For http://stackoverflow.com/q/33839037/4856258

FlatMapTest.java

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Objects;
import java.util.stream.IntStream;

import one.util.streamex.IntStreamEx;
import one.util.streamex.MoreCollectors;

public class FlatMapTest {
	public static void main(String[] args) {
		List<List<String>> lists = Arrays.asList(Arrays.asList("A", "B", "C"),
				Arrays.asList("D", "E", "F", "G"), Arrays.asList("H", "I"),
				Collections.nCopies(5, "J"));
		System.out.println("Indices are "
				+ Arrays.toString(indices(lists, "J")));
		System.out.println("---");
		System.out.println("Indices are "
				+ Arrays.toString(indicesEx(lists, "J")));
	}

	public static int[] indices(List<? extends List<?>> lists, Object o) {
		return IntStream
				.range(0, lists.size())
				.boxed()
				.<int[]> flatMap(
						i -> IntStream.range(0, lists.get(i).size()).mapToObj(
								j -> new int[] { i, j })).parallel()
				.filter(a -> {
					System.out.println(Arrays.toString(a)); // For testing only
					return Objects.equals(o, lists.get(a[0]).get(a[1]));
				}).findAny().orElse(null);
	}

	public static int[] indicesEx(List<? extends List<?>> lists, Object o) {
		return IntStreamEx
				.range(0, lists.size())
				.boxed()
				.<int[]> flatMap(
						i -> IntStream.range(0, lists.get(i).size()).mapToObj(
								j -> new int[] { i, j })).parallel()
				.filter(a -> {
					System.out.println(Arrays.toString(a)); // For testing only
					return Objects.equals(o, lists.get(a[0]).get(a[1]));
				}).unordered().collect(MoreCollectors.first()).orElse(null);
	}
}

result.txt

[0, 0]
[0, 1]
[0, 2]
[2, 0]
[2, 1]
[3, 0]
[3, 1]
[3, 2]
[3, 3]
[3, 4]
[1, 0]
[1, 1]
[1, 2]
[1, 3]
Indices are [3, 0]
---
[1, 0]
[0, 0]
[0, 1]
[0, 2]
[2, 0]
[3, 0]
[2, 1]
[1, 1]
[1, 2]
[1, 3]
Indices are [3, 0]