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);

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());
}

Since Java 16, you can use Stream.toList():

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

You may write your own utility method for this as well:

public static <E> Collection<E> makeCollection(Iterable<E> iter) {
    Collection<E> list = new ArrayList<E>();
    for (E item : iter) {
        list.add(item);
    }
    return list;
}

IteratorUtils from commons-collections may help (although they don t support generics in the latest stable version 3.2.1):

@SuppressWarnings("unchecked")
Collection<Type> list = IteratorUtils.toList(iterable.iterator());

Version 4.0 (which is in SNAPSHOT at this moment) supports generics and you can get rid of the @SuppressWarnings.

Update: Check IterableAsList from Cactoos.

When you get your Iterable from Spring Data you have a couple of additional alternatives.

1. You can override the method that returns the Iterable in the repository with a version that returns a List, Set or Streamable. This way Spring Data is doing the conversion for you.

2. You may do so in a super interface of your repositories so you don t have to repeat the override in all your repository interfaces.

3. If you happen to use Spring Data JPA this is already done for you in JpaRepository

4. You may do the conversion using the just mentioned Streamable yourself:

   Iterable<X> iterable = repo.findAll();
   List<X> list = Streamable.of(iterable).toList();
   

And since you mention being upset, maybe a little background for the decision to use Iterable help as well.

1. It is expected that it is actually fairly rare to actually require a Collection so in many cases it shouldn t make a difference.
2. Using the overriding mechanics one can return different types which wouldn t be possible with a more specific return type like Collection. This would make it impossible to return a Streamable which is intended for cases where a store may decide to return a result before all elements have been fetched.
3. Streamable would actually be a flexible return type, since it offers easy conversions to List, Set, Stream and is itself an Iterable. But this would require you to use a Spring Data specific type in your application which many users wouldn t like.

There is a section about this in the reference documentation.

From CollectionUtils:

List<T> targetCollection = new ArrayList<T>();
CollectionUtils.addAll(targetCollection, iterable.iterator())

Here are the full sources of this utility method:

public static <T> void addAll(Collection<T> collection, Iterator<T> iterator) {
    while (iterator.hasNext()) {
        collection.add(iterator.next());
    }
}

I came across a similar situation while trying to fetch a List of Projects, rather than the default Iterable<T> findAll() declared in CrudRepository interface. So, in my ProjectRepository interface (which extends from CrudRepository), I simply declared the findAll() method to return a List<Project> instead of Iterable<Project>.

package com.example.projectmanagement.dao;

import com.example.projectmanagement.entities.Project;
import org.springframework.data.repository.CrudRepository;
import java.util.List;

public interface ProjectRepository extends CrudRepository<Project, Long> {

    @Override
    List<Project> findAll();
}

This is the simplest solution, I think, without requiring conversion logic or usage of external libraries.

I use FluentIterable.from(myIterable).toList() a lot.

While at it, do not forget that all collections are finite, while Iterable has no promises whatsoever. If something is Iterable you can get an Iterator and that is it.

for (piece : sthIterable){
..........
}

will be expanded to:

Iterator it = sthIterable.iterator();
while (it.hasNext()){
    piece = it.next();
..........
}

it.hasNext() is not required to ever return false. Thus in the general case you cannot expect to be able to convert every Iterable to a Collection. For example you can iterate over all positive natural numbers, iterate over something with cycles in it that produces the same results over and over again, etc.

Otherwise: Atrey s answer is quite fine.

This is not an answer to your question but I believe it is the solution to your problem. The interface org.springframework.data.repository.CrudRepository does indeed have methods that return java.lang.Iterable but you should not use this interface. Instead use sub interfaces, in your case org.springframework.data.mongodb.repository.MongoRepository. This interface has methods that return objects of type java.util.List.

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.

As soon as you call contains, containsAll, equals, hashCode, remove, retainAll, size or toArray, you d have to traverse the elements anyway.

If you re occasionally only calling methods such as isEmpty or clear I suppose you d be better of by creating the collection lazily. You could for instance have a backing ArrayList for storing previously iterated elements.

I don t know of any such class in any library, but it should be a fairly simple exercise to write up.

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.

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

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

Here s an SSCCE for a great way to do this in Java 8

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class IterableToCollection {
    public interface CollectionFactory <T, U extends Collection<T>> {
        U createCollection();
    }

    public static <T, U extends Collection<T>> U collect(Iterable<T> iterable, CollectionFactory<T, U> factory) {
        U collection = factory.createCollection();
        iterable.forEach(collection::add);
        return collection;
    }

    public static void main(String[] args) {
        Iterable<Integer> iterable = IntStream.range(0, 5).boxed().collect(Collectors.toList());
        ArrayList<Integer> arrayList = collect(iterable, ArrayList::new);
        HashSet<Integer> hashSet = collect(iterable, HashSet::new);
        LinkedList<Integer> linkedList = collect(iterable, LinkedList::new);
    }
}

Two remarks

1. There is no need to convert Iterable to Collection to use foreach loop - Iterable may be used in such loop directly, there is no syntactical difference, so I hardly understand why the original question was asked at all.
2. Suggested way to convert Iterable to Collection is unsafe (the same relates to CollectionUtils) - there is no guarantee that subsequent calls to the next() method return different object instances. Moreover, this concern is not pure theoretical. E.g. Iterable implementation used to pass values to a reduce method of Hadoop Reducer always returns the same value instance, just with different field values. So if you apply makeCollection from above (or CollectionUtils.addAll(Iterator)) you will end up with a collection with all identical elements.

Try StickyList from Cactoos:

List<String> list = new StickyList<>(iterable);

Kinda late to the party, but I created a very elegant Java 8 solution that allows converting an Iterable of T to any Collection of T, without any libraries:

public static <T, C extends Collection<T>> C toCollection(Iterable<T> iterable, Supplier<C> baseSupplier) 
{
    C collection = baseSupplier.get();
    
    iterable.forEach(collection::add);
    
    return collection;
}

Usage Example:

Iterable<String> iterable = ...;
List<String> list = toCollection(iterable, ArrayList::new);

You can use Eclipse Collections factories:

Iterable<String> iterable = Arrays.asList("1", "2", "3");

MutableList<String> list = Lists.mutable.withAll(iterable);
MutableSet<String> set = Sets.mutable.withAll(iterable);
MutableSortedSet<String> sortedSet = SortedSets.mutable.withAll(iterable);
MutableBag<String> bag = Bags.mutable.withAll(iterable);
MutableSortedBag<String> sortedBag = SortedBags.mutable.withAll(iterable);

You can also convert the Iterable to a LazyIterable and use the converter methods or any of the other available APIs available.

Iterable<String> iterable = Arrays.asList("1", "2", "3");
LazyIterable<String> lazy = LazyIterate.adapt(iterable);

MutableList<String> list = lazy.toList();
MutableSet<String> set = lazy.toSet();
MutableSortedSet<String> sortedSet = lazy.toSortedSet();
MutableBag<String> bag = lazy.toBag();
MutableSortedBag<String> sortedBag = lazy.toSortedBag();

All of the above Mutable types extend java.util.Collection.

Note: I am a committer for Eclipse Collections.

If you could update to Spring Data 3, this has been addressed there. There is a new interface ListCrudRepository which do exactly what you want.

Here is the interface from https://spring.io/blog/2022/02/22/announcing-listcrudrepository-friends-for-spring-data-3-0:

public interface ListCrudRepository<T, ID> extends CrudRepository<T, ID> {
    <S extends T> List<S> saveAll(Iterable<S> entities);
    List<T> findAll();
    List<T> findAllById(Iterable<ID> ids);
}

Note in version 3 you must implement two interfaces

So in version 2:

public interface PersonRepository<Person, Long> extends 
   PagingAndSortingRepository<Person, Long> {}

In version 3 should be changed to:

public interface PersonRepository<Person, Long> extends
    PagingAndSortingRepository<Person, Long>,ListCrudRepository<Person, Long> {}

Other changes are mentioned in https://spring.io/blog/2022/02/22/announcing-listcrudrepository-friends-for-spring-data-3-0