How do I make an advanced stream that both matches and compares where no match at the same time?

I have two lists (List1 and List2) of Person objects:

public class Person {

  ObjectId id;
  private List<String> names;
  private Integer age;

}

I want to compare the Lists, and wherever there is a match on id, I want to check if the age matches. If age does match, then I don't want to do anything but if there is a mismatch in age then I want to return the names.

So I should end up with one Set (no duplicates) that contains names from all the objects that has ids in both lists but a mismatch in age.

You could filter both lists, concatnate them and flatmap to the names. Something like:

import java.util.List;
import java.util.Objects;
import java.util.Set;
import java.util.function.BiPredicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;

....


List<Person> list1 = //your first list
List<Person> list2 = //your second list

BiPredicate<Person,Person> sameIdDiffAge = (p1,p2) -> Objects.equals(p1.getId(),  p2.getId()) &&
                                                     !Objects.equals(p1.getAge(), p2.getAge());
Set<String> result =
        Stream.concat(list1.stream().filter(p1 -> list2.stream().anyMatch(p2 -> sameIdDiffAge.test(p1,p2))),
                      list2.stream().filter(p1 -> list1.stream().anyMatch(p2 -> sameIdDiffAge.test(p1,p2))))
        .flatMap(p -> p.getNames().stream())
        .collect(Collectors.toSet());