java - stram - Easy way to convert Iterable to Collection

java iterable to stream (11)

IteratorUtils from commons-collections may help (although they don't support generics in the latest stable version 3.2.1):

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.

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

Methods of this library return Iterable<T>, while the rest of my code expects Collection<T>.

Is there any utility method somewhere that will let me quickly convert one to the other? I would like to avoid creating a bunch of foreach loops in my code for such a simple thing.

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.

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()) {

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;

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();
        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);

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


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.


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

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

    Collection<String> collection = toCollection(new Iterable<String>() {
        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.

In JDK 8, without depending on additional libs:

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

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


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


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

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.

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.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.

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) {
    return list;