@Override
    public Class<S> getRealClass() throws IOException {
      return (Class<S>) conf.getClass(SerializationSubclassKey, null,
          Object.class);
    }
Implements configurable so it can use the configuration to find the right
classes Note: ReflectionUtils will automagically call setConf with the
right configuration.
ql/src/java/org/apache/hadoop/hive/ql/io/FlatFileInputFormat.java