Sorting a List in parallel without creating a temporary array in Java 8

Question

Java 8 provides java.util.Arrays.parallelSort, which sorts arrays in parallel using the fork-join framework. But there\'s no corresponding Collections.parallelSort

Answer 1

I think you are doomed to use a custom List implementation augmented with your own parallelSort or else change all your other code to store the big data in Array types.

This is the inherent problem with layers of abstract data types. They're meant to isolate the programmer from details of implementation. But when the details of implementation matter - as in the case of underlying storage model for sort - the otherwise splendid isolation leaves the programmer helpless.

The standard List sort documents provide an example. After the explanation that mergesort is used, they say

The default implementation obtains an array containing all elements in this list, sorts the array, and iterates over this list resetting each element from the corresponding position in the array. (This avoids the n2 log(n) performance that would result from attempting to sort a linked list in place.)

In other words, "since we don't know the underlying storage model for a List and couldn't touch it if we did, we make a copy organized in a known way." The parenthesized expression is based on the fact that the List "i'th element accessor" on a linked list is Omega(n), so the normal array mergesort implemented with it would be a disaster. In fact it's easy to implement mergesort efficiently on linked lists. The List implementer is just prevented from doing it.

A parallel sort on List has the same problem. The standard sequential sort fixes it with custom sorts in the concrete List implementations. The Java folks just haven't chosen to go there yet. Maybe in Java 9.