Question

I am getting a null pointer exception before I can even get to my code. I cannot tell if it will work correctly cause I can’t make it pass this point.

The error is thrown in this line:

if(currentNode.getData() > currentNode.getNext().getData())

Here is my bubble sort:

public static void bubbleSort(DoubleLinkedList list) //static method used to sort the linked list using bubble sort
      {
          int i = 0;
          int j = 0;
          Node currentNode = list.head;
          Node previousNode = currentNode;
          Node tempNext =  currentNode;
          Node tempPrevious = currentNode;


          for(i=1; i<list.getSize(); i++)
          {
              for(j=0; j<list.getSize()-1; j++)
              {
                  if(currentNode.getData() > currentNode.getNext().getData())
                  {
                      if(currentNode == list.head)
                      {
                          Node tempNode = currentNode.getNext();

                          list.head = tempNode;
                          tempNext = tempNode.getNext();

                          tempNode.setNext(currentNode);
                          currentNode.setNext(tempNext);
                          currentNode.setPrevious(tempNode);

                          tempNext.setPrevious(currentNode);
                      }
                      else if(currentNode.getNext() == list.last)
                      {
                          Node tempNode = currentNode.getNext();
                          list.last = currentNode;
                          tempPrevious = currentNode.getPrevious();

                          tempNode.setNext(currentNode);
                          tempNode.setPrevious(tempPrevious);

                          currentNode.setPrevious(tempNode);
                      }
                      else
                      {
                          Node tempNode = currentNode.getNext();
                          tempPrevious = currentNode.getPrevious();
                          tempNext = currentNode.getNext();

                          tempPrevious.setNext(tempNode);
                          tempNext.setPrevious(currentNode);

                          currentNode.setNext(tempNext);
                          tempNode.setPrevious(tempPrevious);
                      }

                  }

                  currentNode = currentNode.getNext();

              }
          }



      }

I thought the limits on my loops would keep the compiler from trying to access a pointer that does not necessarily exist which is what I understand a null pointer exception to be.

If anyone could help me get past this error so that I can test my bubble sort I’d be extremely grateful!

Answer 1

Either currentNode is null or currentNode.getNext() returns null and that’s why you cant call getData() on it. You should always check your nodes for null values before you use them.