Set<String> premiumStrings = new HashSet<>();
Set<String> sortedSet = new TreeSet<>(Comparator.comparing(premiumStrings::contains).thenComparing(Comparator.naturalOrder()));
This doesn't work, because premiumStrings::contains can take any object and not just strings. One can replace it with (String s) -> premiumStrings.contains(s), but is there a way to restrict the parameter type while still using a method reference lambda?
(Specifically, the problem is The method thenComparing(Comparator<? super Object>) in the type Comparator<Object> is not applicable for the arguments (Comparator<Comparable<? super Comparable<? super T>>>).)
Help the compiler a bit with types:
Set<String> sortedSet = new TreeSet<>(
Comparator.<String, Boolean>comparing(premiumStrings::contains).thenComparing(Comparator.naturalOrder()));