Question

So I have a null pointer exception when run. I am supposed to create a generic class that implements a list with chunks of arrays added as needed. Each time I add an element it is to check if there is space in the tail chunk array and if so add the element. Else it needs to add a chunk, adjust the pointers and add the element. My problem so far is that when I go to add the first element it is throwing a null pointer exception. I believe I have instantiated and object and assigned it where needed. If anyone has any insight please feel free to let me know what I am doing wrong or maybe its right in front of my face.

“myChunk.chunk_.add(element);////////////error” is where I am getting the error.

package ChunkList;


import java.util.*;
import java.io.*;

public class chunkList<T> {

    public static void main(String[] args) {

        chunkList<Integer> myList=new chunkList<Integer>();

        for(int i=1; i<24; i++)
        {
            myList.add(i);//////////////////////////////////
            System.out.println("Adding number: "+ i);
        }
        System.out.println("");

        myList.display();


    }


    private chunk head;//changed T to chunk
    private chunk tail;//changed T to chunk
    private int array_size=8;
    private int list_size;

    public chunkList()//Default Constructor
    {
        head=null;
        tail=null;
        list_size=0;
    }

    //public chunkList(chunkList copy){}// a copy constructor.... don't think I need.

    class chunk//  added <T>
    {
        //T[] chunk_arr  = new T[array_size];// illegal operation
        //ArrayList<T> chunk_ = new ArrayList<T>(array_size);
        ArrayList<T> chunk_; 

        private int chunk_size; //may need to change to public
        chunk nextChunk;//changed T to chunk
        chunk prevChunk;//changed T to chunk

        public chunk()//default constructor
        {
            chunk_ = new ArrayList<T>(array_size);
            chunk_size=0;
            nextChunk=null;
            prevChunk=null;
        }
    }

    public void add(T element)
    {
        if(this.tail==null)//empty chunk list
        {
            chunk myChunk=new chunk();//instantiate

            //myChunk.prevChunk=null;//changed from head to null
            //myChunk.nextChunk=null;//changed from tail to null
            head=myChunk;
            tail=myChunk;
            //head.nextChunk=null;
            //head.prevChunk=null;

            myChunk.chunk_.add(element);////////////error
            list_size++;
            myChunk.chunk_size=1;
        }
        else if (this.tail.chunk_size<array_size)//adds the element to the last chunk in list
        {
            this.tail.chunk_.add(element);//add element

            list_size++;
            this.tail.chunk_size++;//increase individual chunk array size
        }
        else// create new chunk, relink chunks, add element
        {
                        chunk myChunk=new chunk();

        myChunk.chunk_size=1;   
        list_size++;
        myChunk.chunk_.add(element);

        tail.nextChunk=myChunk;
        myChunk.prevChunk=tail;
        tail=myChunk;
        }}

     public int size()
         {return list_size;}

     public void display()
     {
         chunk my_chunk=head;

         if(my_chunk==null)
         {
             System.out.print("Empty Chunk List");
             return;
         }

         for(int i=0;i<list_size;  )
         {
             for(int j=0; j<my_chunk.chunk_size;  j++)
             {
                 System.out.println(my_chunk.chunk_.get(j));
                 i++;
             }
             if(my_chunk.nextChunk!=null)
             my_chunk=my_chunk.nextChunk;
         }
     }


}

So thanks Olivier Jacot-Descombes , I fixed one problem with the code and it now adds the first chunk BUT it is throwing NPE when it tries to create the next chunk. I will look at it and be back if i need more help. Thanks All.

P.S. The add method on this was incorrectly linked together in the last else statement.

Answer 1

Your code is very strange

• There is a public static void main(String[] args) inside the class chunkList<T>. This makes no sense.

• You declare a chunkList<Integer> instead of chunkList<int>.

• You re-declare a chunk<T> head and chunk<T> tail in the constructor. The code should simply be head = null; without chunk<T>.

• In the constructor of chunk you do the same thing again with ArrayList<T> chunk_ = ....

I could tell more things; however, I think that you should start by fixing these things to begin with.