Override iterator() defined in Iterable */
public ListIterator<E> listIterator(int index) {
Node<E> current = head; // Current index
for (int i = 0; i < index; i++) { // sets current int to the parameter
current = current.next;
}
return list.listIterator();
}
@Override
public int size()
{
return size;
}
public class Node<E> {
E element;
Node<E> next;
Node<E> previous;
public Node(E element) {
this.element = element;
}
}
}
这是我原来的课程，我将在下面包含我的测试用例，但首先让我解释一下我的问题。我正在尝试创建一个双向链表并向后迭代它。但是，我只是通过向列表中添加元素来获得Null指针异常。我现在已经查看了我的addFirst方法的代码部分大约2个小时，并且没有看到任何逻辑错误（并不意味着没有），请帮忙！
这是我承诺的测试用例。
public class TestMyLinkedList {
/** Main method */
public static void main(String[] args) {
// Create a list for strings
MyTwoWayLinkedList<String> list = new MyTwoWayLinkedList<String>();
// Add elements to the list
list.add("America"); // Add it to the list
System.out.println("(1) " + list);
list.add(0, "Canada"); // Add it to the beginning of the list
System.out.println("(2) " + list);
list.add("Russia"); // Add it to the end of the list
System.out.println("(3) " + list);
list.addLast("France"); // Add it to the end of the list
System.out.println("(4) " + list);
list.add(2, "Germany"); // Add it to the list at index 2
System.out.println("(5) " + list);
list.add(5, "Norway"); // Add it to the list at index 5
System.out.println("(6) " + list);
list.add(0, "Poland"); // Same as list.addFirst("Poland")
System.out.println("(7) " + list);
// Remove elements from the list
list.remove(0); // Same as list.remove("Australia") in this case
System.out.println("(8) " + list);
list.remove(2); // Remove the element at index 2
System.out.println("(9) " + list);
list.remove(list.size() - 1); // Remove the last element
System.out.print("(10) " + list + "\n(11) ");
for (String s: list)
System.out.print(s.toUpperCase() + " ");
list.clear();
System.out.println("\nAfter clearing the list, the list size is "
+ list.size());
}
}
答案 0 :(得分：0)
我不完全确定您在自己的双链接列表实现中使用LinkedList的原因。关于您关于addFirst方法的问题，我有以下评论以及如何处理此解决方案的示例。
调用addFirst方法时，head为null。
Head尚未初始化为新节点。
因此newNode.next = head;实际上是newNode.next = null;我想象你的空指针异常！
public void addFirst (E e)
{
Node<E> newNode = new Node<E>(e); //create new node
if (head != null){ //if head exists
newNode.next = head; //the new node's next link becomes the old head
}
head = newNode; //the new head is the new node
if (tail == null){ //if the tail is non existent ie head the only object in list
tail = head; //the head and the tail are the same
head.next = tail; //the 'next' value of head will be tail
}
head.prev = tail; //the previous node to head will always be tail
size++;
}
}