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Thread: Priority Queue Null Pointer Exception

  1. #1
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    Default Priority Queue Null Pointer Exception

    Hello!

    I am currently working on an assignment and I have been stuck with a NullPointerException.

    I am making a Priority Queue (PQ) as one part of the assignment i'm working on.

    Firstly, if I add nodes into the queue in ascending order I do not get an error while adding to the queue (1,3,5,6, etc), but I do get the error when putting them out of order (4,2,5).

    So this tells me there is a problem in my balance function.

    Secondly, when removing Nodes from the PQ (input ascending order 4,5,6) it keeps removing the first one only (4,4,4). So there is some problem with my "leave" function.

    This is the exact error:
    "java.lang.NullPointerException
    at HEAP.PQ.balance(PQ.java:68)
    at HEAP.PQ.enter(PQ.java:54)
    at HEAP.Dijkstras.<init>(Dijkstras.java:28)
    at HEAP.Dijkstras.main(Dijkstras.java:54)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Nativ e Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknow n Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Un known Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.ru nCommand(JavacCompiler.java:272)"



    Any help with finding where the error is occurring would be greatly appreciated!

    Also any other suggestions are welcomed.

    Thanks in advance!

    Here is my PQ class:

    public class PQ
    {
      public Node[] array = new Node[30];
      public int max = 0;
     
      public PQ()
      {
     
      }
     
      //function to check if the PQ is empty, returns true if it is
      public boolean isEmpty()
      {
        if(array[1] == null)
          return true;
        else
          return false;
      }
     
      //function to add to the PQ
      public void enter(Node n)
      {
        int count = 1;
     
        if(array[1] == null)
        {
          array[1] = n;
        }
     
        else
        {
        while(array[count] != null)
        {
          count++;
          System.out.println(count);
        }
     
        array[count] = n;
        }
     
        max = max + 1;
        balance(max);
      }
     
      //function to balance the PQ after an add
      private void balance(int count)
      {
        Node temp;
        if(array[2] == null && array[3] == null)
        {
          return;
        }
     
        else
        {
          while(array[count/2].key > array[count].key)
          {
            if(count == 1)
            {
              if(array[1].key > array[count].key)
              {
                temp = array[1];
                array[1] = array[count];
                array[count] = temp;
              }
     
              break;
            }
     
            temp = array[count/2];
            array[count/2] = array[count];
     
            array[count] = temp;
            count = count/2;
     
          }
        }
      }
     
      //function to remove from the PQ
      public Node leave()
      {
        Node temp;
     
        //check if array only has 1 node
        if(array[2] == null && array[3] == null)
        {
          temp = array [1];
          array[1] = null;
          max = max - 1;
          return temp;
        }
     
        temp = array [1];
          array[1] = array[max];
          array[max] = null;
          topBal(1);
          max = max - 1;
          return temp;
      }
     
      //function to balance PQ after remove
      private void topBal(int temp)
      {
        Node temp2;
     
        if(array[1] == null)
          return;
     
        else
        {
          //only if right is null, if so check if left is greater, if so swap
          if(array[temp*2] != null && array[(temp*2)+1] == null)
          { 
     
            if(array[temp*2].key > array[temp].key)
            {
             temp2 = array[temp]; 
             array[temp] = array[temp*2];
             array[temp*2] = temp2;
             topBal(temp*2);
            }
          }
     
           //only if left is null, if so check if right is greater, if so swap
          if(array[temp*2] == null && array[(temp*2)+1] != null)
          { 
            if(array[(temp*2)+1].key > array[temp].key)
            {
             temp2 = array[temp]; 
             array[temp] = array[(temp*2) + 2];
             array[(temp*2)+1] = temp2;
             topBal((temp*2)+1);
            }
          }
     
          //if both are not null, check which is larger & if they are larger then current node, swap if so
          if(array[temp] != null && array[temp*2] != null && array[(temp*2) + 1] !=null)
          {
            //if left is larger
           if(array[temp*2].key > array[(temp*2)+1].key && array[temp*2].key > array[temp].key)
           {
             temp2 = array[temp];
             array[temp] = array[temp*2];
             array[temp*2] = temp2;
             topBal(temp*2);
           }
     
           //if right is larger
           if(array[temp*2].key < array[(temp*2)+1].key && array[(temp*2)+1].key > array[temp].key)
           {
             temp2 = array[temp]; 
             array[temp] = array[(temp*2) + 2];
             array[(temp*2)+1] = temp2;
             topBal((temp*2)+1); 
           }
          }
        }
      }
    }

  2. The Following User Says Thank You to OverZealous For This Useful Post:

    ranjeet1523 (March 8th, 2014)


  3. #2
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    Default Re: Priority Queue Null Pointer Exception

    at HEAP.PQ.balance(PQ.java:68)

    Something on line 68 is null. Add print statements before this line to find out what is null. Once you know what is null then trace your code and find out why it is null.
    Improving the world one idiot at a time!

  4. #3
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    Default Re: Priority Queue Null Pointer Exception

    Thank you sir, I didn't know that was the line the error was on!

    I will check it out and let you know how it goes.

  5. #4
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    Default Re: Priority Queue Null Pointer Exception

    Thanks, I found what was causing the problem for this!

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