Is it better to use System.arraycopy(...) than a for loop for copying arrays?
public void testHardCopyBytes()
{
byte[] bytes = new byte[0x5000000]; /*~83mb buffer*/
byte[] out = new byte[bytes.length];
for(int i = 0; i < out.length; i++)
{
out[i] = bytes[i];
}
}
public void testArrayCopyBytes()
{
byte[] bytes = new byte[0x5000000]; /*~83mb buffer*/
byte[] out = new byte[bytes.length];
System.arraycopy(bytes, 0, out, 0, out.length);
}
I know JUnit tests aren't really the best for benchmarking, but
testHardCopyBytes took 0.157s to complete
and
testArrayCopyBytes took 0.086s to complete.
I think it depends on the virtual machine, but it looks as if it copies blocks of memory instead of copying single array elements. This would absolutely increase performance.
EDIT:
It looks like System.arraycopy 's performance is all over the place.
When Strings are used instead of bytes, and arrays are small (size 10),
I get these results:
String HC: 60306 ns
String AC: 4812 ns
byte HC: 4490 ns
byte AC: 9945 ns
Here is what it looks like when arrays are at size 0x1000000. It looks like System.arraycopy definitely wins with larger arrays.
Strs HC: 51730575 ns
Strs AC: 24033154 ns
Bytes HC: 28521827 ns
Bytes AC: 5264961 ns
How peculiar!
Thanks, Daren, for pointing out that references copy differently. It made this a much more interesting problem!
Arrays.copyOf(T[], int)
is easier to read.
Internaly it uses System.arraycopy()
which is a native call.
You can't get it faster!