code: public final boolean isCached(@NonNull final KeyType key) {
          Condition.INSTANCE.ensureNotNull(key, "The key may not be null");
  
          synchronized (cache) {
              return cache.get(key) != null;
          }
      }
docs: Returns, whether the data, which corresponds to a specific key, is currently cached, or not.
  
  @param key
  The key, which corresponds to the data, which should be checked, as an instance of
  the generic type KeyType. The key may not be null
  @return True, if the data, which corresponds to the given key, is currently cached, false
  otherwise