Scala 中object和class的區別

Scala中沒有靜態類型，但是有有“伴侶對象”，起到類似的作用。

Scala中類對象中不可有靜態變量和靜態方法，但是提供了“伴侶對象”的功能：在和類的同一個文件中定義同名的Object對象：(須在同一文件中；main方法定義在Object對象中)

private[spark] class Client(
    val args: ClientArguments,
    val hadoopConf: Configuration,
    val sparkConf: SparkConf)
  extends Logging {...}

object Client extends Logging {
  def main(argStrings: Array[String]) {
    if (!sys.props.contains("SPARK_SUBMIT")) {
      logWarning("WARNING: This client is deprecated and will be removed in a " +
        "future version of Spark. Use ./bin/spark-submit with \"--master yarn\"")
    }

    // Set an env variable indicating we are running in YARN mode.
    // Note that any env variable with the SPARK_ prefix gets propagated to all (remote) processes
    System.setProperty("SPARK_YARN_MODE", "true")
    val sparkConf = new SparkConf

    val args = new ClientArguments(argStrings, sparkConf)
    // to maintain backwards-compatibility
    if (!Utils.isDynamicAllocationEnabled(sparkConf)) {
      sparkConf.setIfMissing("spark.executor.instances", args.numExecutors.toString)
    }
    new Client(args, sparkConf).run()
  }
 ......
}