Java Interlude: Built-in Sorts
The Java Collections.sort() method implements (a variant) of merge sort.
The Java Arrays.sort() method implements the quicksort, selecting the pivot as the median of the three elements (first, last and middle element) when $n \leq 40$, and the median of nine equally spaced elements when $n > 40$. It also switches to the insertion sort when $n < 7$.
Why Java uses quicksort to sort a collection (array) of primitives but for collections of reference variables, it uses merge sort?
The Java API guarantees a stable sorting, which merge sort offers but quicksort doesn't offer. However, when sorting primitive values by their natural order, you would not notice a difference as primitive values have no identity. Therefore, quicksort can be used for primitive arrays and will be used when it is considered more efficient.