code: public static JsonObject getObject(JsonObject object, String field) {
          final JsonValue value = object.get(field);
          throwExceptionIfNull(value, field);
          return value.asObject();
      }
docs: Returns a field in a Json object as an object.
  Throws IllegalArgumentException if the field value is null.
  
  @param object the Json object
  @param field the field in the Json object to return
  @return the Json field value as a Json object