Java tutorial
import java.util.ArrayList; import java.util.List; import java.util.stream.IntStream; public class Main { public static void main(String[] args) throws Exception { IntStream.range(1, 4).mapToObj(num -> new Foo("Foo" + num)) .peek(f -> IntStream.range(1, 4).mapToObj(num -> new Bar("Bar" + num + " <- " + f.name)) .forEach(f.bars::add)) .flatMap(f -> f.bars.stream()).forEach(b -> System.out.println(b.name)); } static class Foo { String name; List<Bar> bars = new ArrayList<>(); Foo(String name) { this.name = name; } } static class Bar { String name; Bar(String name) { this.name = name; } } }