Sorting a part of Java ArrayList

use the subList [inherited from AbstractList] method in ArrayList. And then use Collections.sort() on that sub-list. That is if writing a highly optimised custom sort function is truly hard work.

Collections.sort(list.subList(0,3));

Note: '3' here is excluded from sorting

It is described in the documentation:

public List subList(int fromIndex, int toIndex)

Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive.