It implements an Iterable that take elements from a list of Iterable in a round robin fashion. The test is most explicative than the description.

RoundRobinIterable.java

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.Iterables.filter;
import static com.google.common.collect.Iterators.cycle;
import static java.util.Arrays.asList;

import java.util.Iterator;

import com.google.common.base.Function;
import com.google.common.base.Predicate;
import com.google.common.collect.FluentIterable;
import com.google.common.collect.UnmodifiableIterator;


public class RoundRobinIterable<T> implements Iterable<T> {
 
    private final Iterable<Iterable<T>> iterables;
 
    public RoundRobinIterable(Iterable<Iterable<T>> iterables) {
 
        checkNotNull(iterables);
 
        this.iterables = iterables;
    }
 
    @SafeVarargs
    public static <T> RoundRobinIterable of(Iterable<T>... xs) {
        return new RoundRobinIterable<>(asList(xs));
    }
 
    @Override
    public Iterator<T> iterator() {

        final Iterator<Iterator<T>> it = cycle(filter(
                FluentIterable.from(iterables)
                .transform(RoundRobinIterable.<T>iteratorFn())
                .toList(),
                hasNextPr()
        ));

        return new UnmodifiableIterator<T>() {
            @Override
            public boolean hasNext() {
                return it.hasNext();
            }

            @Override
            public T next() {
                return it.next().next();
            }
        };

    }

    private enum HasNext implements Predicate<Iterator<?>> {
        INSTANCE;

        @Override
        public boolean apply(Iterator<?> input) {
            return input.hasNext();
        }
    }

    private static Predicate<Iterator<?>> hasNextPr() {
        return HasNext.INSTANCE;
    }

    private static <T> Function<Iterable<T>, Iterator<T>> iteratorFn() {
        return new Function<Iterable<T>, Iterator<T>>() {
            @Override
            public Iterator<T> apply(Iterable<T> input) {
                return input.iterator();
            }
        };
    }

}

RoundRobinIterableTest.java

import static com.google.common.collect.Lists.newArrayList;
import static java.util.Arrays.asList;
import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.equalTo;

import java.util.List;

import org.junit.Test;

/**
 * @author Filippo De Luca - me@filippodeluca.com
 */
public class RoundRobinIterableTest extends AbstractUnitTest {

    @Test
    public void iteratorShouldBeWorkAsExpected() {

        List<Integer> numbers = newArrayList(RoundRobinIterable.of(asList(1,2,3,9), asList(4,5), asList(6,7,8)));
        List<Integer> expected = newArrayList(1, 4, 6, 2, 5, 7, 3, 8, 9);

        assertThat(numbers, equalTo(expected));
    }
}