DList.java 这包含方法，我们得到了前 4 个方法，最后 4 个我们必须自己编写，坦率地说，我刚刚弄糊涂了。运行程序时出现此错误
Exception in thread "main" java.lang.NullPointerException
at javaapplication13.DList.remove(Dlist.java:41)
at javaapplication13.Driver.main(Driver.java:18)
Java Result: 1
我知道这个错误是什么，但我觉得我最终实施了错误的方法，而且我的教授没有直接通过示例。
DList./java，这些是方法
package javaapplication13;
import java.util.NoSuchElementException;
/**
*
* @author thomas
*/
class DList<E> {
public DLink<E> head , tail , curr ;
private int cnt ;
/**
* Insert "it" at current position
*/
public void insert(E it) {
curr.setNext(new DLink<E>(it, curr, curr.next()));
curr.next().next().setPrev(curr.next());
cnt++;
}
/**
* Append "it" to list
*/
public void append(E it) {
tail.setPrev(new DLink<E>(it, tail.prev(), tail));
tail.prev().prev().setNext(tail.prev());
cnt++;
}
/**
* Remove and return current element
*/
public E remove() {
if (curr.next() == tail) {
return null; // Nothing to remove
}
E it = curr.next().element(); // Remember value
curr.next().next().setPrev(curr);
curr.setNext(curr.next().next()); // Remove from list
cnt--; // Decrement the count
return it; // Return value removed
}
public void moveToPrev() {
if (curr != head) // Can’t back up from list head
{
curr = curr.prev();
}
}
public void moveToPos(int pos)
{
assert (pos>=0) && (pos<=cnt) : "Position out of range";
curr = head;
for(int i=0; i<pos; i++) {curr = curr.next();
}}
public void moveToValue(E it)
{
while (it!=curr)
{
}
}
public void insertBefore(E it) {
curr.setNext(new DLink<E>(it, curr, curr.prev()));
curr.next().prev().setPrev(curr.next());
cnt++;
}
public void removePrev()
{
moveToPrev();
if (curr==null) {
throw new IndexOutOfBoundsException("empty list.");
}
if (curr == head) {
throw new NoSuchElementException("cannot remove the head");
}
remove();
}
public void printAll()
{
while(curr != null){
System.out.print(curr +" ");
curr = curr.next;
}
System.out.println();
}
}
DLink.java，这个也是给我们的。
package javaapplication13;
/**
* Doubly linked list node
*/
class DLink<E> {
public E element; // Value for this node
public DLink<E> next, prev; // Pointer to next node in list
// Pointer to previous node
/**
* Constructors
*/
/** Constructors */
DLink(E it, DLink<E> p, DLink<E> n)
{ element = it; prev = p; next = n; }
DLink(DLink<E> p, DLink<E> n) { prev = p; next = n; }
/** Get and set methods for the data members */
DLink<E> next() { return next; }
DLink<E> setNext(DLink<E> nextval)
{ return next = nextval; }
DLink<E> prev() { return prev; }
DLink<E> setPrev(DLink<E> prevval)
{ return prev = prevval; }
E element() { return element; }
E setElement(E it) { return element = it; }
}
Driver.java ，这是教师提供的用于测试的文件
public class Driver{
public static void main(String args[])
{
DList<Integer> dlList = new DList<Integer>();
System.out.println("removing from empty list");
dlList.remove();
dlList.remove();
dlList.insert(30);
dlList.insert(20);
dlList.insert(10);
dlList.append(40);
dlList.append(50);
dlList.append(60);
dlList.printAll();
System.out.println("Moving to Position 3..");
dlList.moveToPos(3);
dlList.append(200);
dlList.insert(180);
dlList.insert(140);
dlList.insert(120);
System.out.println("Moving to Previous Position..");
dlList.moveToPrev();
dlList.printAll();
System.out.println("Removing node..");
dlList.remove();
dlList.printAll();
System.out.println("Moving to First Position..");
dlList.moveToStart();
System.out.println("Removing node..");
dlList.remove();
dlList.printAll();
System.out.println("Moving to Value 2..");
dlList.moveToValue(2);
System.out.println("Moving to Value 120..");
dlList.moveToValue(120);
System.out.println("Inserting 25..");
dlList.insert(25);
dlList.printAll();
System.out.println("Removing previous node..");
dlList.removePrev();
dlList.printAll();
System.out.println("Moving to First Position..");
dlList.moveToStart();
System.out.println("Inserting 45..");
dlList.insert(45);
System.out.println("Insert before 35..");
dlList.insertPrev(35);
dlList.printAll();
System.out.println("Moving to 50..");
dlList.moveToValue(50);
System.out.println("Inserting 55..");
dlList.insert(55);
dlList.printAll();
System.out.println("Removing previous nodes twice.");
dlList.removePrev();
dlList.removePrev();
dlList.printAll();
System.out.println("Removing nodes thrice.");
dlList.remove();
dlList.printAll();
dlList.remove();
dlList.printAll();
dlList.remove();
dlList.printAll();
}
}