Bubble Sort for Dual Linked Lists in Java

Question

I'm attempting to implement bubble sort for dual linked lists, but I get a NullPointerException.

Here is the code:

public void bubbleSort()
{           
    DLLNode<E> tmp = this.first; // (1)
    DLLNode<E> tmp2 = null; // (1)
    boolean flag = true;
    while(flag)
    {
        // (2)
        flag = false;
        while(tmp.succ!=null)
        {

            if(tmp.element2.compareTo(tmp.succ.element2)<0)
            {
                tmp2.element2 = tmp.element2;
                tmp.element2 = tmp.succ.element2;
                tmp.succ.element2 = tmp2.element2;
                tmp2.element1 = tmp.element1;
                tmp.element1 = tmp.succ.element1;
                tmp.succ.element1 = tmp2.element1;
                flag = true;
            }
            tmp = tmp.succ;
        }
    }
}

Another thing: this method that I used I think will iterate the list only once, and won't turn back at the beginning. I thought moving tmp and tmp2 from (1) to (2) would solve the problem. But I still receive a NullPointerException.

This seems to be the problem: `DLLNode tmp2 = null; // (1)` (you create a null pointer). Later on, you `tmp2.element2 = tmp.element2;`. You can't access `.element2` on `null`. Produce a [MCVE](https://stackoverflow.com/help/mcve) to receive a working answer. — ggorlen, Nov 09 '18 at 15:05
@ggorlen it works my friend. Thank you for the answer and for the edit. — VL4DiMiRG, Nov 09 '18 at 18:58

score 0 · Accepted Answer · answered Nov 09 '18 at 19:08

This line seems to be the problem:

DLLNode<E> tmp2 = null; // (1)

Here, you've created a null pointer. A few lines later, without any further assignments to tmp2, you attempt to access tmp2.element2 using this null pointer, which crashes the program:

tmp2.element2 = tmp.element2;

Re-evaluate your design and ensure all of your objects have been initialized before trying to access their properties or methods.

Bubble Sort for Dual Linked Lists in Java

1 Answers1