我正在为下周的考试而学习，我希望有人能帮助我在双向链表中添加和删除元素。这个逻辑对我来说很有意义，不幸的是我还不太擅长编码（只是我的第二门计算机科学课程）。我浏览了整个网络和我的文本，但我似乎找不到与我的问题足够接近的例子。下面我将发布我的教授要求我完成的工作，以及我当前的代码。预先感谢您的任何帮助。
更新
我也遇到了添加问题，但是有人能够帮助我，但是我的删除方法仍然存在问题。我觉得奇怪的是，我的教授有一个返回类型不是 void 的 remove 方法。有人可以解释一下吗？无论如何，我的更新代码如下。
来自教授：
按照课堂讲座的指示填写 CS401DblLinkedListImpl.java 中缺少的代码。
要测试您的代码，请创建一个双向链表并在其中输入以下 String 类型的元素：
比尔、罗汉、詹姆斯、克里希纳、哈维尔、丽莎
(a) 从头开始打印链表。
(b) 从末尾开始打印链表。
(c) 删除 Bill 并从头开始打印链表。
(d) 删除 Lisa 并从 end 开始打印链表。
(e) 移除 Krishna 并从头开始打印链表。
下面这组代码是我的用户定义类，我将测试的方法是 boolean add(E e)、E remove(int n)、void print_from_beginning() 和 void print_from_end()：
package week6;
import java.util.Iterator;
public class CS401DblLinkedListImpl<E> //implements CS401CollectionInterface<E>
{
private LinkEntry<E> head = null;
private LinkEntry<E> tail = null;
private int size = 0;
public CS401DblLinkedListImpl()
{
head = tail = null;
}
public boolean is_empty()
{
if (head == null)
return true;
return false;
}
public int size()
{
int count = 0;
for (LinkEntry<E> current = head; current != null; current = current.next)
count++;
return count;
}
/*
* Add e to the end of the doubly linked list.
* Returns true - if e was successfully added, false otherwise.
*/
public boolean add(E e)
{
LinkEntry<E> new_element = new LinkEntry<E>();
new_element.element = e;
if (head == null)
{
new_element.next = head;
head = new_element;
tail = head;
}
else
{
tail.next = new_element;
new_element.previous = tail;
tail = new_element;
}
return true;
}
/*
* Remove the nth element in the list. The first element is element 1.
* Return the removed element to the caller.
*/
public E remove(int n)
{
LinkEntry<E> current = new LinkEntry<E>();
int i = 0;
while (n == i++)
{
current.previous.next = current.next;
if (current.next == null)
{
current.next.previous = current.previous;
}
}
return (E) current;
}
/*
* Print the doubly linked list starting at the beginning.
*/
public void print_from_beginning()
{
LinkEntry<E> current = new LinkEntry<E>();
for (current = head; current != null; current = current.next)
{
System.out.print(current.element + " ");
}
}
/*
* Print the doubly linked list starting the end.
*/
public void print_from_end()
{
LinkEntry<E> current = new LinkEntry<E>();
for (current = tail; current != null; current = current.previous)
{
System.out.print(current.element + " ");
}
}
/* ------------------------------------------------------------------- */
/* Inner classes */
protected class LinkEntry<E>
{
protected E element;
protected LinkEntry<E> next;
protected LinkEntry<E> previous;
protected LinkEntry() { element = null; next = previous = null; }
}
/* ------------------------------------------------------------------- */
protected class CS401DblLinkedListImplIterate<E> implements Iterator<E>
{
protected LinkEntry<E> next;
protected CS401DblLinkedListImplIterate()
{
next = (LinkEntry<E>) head;
}
@Override
public boolean hasNext() {
// TODO Auto-generated method stub
return false;
}
@Override
public E next() {
// TODO Auto-generated method stub
return null;
}
@Override
public void remove() {
// TODO Auto-generated method stub
}
}
} /* CS401LinkedListImpl<E> */
下面是我测试方法的主要课程：
package week6;
import java.util.*;
public class App {
public static <E> void main(String[] args) {
/*
* Part 1 of lab 6: Testing CS401DblLinkedListImpl
*/
System.out.println("Testing Lab 6: Part 1...");
CS401DblLinkedListImpl<String> list = new CS401DblLinkedListImpl<String>();
list.add("Bill");
list.add("Rohan");
list.add("James");
list.add("Krishna");
list.add("Javier");
list.add("Lisa");
System.out.println("List size after all names are added: " + list.size());
//a. Print the linked list starting at the beginning.
System.out.println("\nPrint the linked list starting at the beginning:");
list.print_from_beginning();
//b. Print the linked list starting at the end.
System.out.println("\nPrint the linked list starting at the end:");
list.print_from_end();
//c. Remove Bill and print the linked list starting from beginning.
System.out.println("\nRemove Bill and print the linked list starting from beginning:");
list.remove(0);
list.print_from_beginning();
//d. Remove Lisa and print the linked list starting from end.
System.out.println("\nRemove Lisa and print the linked list starting from end:");
list.remove(4);
list.print_from_end();
//e. Remove Krishna and print the linked list starting from the beginning.
System.out.println("\nRemove Krishna and print the linked list starting from the beginning:");
list.remove(2);
list.print_from_beginning();
System.out.println("\nList size: " + list.size());
}
}
最后，下面是我运行程序时得到的结果：
Testing Lab 6: Part 1...
List size after all names are added: 6
Print the linked list starting at the beginning:
Bill Rohan James Krishna Javier Lisa
Print the linked list starting at the end:
Lisa Javier Krishna James Rohan Bill
Remove Bill and print the linked list starting from beginning:
Exception in thread "main" java.lang.NullPointerException
at week6.CS401DblLinkedListImpl.remove(CS401DblLinkedListImpl.java:67)
at week6.App.main(App.java:34)
仅供参考，CS401DblLinkedListImpl.java:67 指的是我的删除方法中的以下代码行：
current.previous.next = current.next;
对此的任何帮助将不胜感激。我觉得我有点接近答案，只需要澄清一下。谢谢。