Java Collections Framework's sorting algorithms

Question

I'm trying to understand how Java Collections Framework sorts its collections by default and I got confused, because I read all the collections are being sorted using merge sort. But as I took a look at Array class I saw this: «Implementors should feel free to substitute other algorithms, so long as the specification itself is adhered to. (For example, the algorithm used bysort(Object[]) does not have to be a mergesort, but it does have to be stable.)» Which means it also uses other sorting algorithms. So how exactly are the collections being sorted?

Unless you are implementing Arrays class yourself, it is using merge sort. Also, Arrays is not the same as Collections — Lucas
– Lucas, Commented Mar 19, 2013 at 15:52
Collections.sort delegates to Arrays.sort, which uses TimSort these days. — Louis Wasserman
– Louis Wasserman, Commented Mar 19, 2013 at 16:44

Joachim Sauer · Accepted Answer · 2013-03-19 15:54:26Z

5

The code to sort collections is delivered with the JRE/JDK.

Anyone who implements the JRE/JDK can choose to implement it in any way he wants, as long as it's conforming (i.e. it actually sorts the collection correctly and the sorting is stable).

Some implementations might choose merge-sort, others might choose something else. No specific implementation is required.

answered Mar 19, 2013 at 15:54

Joachim Sauer

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