为什么Java的Arrays.sort方法对不同类型使用两种不同的排序算法？

Java 6's Arrays.sort method uses Quicksort for arrays of primitives and merge sort for arrays of objects. I believe that most of time Quicksort is faster than merge sort and costs less memory. My experiments support that, although both algorithms are O(n log(n)). So why are different algorithms used for different types? method uses Quicksort for