import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

interface Person {
  int age();
}

interface People<T> {
  T fromList(List<Person> ps);
  List<Integer> getAges(T people);
  People<?> module = new People<List<Person>>() {
    public List<Person> fromList(List<Person> ps) {
      return ps;
    }
    public List<Integer> getAges(List<Person> people) {
      return people.stream().map(Person::age).collect(Collectors.toList());
    }
  };
}

// run with:
// javac Program.java
// java -cp . Program
public class Program<T> {
  People<T> P;
  Program(People<T> P) {this.P = P;}
  void run() {
    T people = P.fromList(Arrays.asList(() -> 21, () -> 37));
    System.out.println(P.getAges(people)); // print "[21, 37]"
  }
  public static void main(String[] args) {
    new Program<>(People.module).run();
  }
}