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Thread: NullPointerException... Desperately need help

  1. #1
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    Join Date
    Oct 2017
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    Default 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");
    	}
    }
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    Last edited by mikbferguson; October 18th, 2017 at 02:28 AM.

  2. #2
    Member
    Join Date
    Apr 2014
    Posts
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    Thanked 6 Times in 6 Posts

    Default Re: NullPointerException... Desperately need help

    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.
    Last edited by BinaryDigit09; October 20th, 2017 at 01:14 PM. Reason: Forum commit fail

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