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