Question

I am trying to create a bubble sort on a doubly linked linked list in Java but am getting Null Pointer Exception errors. I believe it to have an issue with when I call the getPrevious method on the head which of course has a value of null. However, I cannot think how to do the bubble sort without accessing the getPrevious method for the other nodes.

I can implement an if statement to check if its the head or tail of the list first, but I feel like there is a smarter way to do this.

I also have been unable to run a successful build of this, so am not even sure the code will work. If you have a different idea of how to implement this please let me know.

Any suggestions are welcome!

 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=0; i<list.getSize(); i++)
      {
          for(j=0; j<list.getSize()-1; i++)
          {
              if(currentNode.getData() > currentNode.getNext().getData())
              {
                  tempNext = currentNode.getNext().getNext();
                  tempPrevious = currentNode.getPrevious();
                  currentNode.getPrevious().setNext(currentNode.getNext());
                  currentNode.getNext().setNext(currentNode);
                  currentNode.setPrevious(currentNode.getNext());
                  currentNode.setNext(tempNext);

              }

              currentNode = currentNode.getNext();

          }
      }



  }

Answer 1

So you have a double linked list. I assume each element contains some information… say an integer. It must also contain two pointers: one to the previous element and one to the next element.

Assuming this is true, notice that you don’t have to modify the pointers because they already point from one element to another. all you have to do is sort the values of the list elements so that the first item in the list has the lowest value, the second has the second lowest value and so on.

You can do it like this:

public static void bubbleSort(DoubleLinkedList list) //static method used to sort the linked list using bubble sort {
      int i = 0;
      Node currentNode = list.head;
      Node auxNode;
      int foundChange = 1;
      while(foundChange) {
        foundChange = 0;
        for(i=0; i<list.getSize()-1; i++) {
          if (currentNode.getData() > currentNode.getNext().getData()) {
            auxNode.setData(currentNode.getData());
            currentNode.setData(currentNode.getNext.getData());
            currentNode.getNext.setData(auxNode.getData());
            foundChange = 1;
          }
          currentNode = currentNode.getNext();
        }

}

If you haven’t defined the setData method yet, then do so. It must be similar to getData, but it will set the data of an object to the value it gets as a parameter instead of returning the value of the data in that object.