code: public static FastWritable tryGetInstance(String name, Configuration conf) {
      if (name.length() != NAME_LEN) {
        // we use it to fast discard the request without doing map lookup
        return null;
      }
      FastWritable fw = register.get(name);
      return fw == null ? null : fw.getFastWritableInstance(conf);
    }
docs: Tries to get an instance given the name of class. If the name is registered
  as FastWritable, the instance is obtained using the registry.