Java: sorting an ArrayList in place

You can extract the underlying array (e.g. reflection) and perform a Arrays.sort(array, 0, list.size()) on it.

Java 7 does not copy the array in Arrays.sort() before sorting the array. In Java 6 it does which means that Collections.sort() in Java 6 actually copies the underlying array TWICE to perform the sort.

As of Java 8 List interface has default void sort(Comparator<? super E> c) API, using which you can sort an instance in place.

The list must be modifiable, iterable and its elements comparable to each other.

Collections.sort() was made to work with any List implementation, and that's why it's not working in place (merging LinkedList in place is difficult and sacrifices stability).

If you are really worried about sorting in-place you'll have to roll out your own sort funcion. It's not really worth your time unless your List is very long.

Arrays.sort(Object[]) does the same mergesort and is called internally by Collections.sort() (at least in openjdk)