Is it better to use System.arraycopy(…) than a for loop for copying arrays?

Question

I want to create a new array of objects putting together two smaller arrays.

They can\'t be null, but size may be 0.

I can\'t chose between these two ways: a

Answer 1

Instead of relying on speculation and possibly outdated information, I ran some benchmarks using caliper. In fact, Caliper comes with some examples, including a CopyArrayBenchmark that measures exactly this question! All you have to do is run

mvn exec:java -Dexec.mainClass=com.google.caliper.runner.CaliperMain -Dexec.args=examples.CopyArrayBenchmark

My results are based on Oracle's Java HotSpot(TM) 64-Bit Server VM, 1.8.0_31-b13, running on a mid-2010 MacBook Pro (macOS 10.11.6 with an Intel Arrandale i7, 8 GiB RAM). I don't believe that it's useful to post the raw timing data. Rather, I'll summarize the conclusions with the supporting visualizations.

In summary:

• Writing a manual for loop to copy each element into a newly instantiated array is never advantageous, whether for short arrays or long arrays.
• Arrays.copyOf(array, array.length) and array.clone() are both consistently fast. These two techniques are nearly identical in performance; which one you choose is a matter of taste.
• System.arraycopy(src, 0, dest, 0, src.length) is almost as fast as Arrays.copyOf(array, array.length) and array.clone(), but not quite consistently so. (See the case for 50000 ints.) Because of that, and the verbosity of the call, I would recommend System.arraycopy() if you need fine control over which elements get copied where.

Here are the timing plots: