Array initialization - performance differences

Question

If I run this program the output is:

init: 1239.0 toCharArray: 1343.6 arraycopy: 1583.8

The first question is:
Why init() is faster than toCharArray()? Is there a compiler optimization here? (I am using Java 1.8.0_20)

The second question is:
Why toCharArray() is faster than arraycopy()? I copied arraycopy() from here: String.toCharArray().

public class MyClass { private static char[] SRC = { 'a', 'b', 'c', 'd', 'e', 'f', 'g' }; public static void main(String[] args) { long start = 0; int init = 0, toCharArray = 0, arraycopy = 0; for (int j = 0; j < 5; j++) { start = System.currentTimeMillis(); for (int i = 0; i < 100000000; i++) init(); init += (System.currentTimeMillis() - start); start = System.currentTimeMillis(); for (int i = 0; i < 100000000; i++) toCharArray(); toCharArray += (System.currentTimeMillis() - start); start = System.currentTimeMillis(); for (int i = 0; i < 100000000; i++) arraycopy(); arraycopy += (System.currentTimeMillis() - start); } System.out.println("init: " + init / 5.0); System.out.println("toCharArray: " + toCharArray / 5.0); System.out.println("arraycopy: " + arraycopy / 5.0); } private static void init() { char[] c = { 'a', 'b', 'c', 'd', 'e', 'f', 'g' }; doSomething(c); } private static void toCharArray() { char[] c = "abcdefg".toCharArray(); doSomething(c); } private static void arraycopy() { char[] c = new char[SRC.length]; System.arraycopy(SRC, 0, c, 0, SRC.length); doSomething(c); } private static void doSomething(char[] c) { for (int i = 0; i < c.length; i++) c[i] = ' '; } }

EDIT

Caliper result:

init: min=11.90, 1st qu.=12.27, median=12.48, mean=12.44, 3rd qu.=12.54, max=13.16 toCharArray: min=13.10, 1st qu.=13.21, median=13.39, mean=13.49, 3rd qu.=13.78, max=14.27 arraycopy: min=15.42, 1st qu.=15.49, median=15.51, mean=15.51, 3rd qu.=15.55, max=15.58

These results can be ignored as you haven't warmed the JVM. Either learn how to microbench Java or use a testing library like Caliper. — Boris the Spider
– Boris the Spider, Commented Sep 19, 2014 at 22:49
Yup, test is flawed, if you switch toCharArray() and arraycopy() around I bet the results will be different — Steve Kuo
– Steve Kuo, Commented Sep 19, 2014 at 23:03
I changed the order: arraycopy: 1584.2 toCharArray: 1364.0. But thanks for the tip, I will try Caliper! — Krayo
– Krayo, Commented Sep 19, 2014 at 23:13

Steve Kuo · Accepted Answer · 2014-09-20 00:23:16Z

For "abcdefg".toCharArray(), String.toCharArray()'s source code is

public char[] toCharArray() { char result[] = new char[count]; getChars(0, count, result, 0); return result; }

getChars calls System.arraycopy so its performance with your arraycopy() should be the same. However, String's getChars copies from its internal char[] field, which is declared as final

private final char value[];

Where as your arraycopy() copies from SRC, which is non-final

private static char[] SRC = { 'a', 'b', 'c', 'd', 'e', 'f', 'g' };

This is just a guess, but trying making SRC final and see what happens.

You're right! After I make SRC final the result is: toCharArray: 1332.2 arraycopy: 1322.6. Thanks! So here is optimization. And do you have any idea why init() a bit faster than others?

Community · Accepted Answer · 2017-05-23 12:20:28Z

My first guess for my first question was the following:

I thought that the difference between init() and toCharArray() is that the init() skips array initialization with default values. But then I checked the bytecode and I haven't seen any difference.

Later I found this, and when I modified my test to work with a larger array, I realized that toCharArray() is faster!

To my second question I got the answer (thanks again).

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