CODEWITHSUNDEEP
javanullpointerexceptionresizehashtable
user3011391
user3011391

Reputation: 67

Re-Sizing Hash Table

I'm attempting to resize my hash table however; I am keep getting a NullPointerException. I know if the size is greater than 0.75 then the table size has to double, if it's less than 0.50 then the table size is halved. So far I have this..

public boolean add(Object x)
{
  int h = x.hashCode();
  if (h < 0) { h = -h; }
  h = h % buckets.length;

  Node current = buckets[h];
  while (current != null)
  {
     if (current.data.equals(x)) { return false; }
        // Already in the set
     current = current.next;
  }
  Node newNode = new Node();
  newNode.data = x;
  newNode.next = buckets[h];
  buckets[h] = newNode;
  currentSize++;
  double factor1 = currentSize * load1; //load1 = 0.75
  double factor2 = currentSize * load2; //load2 = 0.50
  if (currentSize > factor1) { resize(buckets.length*2); }
  if (currentSize < factor2) { resize(buckets.length/2); }

  return true;
}

Example. Size = 3. Max Size = 5
if we take the Max Size and multiply by 0.75 we get 3.75.
this is the factor that says if we pass it the Max Size must double
so if we add an extra element into the table the size is 4 and is > 3.75 thus the new Max Size is 10.
However; once we increase the size, the hashcode will change with the addition of a new element, so we call resize(int newSize)

private void resize(int newLength)
{
  //   
   HashSet newTable = new HashSet(newLength);

   for (int i = 0; i < buckets.length; i++) {
       newTable.add(buckets[i]);
   }
}

Here is my constructor if the buckets[i] confuses anyone. 

public HashSet(int bucketsLength)
{
  buckets = new Node[bucketsLength];
  currentSize = 0;
}

I feel that the logic is correct, unless my resize method is not retrieving the elements.

Upvotes: 0

Views: 3556

Answers (1)

Blub
Blub

Reputation: 3822

If that is all your code for resize(), then you are failing to assign newTable to a class attribute, i.e. your old table. Right now you fill it with data and then don't do anything with it, since it is defined inside resize and therefore not available outside of it.

So you end up thinking you have a larger table now, but in fact you are still using the old one ;-)

Upvotes: 2

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