What sort does Java Collections.sort(nodes) use?

Question

I think it is MergeSort, which is O(n log n).

However, the following output disagrees:

-1,0000000099000391,0000000099000427
1,0000000099000427,000000         


        

Answer 1

I've written some stuff you may be interested in about the Java sort algorithm and taken some performance measurements of Collections.sort(). The algorithm at present is a mergesort with an insertion sort once you get down to a certain size of sublists (N.B. this algorithm is very probably going to change in Java 7).

You should really take the Big O notation as an indication of how the algorithm will scale overall; for a particular sort, the precise time will deviate from the time predicted by this calculation (as you'll see on my graph, the two sort algorithms that are combined each have different performance characteristics, and so the overall time for a sort is a bit more complex).

That said, as a rough guide, for every time you double the number of elements, if you multiply the expected time by 2.2, you won't be far out. (It doesn't make much sense really to do this for very small lists of a few elements, though.)