SetNarrowTest.java

import javaslang.collection.*;
import javaslang.*;

public class SetNarrowTest {
    static class PersonBase {
        public final String name;

        public PersonBase(String name) {
            this.name = name;
        }

        @Override
        public boolean equals(Object other) {
            return other instanceof PersonBase &&
                ((PersonBase)other).name.equals(name);
        }

        @Override
        public int hashCode() {
            return name.hashCode();
        }
    }

    static class Person extends PersonBase {
        public final int age;

        public Person(String name, int age) {
            super(name);
            this.age = age;
        }

        @Override
        public boolean equals(Object other) {
            return other instanceof Person &&
                ((Person)other).name.equals(name) &&
                ((Person)other).age == age;
        }

        @Override
        public int hashCode() {
            return Tuple.of(Integer.toString(age), super.hashCode()).hashCode();
        }
    }

    public static void main(String[] argv) {
        final Set<? extends PersonBase> persons = HashSet.of(new Person("a", 1), new Person("a", 2));
        System.out.println(persons.size());
        final Set<PersonBase> personBases = Set.narrow(persons);
        System.out.println(personBases.size());
    }
}

Thank you! Yes, that's our use-case and it looks alright. Maybe something obscure can occur when mixing instances of PersonBase and Person in that Set, but that would be an error by itself and has nothing to do with narrowing.