@CanIgnoreReturnValue
  public Caffeine<K, V> weakKeys() {
    requireState(keyStrength == null, "Key strength was already set to %s", keyStrength);
    keyStrength = Strength.WEAK;
    return this;
  }
@Test
  public void weakKeys() {
    var cache = Caffeine.newBuilder().weakKeys().build();
    assertThat(cache).isNotNull();
  }