Question

Is there something like this in any standard library (e.g. apache-commons, guava) ?

public static <T> List<T> toList(Iterable<T> iterable) {
    if (iterable instanceof List)
        return (List<T>)iterable;

    if (iterable instanceof Collection)
        return new ArrayList<T>((Collection<T>)iterable);

    List<T> result = new ArrayList<T>();
    for (T item : iterable)
        result.add(item);

    return result;
}

Answer 1

I don’t think so, because your implementation does two completely different things:

• If the argument is a list, it returns it. The returned list will therefore be a “live view” of the argument. Changes to each of the lists are visible in the other.
• If the argument is not a list, it returns a copy of it. The returned list will be independent of the argument.

These two things are so different that no sane general-purpose library would throw them together in one method.