Java中链接的插入列表（调试）

Question

I am trying to build a linked list as is shown below:

**list-> [C] - [B] - [A] -> NULL**

Here is my main:

import java.util.*; 

class LinkedListF
{
    public static void main (String [] args)
    {    
        Node aNode = new Node ("A", null);
        Node bNode = new Node ("B", aNode);
        Node list = new Node ("C",bNode);  

        DynamicList dynamicList = new DynamicList();   
        dynamicList.insertFirst("A");
        dynamicList.insertFirst("C");
        dynamicList.insertAfter(list, "B");
        //the line above is where I am struggling with.

        System.out.println(dynamicList.getList().getInfo());
        System.out.println(dynamicList.getList().getNext().getInfo());
        System.out.println(dynamicList.getList().getNext().getNext().getInfo());
    }
}

The problem I am having now, as I mark in the codes, is when the code " dynamicList.insertAfter(list, "B") " runs, it does not insert "B" after the Node "list" in the dynamicList that I construct. I am very confused about what is happening here.

My output looks like this:

----jGRASP exec: java LinkedListF

C
A
Exception in thread "main" java.lang.NullPointerException
    at LinkedListF.main(LinkedListF.java:27)

 ----jGRASP wedge2: exit code for process is 1.
 ----jGRASP: operation complete.

The line 27 is my third System.out.print statement.

Below is the insertAfter method in my DynamicList class:

public void insertAfter(Node p, String x)
{
    if (isEmpty())
    {
        System.out.println("VOID INSERTION.");
        System.exit(1);
    }

    Node q = new Node (x, p.getNext());
    p.setNext(q);
}

Below is the Node class that I use to construct each node:

class Node
{
    private String data;
    private Node next;

    public Node (String data, Node next)
    {
        this.data = data;
        this.next = next;
    }

    String getInfo()
    {
        return data;
    }

    Node getNext()
    {
        return next;
    }

    void setInfo(String data)
    {
        this.data = data;
    }

    void setNext(Node next)
    {
        this.next = next;
    }
}

Answer 1

Problem

The problem is that you are creating several bare Node objects, which are not part of dynamicList , so your list is shorter than you think it is, and your print statements are running off the end.

Let's look at what your code is actually doing:

 Node aNode = new Node ("A", null);
 Node bNode = new Node ("B", aNode);
 Node list = new Node ("C", bNode);  

This will create three Node s which are in a chain

list -> bNode -> aNode -> null

You then create a DynamicList and add a couple String s to it:

 DynamicList dynamicList = new DynamicList();   
 dynamicList.insertFirst("A");
 dynamicList.insertFirst("C");

You haven't shown the code for insertFirst(String) but we can safely assume it creates a new Node for each String you add, which leaves you with this:

(stand-alone)  list -> bNode -> aNode -> null
(dynamicList)  "C" -> "A" -> null

Now let's examine what happens when you dynamicList.insertAfter(list, "B");

First the method checks to see if the list is empty (which it isn't, since you've added "A" and "C" to it). However, it then creates a new Node based on the String you give it and inserts it after the parameter Node . Nowhere does it check to see if that Node actually exists in the DynamicList .

This means that you end up with this:

(stand-alone)  list -> "B" -> bNode -> aNode -> null
(dynamicList)  "C" -> "A" -> null

This is what is leading to the NPE when you try to print:

System.out.println(dynamicList.getList().getInfo());
(dynamicList)  "C" -> "A" -> null
               ^^^                  prints "C"

System.out.println(dynamicList.getList().getNext().getInfo());
(dynamicList)  "C" -> "A" -> null
                      ^^^           prints "A"

System.out.println(dynamicList.getList().getNext().getNext().getInfo());
(dynamicList)  "C" -> "A" -> null
                             ^^^    null has no info to get

Solution

Instead of calling the insert method which takes a String and creates a new Node , you need to insert the Node s you've already created (assuming such methods exist):

 Node aNode = new Node ("A", null);
 Node list = new Node ("C", aNode);
 // creates list ("C") -> aNode ("A") -> null

 DynamicList dynamicList = new DynamicList();
 dynamicList.insertFirst(list);
 // dynamicList = list ("C") -> aNode ("A") -> null
 dynamicList.insertAfter(list, "B");
 // dynamicList = list ("C") -> "B" -> aNode ("A") -> null

An even simpler solution would be to never manually create any Node and just let the DynamicList do all the work:

 DynamicList dynamicList = new DynamicList();
 dynamicList.insertFirst("A");
 dynamicList.insertFirst("B");
 dynamicList.insertFirst("C");
 // dynamicList = "C" -> "B" -> "A" -> null