import com.google.common.base.Function; import java.util.ArrayList; import java.util.List; import java.util.stream.Stream; class A { void c() { ArrayList<String> strings = new ArrayList<String>(); Stream<String> it = strings.stream(); int i = (int) it.flatMap(f -> getFunction().apply(f).stream()).count(); } Function<String, List<String>> getFunction() { return null; } }