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

Asked 2010-04-07T03:00:22.127
89

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?

java arrays performance
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2 Answers

121

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;
}
answered 2010-04-07T03:21:52.197
12

Have you looked at the Sun's implementation of Arrays.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;
}

As can be seen, it uses System.arraycopy() internally, so the performance would be the same.

answered 2010-04-07T10:25:40.967

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