調試鏈表實現

Question

DList.java包含了方法，我們給了前4個方法，最後4個我們必須自己寫，而坦率地說，我剛剛得到了自己的困惑。我上運行的程序

Exception in thread "main" java.lang.NullPointerException 
    at javaapplication13.DList.remove(Dlist.java:41) 
    at javaapplication13.Driver.main(Driver.java:18) 
Java Result: 1 

這個錯誤，我知道這個錯誤是什麼，但我覺得我實現的方法錯了，在結束我的教授不直接經過的例子。

DList./java，這些都是方法

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，這也賦予了我們。

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，這是教師提供與

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(); 

} 
} 

Answer 1

curr.next()爲空，以測試該文件。你沒有初始化它。所以調用下一個方法將會給NullPointerException