My SelectionSort method does not work. Why?

Question

I am trying to use the selection sort algorithm in a version of the doubly linked list that I wrote myself. For this question we can assume that there are no errors elsewhere other than the code that I post (at least, none relevant to the question). I have done plenty of testing.

here is my method:

public void selectionSort(){

    ListItem front = head;
    ListItem current;
    T currentLowest;
    T potentialLowest;
    int lowestIndex = 0;
    for (int a = 0; a<count-1; a++){
        System.out.println("a: "+a);
        currentLowest = (T) front.content;
        front = front.next;
        current = front.next;
    for(int i = a+1; i<count; i++){
        System.out.println("i: "+i);
**(29)**    potentialLowest = (T) current.content;
        if (potentialLowest.compareTo(currentLowest)==-1)
        {
            currentLowest = (T) current.content;
            lowestIndex = i;
        }
        if(current.next == null)break;

        current = current.next;
    }
    System.out.println("swapped"+a+","+lowestIndex);
    swap(a, lowestIndex);
}

}

It is sorting a list of 100 integers. Here is the last bit of output before I receive a null pointer on line 29 (marked).

swapped95,97

a: 96 i: 97 i: 98

swapped96,97

a: 97 i: 98

swapped97,97

a: 98 i: 99 (null pointer)

I had this working earlier but it was horribly optimized. After making some changes, I'm stuck with this. Any ideas?

Thanks for your time.

Answer 1

Well you're trying to access the content of a null element. When you're on the last element, your "current" will null when you set it to next.

I think I'm a little too tired to provide a fix for it, but you should be able to compare your old (working) code to it and spot the fix.

Answer 2

I think the problem might arise in the first iteration of your sorting loop. Considering the first line in this function ( ListItem front = head ) points the front to the first element of the list, it seems that by calling: front = front.next; current = front.next; front = front.next; current = front.next; you actually 'skip' the element at index 1 in the list and start your comparison loop of the element at index 2.

For example, if your (unsorted) list looks like this:

[54, 11, 25, 34]

It will look like

[25, 11, 54, 34]

after the first iteration of your sorting algorithm. Since the next iteration will start at index 1, the element 11 will never be placed at index 0 even though it is the lowest element in the list.

It might be this inaccuracy which causes the null pointer problem at the end of the list. I would consider putting the statement front = front.next; after the inner for loop and before the swap(a, lowestIndex); statement. This will prevent the possible error in the first iteration and might solve your problem.