code: @SuppressWarnings("unchecked")
      public <T, E> RingbufferContainer<T, E> getContainerOrNull(int partitionId, ObjectNamespace namespace) {
          final Map<ObjectNamespace, RingbufferContainer> partitionContainers = containers.get(partitionId);
          return partitionContainers != null ? partitionContainers.get(namespace) : null;
      }
docs: Returns the ringbuffer container if it already exists for the
  given {@code partitionId} and {@code namespace}. Returns {@code null}
  if it doesn't exist.
  
  @param partitionId the partition ID of the ringbuffer container
  @param namespace   the namespace of the ringbuffer container
  @param <T>         the type of items in the ringbuffer container
  @param <E>         the type of items in the ringbuffer
  @return the ringbuffer container or {@code null} if it has not been created