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

The toArray method in ArrayList, Bloch uses both System.arraycopy and Arrays.copyOf to copy an array.

public <T> T[] toArray(T[] a) {
    if (a.length < size)
        // Make a new array of a's runtime type, but my contents:
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}

How can I compare these two copy methods and when should I use which?

Answer

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;
}