将Node添加到指定索引Java的双向链接列表时发生NullPointerException

Question

I cant figure out where I'm going wrong at all, I've checked and rechecked my add(int index, T obj) method countless times, still getting the same error. Here's my code, any pointers would be GREATLY appreciated; this problem has been holding up my project for a day or so at least now.

package edu.neumont.csc250;


class LinkedList<T> implements List<T>{

    Node<T> head;
    Node<T> tail;
    int listCount;

    public LinkedList(){
        head = null;
        listCount = 0;
    }

    @Override
    public T get(int index) throws IllegalArgumentException {
        if(index > size() - 1 || index < 0){
            throw new IllegalArgumentException();
        }
        else{           
            Node<T> current = head;
            for(int i = 0; i < index; i++)
            {               
                current = current.next;
            }
            if(current.content != null){
                return current.content;
            }
            else{
                System.out.println("Null value.");
                return null;
            }
        }
    }

    @Override
    public void add(T obj) {
        if(head == null){
            head = new Node<T>(obj);
            head.next = null;
            tail = head;
            listCount++;
        }
        else{
            if(head.next == null){
                head.next = new Node<T>(obj);
                //head.next.next = null;
                tail = head.next;
                tail.prev = head;
                listCount++;
            }
            else{
                tail.next = new Node<T>(obj);
                tail.next.prev = tail;
                tail = tail.next;
                tail.next = null;
                listCount++;
            }
        }
    }

    @Override
    public void add(int index, T obj) throws IllegalArgumentException {
        // TODO Auto-generated method stub      
        Node<T> temp = new Node(obj);
        Node<T> current = head;

        for(int i = 0; i<=index; i++){
            current = current.next;
        }
        temp.prev = current.prev;
        current.prev = temp;
        current.prev.next = current;

        if(index == 0){
            head = current.prev;
        }
        else if(index == size()+1){
            tail = current.next;
        }

        listCount++;
    }

    @Override
    public void replace(int index, T obj) throws IllegalArgumentException {
        // TODO Auto-generated method stub
        if(index > size() - 1 || index < 0){
            throw new IllegalArgumentException();
        }
        else{
            //get(index)
        }

    }

    @Override
    public T remove() {
        head = head.next;
        listCount--;

        return null;
    }

    @Override
    public T remove(int index) throws IllegalArgumentException {
        // TODO Auto-generated method stub
        if(index > size() - 1 || index < 0){
            throw new IllegalArgumentException();
        }
        else{

            listCount--;
        }

        return null;
    }

    @Override
    public int size() {
        return listCount;
    }

    public static void main(String[] args){
        LinkedList<String> list = new LinkedList<String>();
        list.add("Red");
        list.add("Orange");
        list.add("Yellow");
        list.add("Green");
        list.add("Blue");
        list.add("Purple");

        for(int a = 0; a < list.size(); a++){
            System.out.println(list.get(a));
        }       
//      System.out.println(list.get(0).toString());
//      System.out.println(list.get(1).toString());
//      System.out.println(list.get(2).toString());
//      System.out.println(list.get(3).toString());
//      System.out.println(list.get(4).toString());
//      System.out.println(list.get(5).toString());
        System.out.println("There are " + list.size() + " colors in the list.");

        list.remove();

        for(int b = 0; b < list.size(); b++){
            System.out.println(list.get(b));
        }
//      System.out.println(list.get(0).toString());
//      System.out.println(list.get(1).toString());
//      System.out.println(list.get(2).toString());
//      System.out.println(list.get(3).toString());
//      System.out.println(list.get(4).toString());
        //System.out.println(list.get(5));
        System.out.println("There are " + list.size() + " colors in the list.");

        list.add(0, "Red");
        System.out.println(list.size());

        for(int c = 0; c < list.size(); c++){
            System.out.println(list.get(c));
        }
//      System.out.println(list.get(0).toString());
//      System.out.println(list.get(1).toString());
//      System.out.println(list.get(2).toString());
//      System.out.println(list.get(3).toString());
//      System.out.println(list.get(4).toString());
//      System.out.println(list.get(5).toString());
        System.out.println("There are " + list.size() + " colors in the list.");
    }

    class Node<T>{
        T content;
        Node<T> next;
        Node<T> prev;

        public Node(T content){
            this.content = content;
        }

        public T getContents(){
            return content;
        }

        public void printNode() {
            System.out.print("{" + content + "} ");
        }
    }
}

Here's my console readout if it's of any help:

Red Orange Yellow Green Blue Purple There are 6 colors in the list. Orange Yellow Green Blue Purple There are 5 colors in the list. 6 Red Yellow Green Blue Purple Exception in thread "main" java.lang.NullPointerException at edu.neumont.csc250.LinkedList.get(LinkedList.java:26) at edu.neumont.csc250.LinkedList.main(LinkedList.java:161)

EDIT: main method isolated, as requested:

public static void main(String[] args){
        LinkedList<String> list = new LinkedList<String>();
        list.add("Red");
        list.add("Orange");
        list.add("Yellow");
        list.add("Green");
        list.add("Blue");
        list.add("Purple");

        for(int a = 0; a < list.size(); a++){
            System.out.println(list.get(a));
        }       
//      System.out.println(list.get(0).toString());
//      System.out.println(list.get(1).toString());
//      System.out.println(list.get(2).toString());
//      System.out.println(list.get(3).toString());
//      System.out.println(list.get(4).toString());
//      System.out.println(list.get(5).toString());
        System.out.println("There are " + list.size() + " colors in the list.");

        list.remove();

        for(int b = 0; b < list.size(); b++){
            System.out.println(list.get(b));
        }
//      System.out.println(list.get(0).toString());
//      System.out.println(list.get(1).toString());
//      System.out.println(list.get(2).toString());
//      System.out.println(list.get(3).toString());
//      System.out.println(list.get(4).toString());
        //System.out.println(list.get(5));
        System.out.println("There are " + list.size() + " colors in the list.");

        list.add(0, "Red");
        System.out.println(list.size());

        for(int c = 0; c < list.size(); c++){
            System.out.println(list.get(c));
        }
//      System.out.println(list.get(0).toString());
//      System.out.println(list.get(1).toString());
//      System.out.println(list.get(2).toString());
//      System.out.println(list.get(3).toString());
//      System.out.println(list.get(4).toString());
//      System.out.println(list.get(5).toString());
        System.out.println("There are " + list.size() + " colors in the list.");
    }

Answer 1

When insert(add()) with index 0, the new element will be inserted behind the first element.
However, the head reference points to the newly inserted element rather than the actual head element after the operation is done.
Thus, a NPE will occur when you iterate your list.
In fact, you can get this from the output of the programe. Notice that the third iteration starts at 'Red' and the 'Orange' diappears.

And the implementation of remove() is not good, for it'll lead to 'Memory Leak'. It just moves the 'head' forward without null-ing the element.