Arrays.sort(Object[] a) - how is it implemented?

Question

Are there any resources on how the mergeSort used by Arrays.sort(Object[] a) is implemented? While it is documented quite good, I have a hard time understanding it (especially why the src and dest are are switched when mergeSort() get's recursively called).

Answer 1

Here is the source of java.util.Arrays.

Actually, you have that source code in the JDK - just open java.util.Arrays in your IDE and the source code + the comments will appear. If you don't have an IDE, look at JDK_HOME\src.zip

Then, put it in your IDE and trace how it works.

• put breakpoints (and run a program in debug mode)
• use System.out.println(..)
• change parts of it to see how they are reflected.
• read the wikipedia article about merge sort
• pay attention to this comment: // Recursively sort halves of dest into src

Answer 2

This article could help you to understand mergesort better before you go into the (optimzed) libary code.

Answer 3

I understand how the algorithm works, but what I don't understand why the recursion in the end always returns the dest array (switching dest and src on any recursive call). Here's the implementation:

       private static void mergeSort(Object[] src,
                                      Object[] dest,
                                      int low,
                                      int high,
                                      int off) {
            int length = high - low;

            // Insertion sort on smallest arrays
            if (length < INSERTIONSORT_THRESHOLD) {
                for (int i=low; i<high; i++)
                    for (int j=i; j>low &&
                             ((Comparable) dest[j-1]).compareTo(dest[j])>0; j--)
                        swap(dest, j, j-1);
                return;
            }


            // Recursively sort halves of dest into src
            int destLow  = low;
            int destHigh = high;
            low  += off;
            high += off;
            int mid = (low + high) >>> 1;
            mergeSort(dest, src, low, mid, -off);
            mergeSort(dest, src, mid, high, -off);

            // If list is already sorted, just copy from src to dest.  This is an
            // optimization that results in faster sorts for nearly ordered lists.
            if (((Comparable)src[mid-1]).compareTo(src[mid]) <= 0) {
                System.arraycopy(src, low, dest, destLow, length);
                return;
            }

            // Merge sorted halves (now in src) into dest
            for(int i = destLow, p = low, q = mid; i < destHigh; i++) {
                if (q >= high || p < mid && ((Comparable)src[p]).compareTo(src[q])<=0)
                    dest[i] = src[p++];
                else
                    dest[i] = src[q++];
            }
        }