What is more efficient: System.arraycopy or Arrays.copyOf?

While System.arraycopy is implemented natively, and is therefore could be¹ faster than a Java loop, it is not always as fast as you might expect. Consider this example:

Object[] foo = new Object[]{...};
String[] bar = new String[foo.length];

System.arraycopy(foo, 0, bar, 0, bar.length);

In this case, the foo and bar arrays have different base types, so the implementation of arraycopy has to check the type of every reference copied to make sure that it is actually a reference to a String instance. That is significantly slower than a simple C-style memcopy of the array contents.

The other point is that Arrays.copyOf uses System.arraycopy under the hood. Therefore System.arraycopy is on the face of it should not be slower² than Arrays.copyOf. But you can see (from the code quoted above) that Arrays.copyOf will in some cases use reflection to create the new array. So the performance comparison is not straightforward.

There are a couple of flaws in this analysis.

1. We are looking at the implementation code from a specific version of Java. These methods may change, invalidating previous assumptions about efficiency.

2. We are ignoring the possibility that the JIT compiler could do some clever special case optimization for these methods. And it apparently this does happen with Arrays.copyOf; see Why is Arrays.copyOf 2 times faster than System.arraycopy for small arrays?. This method is "intrinsic" in current-generation Java implementations, which means that the JIT compiler will ignore what is in the Java source code!

But either way, the difference between the two versions is O(1) (i.e. independent of array size) and relatively small. Therefore, my advice would be to use the version that makes your code easiest to read, and only worry about which one is faster if profiling tells you that it matters.

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^{1 - It could be faster, but it is also possible that the JIT compiler does such a good job of optimizing a hand-coded loop that there is no difference.}

The difference is that Arrays.copyOf does not only copy elements, it also creates a new array. System.arraycopy copies into an existing array.

Here is the source for Arrays.copyOf, as you can see it uses System.arraycopy internally to fill up the new array:

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

If you want an exact copy of an array (say, if you want to do a defensive copy), the most effective way of copying an array is probably using the array object's clone() method:

class C {
    private int[] arr;
    public C(int[] values){
        this.arr = values.clone();
    }
}

I haven't bothered to test the performance of it, but it stands a good chance to be pretty fast since it's all native (allocation and copying in call), and cloning is kind of a special JVM blessed way of copying objects (and it's mostly evil for other purposes) and is likely to be able to take some "shortcuts".

Personally, I'd still use clone if it was slower than any other way of copying, because it's easier to read and nigh-impossible to screw up when writing. System.arrayCopy, on the other hand...