Java: why does Collection.addAll can not accept Iterables?

Question

I wonder why the Collection.addAll() method only accepts other Collections but not Iterables. Why is that?

Any similar method to do that for Iterables?

Answer 1

Presumably because the Collection interface was introduced in Java 1.2 whereas Iterable appeared only in 1.5, and changing the interface would break all existing implementations.

Answer 2

When in doubt, always check Guava (or Commons):

• Guava: Iterables.addAll
• Commons Collections: CollectionUtils.addAll

Answer 3

Others have answered the "why" extensively.

Any similar method to do that for Iterables?

In Java 8 you don't need addAll any more:

Collection<X> coll = ...;
Iterable<X> it = ...;
it.forEach(coll::add); // coll.addAll(it);

Answer 4

Basically because an Iterable may never end (that is, hasNext() return true forever).

Also, to keep congruency, you may think a Collection may add all the elements of another collection, but, an Iterable is not necesarily a collection (it may be anything, like the a ResultSet wrapper for instance).

Answer 5

There are quite a few things in the core JDK which don't work as well with plain Iterables as they might. I'd recommend using Guava to overcome a lot of these shortcomings.