Weak keys, retired for real

Inside Solution, keep static class CacheKey { final String id; ... } and a private static final WeakHashMap<CacheKey, String> store with static void put, static String get, plus: 1. static int weakRetire() — seed 3 key/value pairs from a PRIVATE helper (so the key objects are reachable ONLY through the weak map once the helper returns), call System.gc() twice, then return how many entries remain (probe store.size(), which expunges cleared entries). Correct answer: 0. 2. static int strongRetire() — same seeding, but ALSO retain the keys in a static List<CacheKey> pinned; gc twice; return survivors (correct: 3).

Difficulty: advanced