docs: public static <K, V> ConcurrentMap<K, V> makeDefaultsMap(final Function<K, V> creator) {
          return new ConcurrentHashMap<K, V>() {
              @Override
              public V get(Object keyObj) {
                  V value = super.get(keyObj);
                  if (value == null) {
                      K key = (K) keyObj;
                      value = creator.apply(key);
                      final V existingValue = super.putIfAbsent(key, value);
                      if (existingValue != null) {
                          value = existingValue;
                      }
                  }
                  return value;
              }
          };
      }