code: void setAnnotation(String key, Object object) {
      checkArgument(object != null, "The stored annotation value cannot be null.");
      Preconditions.checkArgument(
          !annotationMap.containsKey(key), "Cannot overwrite the existing annotation '%s'.", key);
      annotationMap.put(key, object);
    }
docs: Sets an annotation for the given key.
  
  @param key the annotation key
  @param object the object to store as the annotation