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!

Answer 2

Arrays.copyOf(T[], int) is easier to read. Internaly it uses System.arraycopy() which is a native call.

You can't get it faster!

Answer 3

It depends on the virtual machine, but System.arraycopy should give you the closest you can get to native performance.

I've worked for 2 years as a java developer for embedded systems (where performance is a huge priority) and everywhere System.arraycopy could be used, I've mostly used it / seen it used in existing code. It's always preferred over loops when performance is an issue. If performance isn't a big issue, I'd go with the loop, though. Much easier to read.

Answer 4

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:

Answer 5

It is not possible that Arrays.copyOf is faster than System.arraycopy since this is the implementation of copyOf:

public static int[] copyOf(int[] original, int newLength) {
    int[] copy = new int[newLength];
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

Answer 6

Executing native methods like Arrays.copyOf(T[], int) does have some overhead but it doesnot mean that it is not fast as you are executing it using JNI.

The easiest way is to write a benchmark and test.

You can check that Arrays.copyOf(T[], int) is faster than your normal for loop.

The benchmark code from here:-

public void test(int copySize, int copyCount, int testRep) {
    System.out.println("Copy size = " + copySize);
    System.out.println("Copy count = " + copyCount);
    System.out.println();
    for (int i = testRep; i > 0; --i) {
        copy(copySize, copyCount);
        loop(copySize, copyCount);
    }
    System.out.println();
}

public void copy(int copySize, int copyCount) {
    int[] src = newSrc(copySize + 1);
    int[] dst = new int[copySize + 1];
    long begin = System.nanoTime();
    for (int count = copyCount; count > 0; --count) {
        System.arraycopy(src, 1, dst, 0, copySize);
        dst[copySize] = src[copySize] + 1;
        System.arraycopy(dst, 0, src, 0, copySize);
        src[copySize] = dst[copySize];
    }
    long end = System.nanoTime();
    System.out.println("Arraycopy: " + (end - begin) / 1e9 + " s");
}

public void loop(int copySize, int copyCount) {
    int[] src = newSrc(copySize + 1);
    int[] dst = new int[copySize + 1];
    long begin = System.nanoTime();
    for (int count = copyCount; count > 0; --count) {
        for (int i = copySize - 1; i >= 0; --i) {
            dst[i] = src[i + 1];
        }
        dst[copySize] = src[copySize] + 1;
        for (int i = copySize - 1; i >= 0; --i) {
            src[i] = dst[i];
        }
        src[copySize] = dst[copySize];
    }
    long end = System.nanoTime();
    System.out.println("Man. loop: " + (end - begin) / 1e9 + " s");
}

public int[] newSrc(int arraySize) {
    int[] src = new int[arraySize];
    for (int i = arraySize - 1; i >= 0; --i) {
        src[i] = i;
    }
    return src;
}

System.arraycopy() uses JNI (Java Native Interface) to copy an array (or parts of it), so it is blazingly fast, as you can confirm here

Answer 7

System.arraycopy() is a native call which does copy operation directly at memory. Single memory copy would be always faster than your for loop