java - 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: are they equivalent or is one more efficient (for example system.arraycopy() copies whole chunks)?

MyObject[] things = new MyObject[publicThings.length+privateThings.length];
System.arraycopy(publicThings, 0, things, 0, publicThings.length);
System.arraycopy(privateThings, 0, things,  publicThings.length, privateThings.length);

or

MyObject[] things = new MyObject[publicThings.length+privateThings.length];
for (int i = 0; i < things.length; i++) {
    if (i<publicThings.length){
        things[i] = publicThings[i]
    } else {
        things[i] = privateThings[i-publicThings.length]        
    }
}

Is the only difference the look of the code?

EDIT: thanks for linked question, but they seem to have an unsolved discussion:

Is it truly faster if it is not for native types: byte[], Object[], char[]? in all other cases, a type check is executed, which would be my case and so would be equivalent... no?

On another linked question, they say that the size matters a lot, for size >24 system.arraycopy() wins, for smaller than 10, manual for loop is better...

Now I'm really confused.

Answer 1

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!