Easy way to convert Iterable to Collection

Question

In my application I use 3rd party library (Spring Data for MongoDB to be exact).

Methods of this library return Iterable, while the rest of my

Answer 1

In Java 8 you can do this to add all elements from an Iterable to Collection and return it:

public static <T> Collection<T> iterableToCollection(Iterable<T> iterable) {
  Collection<T> collection = new ArrayList<>();
  iterable.forEach(collection::add);
  return collection;
}

Inspired by @Afreys answer.

Answer 2

Since RxJava is a hammer and this kinda looks like a nail, you can do

Observable.from(iterable).toList().toBlocking().single();

Answer 3

I didn't see a simple one line solution without any dependencies. I simple use

List<Users> list;
Iterable<IterableUsers> users = getUsers();

// one line solution
list = StreamSupport.stream(users.spliterator(), true).collect(Collectors.toList());

Answer 4

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.

Answer 5

I use my custom utility to cast an existing Collection if available.

Main:

public static <T> Collection<T> toCollection(Iterable<T> iterable) {
    if (iterable instanceof Collection) {
        return (Collection<T>) iterable;
    } else {
        return Lists.newArrayList(iterable);
    }
}

Ideally the above would use ImmutableList, but ImmutableCollection does not allow nulls which may provide undesirable results.

Tests:

@Test
public void testToCollectionAlreadyCollection() {
    ArrayList<String> list = Lists.newArrayList(FIRST, MIDDLE, LAST);
    assertSame("no need to change, just cast", list, toCollection(list));
}

@Test
public void testIterableToCollection() {
    final ArrayList<String> expected = Lists.newArrayList(FIRST, null, MIDDLE, LAST);

    Collection<String> collection = toCollection(new Iterable<String>() {
        @Override
        public Iterator<String> iterator() {
            return expected.iterator();
        }
    });
    assertNotSame("a new list must have been created", expected, collection);
    assertTrue(expected + " != " + collection, CollectionUtils.isEqualCollection(expected, collection));
}

I implement similar utilities for all subtypes of Collections (Set,List,etc). I'd think these would already be part of Guava, but I haven't found it.

Answer 6

Concise solution with Java 8 using java.util.stream:

public static <T> List<T> toList(final Iterable<T> iterable) {
    return StreamSupport.stream(iterable.spliterator(), false)
                        .collect(Collectors.toList());
}