Multi-stream: semi-join filters one list by keys present in another

A semi-join answers "which of these appear over there" without pulling any column across. Collect the other stream's keys into a Set once; the filter is then a hash lookup per row.

Code

record User(String id, String name) {}
record Login(String userId, String at) {}

var users = List.of(new User("u1", "Ava"), new User("u2", "Ben"), new User("u3", "Cara"));
var logins = List.of(new Login("u1", "09:00"), new Login("u3", "09:05"), new Login("u1", "09:40"));

Set<String> loggedIn = logins.stream().map(Login::userId).collect(Collectors.toSet());

users.stream()
     .filter(u -> loggedIn.contains(u.id()))
     .map(User::name)
     .forEach(System.out::println);

Output

Ava
Cara