Thread: NullPointerException... Desperately need help

I am trying to create a Hash table using chaining method.

I create an array of pointers that are all instantiated with data = 0 and pointer = null.

I get the error "Exception in thread "main" java.lang.NullPointerException
at HashChaining.main(HashChaining.java:341)"

I am very confused and been trying to solve this for a while. I know it is a simple fix. I am especially confused because there should be no way that the getData() function would return null, it should only be able to return zero.

The part that throws the exception:

while (i < HashSize)
     {
           Node CurrentNode = HashTable[i];
           if (CurrentNode != null)
           {
                 NodeData = CurrentNode.getData();
           }
           if (NodeData != 0)
           {
                    System.out.println(i + ": ");
                    while(true)
                    {
 
                           System.out.println(CurrentNode.getData() + " ");
                           CurrentNode = CurrentNode.getLink();                                    <----------- Exception occurs on this line.
                           NodeData = CurrentNode.getData();
 
                           if (NodeData == 0)
                           {
                                  break;
                            }
                    }
                     i++;
            }
            else
            {
                     System.out.println(i + ": Empty");
                     i++;
              }			
}

And the entire program:

import java.util.*; 
import java.io.*;
 
/*String UserInputCommands = UserInput.nextLine();
		String[] parts = UserInputCommands.split("\\.");
		int size = Integer.parseInt(parts[0]);
		HashTable = new Node[size];*/
 
class Node
{
    protected int data;
    protected Node link;
 
    /*  Constructor  */
    public Node()
    {
        link = null;
        data = 0;
    }    
    /*  Constructor  */
    public Node(int d,Node n)
    {
        data = d;
        link = n;
    }    
    /*  Function to set link to next Node  */
    public void setLink(Node n)
    {
        link = n;
    }    
    /*  Function to set data to current Node  */
    public void setData(int d)
    {
        data = d;
    }    
    /*  Function to get link to next node  */
    public Node getLink()
    {
        return link;
    }    
    /*  Function to get data from current Node  */
    public int getData()
    {
        return data;
    }
}
 
/* Class linkedList */
class linkedList
{
    protected Node start;
    protected Node end ;
    public int size ;
 
    /*  Constructor  */
    public linkedList()
    {
        start = null;
        end = null;
        size = 0;
    }
    /*  Function to check if list is empty  */
    public boolean isEmpty()
    {
        return start == null;
    }
    /*  Function to insert an element at begining  */
    public void insertAtStart(int val)
    {
		Node nptr = new Node(val, null);    
		size++ ;    
		if(start == null) 
		{
			start = nptr;
			end = start;
		}
		else 
		{
			nptr.setLink(start);
			start = nptr;
		}
    }
    /*  Function to insert an element at end  */
    public void insertAtEnd(int val)
    {
        Node nptr = new Node(val,null);    
        size++ ;    
        if(start == null) 
        {
            start = nptr;
            end = start;
        }
        else 
        {
            end.setLink(nptr);
            end = nptr;
        }
    }
    /*  Function to delete an element at position  */
    public void deleteAtPos(int pos)
    {        
        if (pos == 1) 
        {
            start = start.getLink();
            size--; 
            return ;
        }
        if (pos == size) 
        {
            Node s = start;
            Node t = start;
            while (s != end)
            {
                t = s;
                s = s.getLink();
            }
            end = t;
            end.setLink(null);
            size --;
            return;
        }
        Node ptr = start;
        pos = pos - 1 ;
        for (int i = 1; i < size - 1; i++) 
        {
            if (i == pos) 
            {
                Node tmp = ptr.getLink();
                tmp = tmp.getLink();
                ptr.setLink(tmp);
                break;
            }
            ptr = ptr.getLink();
        }
        size-- ;
    }    
    /*  Function to display elements  */
    public void display()
    {
        System.out.print("\nSingly Linked List = ");
        if (size == 0) 
        {
            System.out.print("empty\n");
            return;
        }    
        if (start.getLink() == null) 
        {
            System.out.println(start.getData() );
            return;
        }
        Node ptr = start;
        System.out.print(start.getData()+ "->");
        ptr = start.getLink();
        while (ptr.getLink() != null)
        {
            System.out.print(ptr.getData()+ "->");
            ptr = ptr.getLink();
        }
        System.out.print(ptr.getData()+ "\n");
    }
}
 
public class HashChaining
{
	public static void main(String[] args)
	{
		int HashSize = 10; //Default Size of Hash
		Node[] HashTable = new Node[HashSize];
 
		System.out.println("\n\n\n============================================================");
		System.out.println("Welcome to the Chaining Hash tool.");
		System.out.println("There are 3 commands: Insertion (X.in) Deletion (Y.del) Search (Z.sch)\n");
		System.out.println("The Hash table is currently empty.\n");
		System.out.println("Enter a series of commands in the format: \"1.in 2.in del 3.in ...\"\n");
 
		Scanner UserInput = new Scanner(System.in);
		String UserInputCommands = UserInput.nextLine();
 
		int i = 0;
		int j = 0;
		int k = 0;
		int NodeData = 0;
		int HashIndex = 0;
		int ContinueRunning = 1;
		int singleCharInteger = 0;
		int TemporaryHashValue = 0;
		int CommandNumberLength = 0;
		int UserInputLength = UserInputCommands.length();
 
		String singleChar = "";
		String CommandNumber = "";
		String[] CommandNumberDigits = new String[3];
		StringReader Commands = new StringReader(UserInputCommands);
 
		/* while (i < HashSize)
		{
			HashTable[i].setData(0);
			i++;
		}
		i = 0; */
 
		while (ContinueRunning == 1)
		{
			while (i < UserInputLength - 1)
			{
				i++;
				try
				{
					singleCharInteger = Commands.read();
					singleChar = Character.toString((char) singleCharInteger);
				}
				catch(IOException err)
				{
					//
				}
 
				if ((singleChar.equals("0")) || (singleChar.equals("1")) || (singleChar.equals("2")) || (singleChar.equals("3")) || (singleChar.equals("4")) || (singleChar.equals("5")) || (singleChar.equals("6")) || (singleChar.equals("7")) || (singleChar.equals("8")) || (singleChar.equals("9")))
				{
					CommandNumberDigits[j] = singleChar;
					j++;
				}
				else if (singleChar.equals("."))
				{
					StringBuilder ConcatenateCommandNumberDigits = new StringBuilder();
					for(String s : CommandNumberDigits)
					{
						ConcatenateCommandNumberDigits.append(s);
					}
 
					CommandNumber = ConcatenateCommandNumberDigits.toString();
					CommandNumberLength = CommandNumber.length();
 
					StringReader RemoveNullsFromCommandNumber = new StringReader(CommandNumber);
 
					while (k <= 3)
					{
						try
						{
							singleCharInteger = RemoveNullsFromCommandNumber.read();
							singleChar = Character.toString((char) singleCharInteger);
							if (singleChar.equals("n"))
							{
								break;
							}
							if (k == 3)
							{
								break;
							}
							k++;
						}
						catch(IOException err)
						{
							System.out.println("There was an error. This is most likely caused by input of a number greater than 3 digits.");
						}
					}
 
					if (k == 1)
					{	
						CommandNumber = CommandNumber.substring(0,1);
					}
					else if (k == 2)
					{
						CommandNumber = CommandNumber.substring(0,2);
					}
 
					k = 0;
 
					try
					{
						singleCharInteger = Commands.read();
						singleChar = Character.toString((char) singleCharInteger);
					}
					catch(IOException err)
					{
						System.out.println("There was an error. This is most likely caused by input of a number greater than 3 digits.");
					}
 
					/////////////////////////////////////////////////////
					if (singleChar.equals("i"))
					{
						TemporaryHashValue = Integer.parseInt(CommandNumber);
						HashIndex = TemporaryHashValue % 10;
 
						Node NewPointer = new Node(TemporaryHashValue, null);
 
						if (HashTable[HashIndex] == null) 
						{
							HashTable[HashIndex] = NewPointer;
						}
						else 
						{
							Node PreviousHead = new Node(HashTable[HashIndex].getData(), null);
							NewPointer.setLink(PreviousHead);
						}
 
 
 
						Arrays.fill(CommandNumberDigits, null);
						j = 0;
					}
 
					/////////////////////////////////////////////////////
					else if (singleChar.equals("d"))
					{
 
 
 
 
						Arrays.fill(CommandNumberDigits, null);
						j = 0;
					}
 
					//////////////////////////////////////////////////////
					else if (singleChar.equals("s"))
					{
 
 
 
 
						Arrays.fill(CommandNumberDigits, null);
						j = 0;
					}
					/////////////////////////////////////////////////////
				}
			}
 
			System.out.println("\nThe Hash table is currently:\n");
 
			i = 0;
			j = 0;
			while (i < HashSize)
			{
				Node CurrentNode = HashTable[i];
				if (CurrentNode != null)
				{
					NodeData = CurrentNode.getData();
				}
				if (NodeData != 0)
				{
					System.out.println(i + ": ");
					while(true)
					{
 
						System.out.println(CurrentNode.getData() + " ");
						CurrentNode = CurrentNode.getLink();
						NodeData = CurrentNode.getData();
 
						if (NodeData == 0)
						{
							break;
						}
					}
					i++;
				}
				else
				{
					System.out.println(i + ": Empty");
					i++;
				}			
			}
 
			System.out.println(Arrays.toString(HashTable));
			System.out.println("\nEnter another command or enter [/] to exit.\n");
			String CommandOrStop = UserInput.nextLine();
			if (CommandOrStop.equals("/"))
			{
				ContinueRunning = 0;
			}
			else
			{
				i = 0;
 
				UserInputCommands = CommandOrStop;
				UserInputLength = UserInputCommands.length();
 
				Commands = new StringReader(UserInputCommands);
			}
		}
		System.out.println("\nThe Hash program has finished");
	}
}

It looks like CurrentNode is null. Are you not getting a NullPointerException on this line, just above the one you marked in comments?

System.out.println(CurrentNode.getData() + " ");

Your HashTable array is populated based on user inputs. Are you sure every element in the array is guaranteed to be non-null?

Node CurrentNode = HashTable[i];

I think you're getting a null 'CurrentNode' object from this. Add a print statement after that line to verify.