为什么textFile用string去接收

/**
   * Read a text file from HDFS, a local file system (available on all nodes), or any
   * Hadoop-supported file system URI, and return it as an RDD of Strings.
   * @param path path to the text file on a supported file system
   * @param minPartitions suggested minimum number of partitions for the resulting RDD
   * @return RDD of lines of the text file
   */
  def textFile(
      path: String,
      minPartitions: Int = defaultMinPartitions): RDD[String] = withScope {
    assertNotStopped()
    hadoopFile(path, classOf[TextInputFormat], classOf[LongWritable], classOf[Text],
      minPartitions).map(pair => pair._2.toString).setName(path)
  }