code: private static void setIfUnset(JobConf conf, String key, String value) {
      if (conf.get(key) == null) {
        conf.set(key, value);
      }
    }
docs: Set the configuration, if it doesn't already have a value for the given
  key.
  @param conf the configuration to modify
  @param key the key to set
  @param value the new "default" value to set