Question

This test

        for (;;) {
            int[] a = new int[10];
            System.gc();
            long t0 = System.currentTimeMillis();
            for (int i = 0; i < 1000000; i++) {
//              int[] b =  a.clone();
                int[] b =  Arrays.copyOf(a, a.length);
            }
            System.out.println(System.currentTimeMillis() - t0);
        }

shows ~50ms for Arrays.copyOf and ~160 ms for clone. Clone is a special native method for making copies, why is it so slow?

I ran the test on my HotSpot Client JVM 1.7.0_11-b21. Note that when the array increases in size, the difference between clone and copyOf disappears.

Answer 1

I’ve found a good article explaining why clone is slow here http://www.javaspecialists.eu/archive/Issue124.html. To put it shortly, it’s because int[].clone simply uses Object.clone and this method makes two checks before copying the array:

1.Check whether instance is normal object or array.

2.Check whether array is of primitives or objects.

I added these checks to the Arrrays.copyOf test

        for (int i = 0; i < 1000000; i++) {
            Class cls = a.getClass();
            if (cls.isArray() && !cls.getComponentType().isAssignableFrom(Object.class)) {
                int[] b = Arrays.copyOf(a, a.length);
            }
        }

and the test showed no difference between clone and Arrays.copyOf. Had it been a specialized version for arrays clone would have been fast.