sorting - Bubble sort in functional style Java 8

Question

How would you implement the following Bubble Sort algorithm in a functional (Java 8) way?

public static final <T extends Comparable<T>> List<T> imperativeBubbleSort(List<T> list) {
    int len = list == null ? 0 : list.size();
    for (int j = len - 1; j > 0; j--) {
        for (int k = 0; k < j; k++) {
            if (list.get(k + 1).compareTo(list.get(k)) < 0) {
                list.add(k, list.remove(k + 1));
            }
        }
    }
    return list;
}

Answer 1

It would depend on what you mean by functional. If you mean just passing around functions as first class objects, then you should change your method signature to be:

public static final <T> List<T> imperativeBubbleSort(List<T> list, Comparator<T> comparisonFunction)

This way the comparison logic can be supplied as an argument.

If you mean going fully functional and not at all procedural, then I would call it an anti-pattern. Despite what you might hear, Java 8 does not fully support functional programming. A key feature that it is missing is tail-call optimization. Without it, the sort of loop-less programming that defines functional programming is likely to crash your JVM for relatively small values.

More information about tail call optimizations and the JVM can be found here: http://www.drdobbs.com/jvm/tail-call-optimization-and-java/240167044