Easy way to convert Iterable to Collection

With Guava you can use Lists.newArrayList(Iterable) or Sets.newHashSet(Iterable), among other similar methods. This will of course copy all the elements in to memory. If that isn't acceptable, I think your code that works with these ought to take Iterable rather than Collection. Guava also happens to provide convenient methods for doing things you can do on a Collection using an Iterable (such as Iterables.isEmpty(Iterable) or Iterables.contains(Iterable, Object)), but the performance implications are more obvious.

In JDK 8+, without using any additional libs:

Iterator<T> source = ...;
List<T> target = new ArrayList<>();
source.forEachRemaining(target::add);

Edit: The above one is for Iterator. If you are dealing with Iterable,

iterable.forEach(target::add);