Sealed interfaces make a switch exhaustive

sealed interface Event permits Login, Purchase, Logout {}
record Login(String user) implements Event {}
record Purchase(String user, int amount) implements Event {}
record Logout(String user) implements Event {}

List<Event> events = List.of(new Login("ava"), new Purchase("ava", 99), new Logout("ava"));

for (Event e : events) {
    String line = switch (e) {                     // no default needed
        case Login l -> l.user() + " signed in";
        case Purchase p -> p.user() + " spent " + p.amount();
        case Logout l -> l.user() + " signed out";
    };
    System.out.println(line);
}

ava signed in
ava spent 99
ava signed out