我正在为网上课程实现链接列表，而且我的添加功能似乎有问题。当我尝试添加第一个元素时，Eclipse会输出null，而对于第二个元素，Eclipse会显示错误。 （我假设因为第一个元素从未添加过，所以不能再添加第二个元素。）
这是我的链接列表的实现：
package textgen;
import java.util.AbstractList;
public class MyLinkedList<E> extends AbstractList<E> {
LLNode<E> head;
LLNode<E> tail;
int size;
/** Create a new empty LinkedList */
public MyLinkedList() {
size = 0;
head = new LLNode<E>();
tail = new LLNode<E>();
head.next = tail;
tail.prev = head;
}
/**
* Appends an element to the end of the list
* @param element The element to add
*/
public boolean add(E element )
{
add(size, element);
return false;
}
/** Get the element at position index
* @throws IndexOutOfBoundsException if the index is out of bounds. */
public E get(int index) throws IndexOutOfBoundsException
{
if(index >= this.size){
throw new IndexOutOfBoundsException("Your index is out of bounds!");
}
LLNode<E> lGet = head;
for(int i = 0; i < index + 1; i++){
lGet = lGet.next;
}
return lGet.data;
}
public void printList(){
LLNode lTemp = head;
while(lTemp.next != tail){
System.out.println(lTemp.next.data);
lTemp = lTemp.next;
}
}
/**
* Add an element to the list at the specified index
* @param The index where the element should be added
* @param element The element to add
*/
public void add(int index, E element ) throws IndexOutOfBoundsException
{
if(index > this.size){
throw new IndexOutOfBoundsException("Oops! Out of bounds!");
}
else{
LLNode<E> nAdd = new LLNode<E>(element);
LLNode<E> nIt = null;
if(index <= size/2) // if the index is closer to the start from the beginning of the list
{
nIt = head;
for(int i = 0; i < index + 1; i++){
nIt = nIt.next;
}
}
else {
nIt = tail;
for(int i = this.size; i > index; i--){
nIt = nIt.prev;
}
}
nIt.prev.next.prev = nAdd;
nAdd.next = nIt.prev.next;
nIt.prev.next = nAdd;
nAdd.prev = nIt.prev;
size++;
}
}
/** Return the size of the list */
public int size()
{
return size;
}
/** Remove a node at the specified index and return its data element.
* @param index The index of the element to remove
* @return The data element removed
* @throws IndexOutOfBoundsException If index is outside the bounds of the list
*
*/
public E remove(int index)
{
// TODO: Implement this method
size--;
return null;
}
/**
* Set an index position in the list to a new element
* @param index The index of the element to change
* @param element The new element
* @return The element that was replaced
* @throws IndexOutOfBoundsException if the index is out of bounds.
*/
public E set(int index, E element)
{
// TODO: Implement this method
return null;
}
}
class LLNode<E>
{
LLNode<E> prev;
LLNode<E> next;
E data;
public LLNode(){
this.data = null;
this.prev = null;
this.next = null;
}
public LLNode(E e)
{
this.data = e;
this.prev = null;
this.next = null;
}
}
This is the main:
包textgen;
public class fixAdd {
public static void main(String [] Arg){
MyLinkedList<String> ll = new MyLinkedList<String>();
ll.add(0, "happy");
ll.add(1, "gilda");
System.out.println(ll);
}
}
And this is the error printed:
Exception in thread "main" java.lang.NullPointerException
at textgen.MyLinkedList.get(MyLinkedList.java:57)
at java.util.AbstractList$Itr.next(Unknown Source)
at java.util.AbstractCollection.toString(Unknown Source)
at java.lang.String.valueOf(Unknown Source)
at java.io.PrintStream.println(Unknown Source)
at textgen.fixAdd.main(fixAdd.java:11)
我已经多次使用我的添加方法，并将其与我在网上找到的其他实现进行比较，一切看起来都井井有条。我完全感到困惑，并希望得到任何帮助。谢谢！
答案 0 :(得分：1)
您应首先尝试实施更简单的add，而不是进行size / 2优化。
您的代码存在一些问题：
get方法中的循环应为for(int i = 0; i < index; i++) add方法编辑：更改add方法以涵盖所有情况：
public void add(int index, E element)
{
if (index > size)
{
throw new IndexOutOfBoundsException("Oops! Out of bounds!");
}
LLNode<E> node = new LLNode<E>(element);
//Add first and last
if(size == 0)
{
head = tail = node;
}
else
{
//Add first
if(index == 0)
{
node.next = head;
head.prev = node;
head = node;
}
//Add last
else if(index == size)
{
node.prev = tail;
tail.next = node;
tail = node;
}
//Add between
else
{
LLNode<E> current = this.head;
for(int i = 0; i < index; i++)
{
current = current.next;
}
node.next = current;
node.prev = current.prev;
current.prev.next = node;
}
}
size++;
}
答案 1 :(得分：0)
此实施的多个问题：
get()方法（通过调用AbstractList.toString()）add(E element)您正在调用add(size, element);并将size作为索引传递，稍后检查if(index > this.size)是否实际问size > size remove() 1}} - 毫无意义。nIt.prev.next.prev = nAdd;
nAdd.next = nIt.prev.next;
nIt.prev.next = nAdd;
nAdd.prev = nIt.prev;
实施中，你只是缩小了尺寸 - 这还不够好，你也应该向后移动尾部（如果列表大小为1 - 也要处理头部）。AbstractList两个建议：
tail - 毕竟您正在创建自己的实施 var $table = $('<table>' + trHTML + '</table>');
或尝试从最后插入以提高效率。只有在你有一个简单的实现工作后 - 尝试改进它。答案 2 :(得分：-1)
请尝试以下代码：
public void insert(int index, T item)
{
if(index == size())
{
add(item);
}
else if(index == 0)
{
MyNode<T> temp = new MyNode<T>();
temp.data = item;
temp.next = head;
head.previous = temp;
head = temp;
count++;
}
temp = head;
for(int i = 0; i < index-1; i++)
{
temp = temp.next;
MyNode<T> myNode = new MyNode<T>();
myNode.data = item;
myNode.next = temp.next;
temp.next = myNode;
count++;
}
}