In Java 8 a default method List.sort() was added. The default implementation uses a slow copyout-sort-copyback algorithm. But specific List implementations can override sort() to provide a better implementation. For example, ArrayList.sort() sorts in-place.

In Java 8 the Arrays.parallelSort() method was also added, which does a parallel, in-place sort on an array.

Unfortunately, there doesn't appear to be an effective way to do a parallel sort on a List. Clearly one could implement the copyout-parallelsort-copyback algorithm, but this is wasteful if the List's internal representation would allow an in-place parallel sort to occur. This suggests the addition of a new default method List.parallelSort() which could be overridden by ArrayList to do in-place sorting in parallel.

See also this StackOverflow answer:

http://stackoverflow.com/a/25962666/1441122