scala - $() 在 Scala 中是什么意思？

Question

当我从 apache-spark 读取代码时遇到这个:

    val alpha = new DoubleParam(this, "alpha", "alpha for implicit preference",
                                ParamValidators.gtEq(0)) 
    /** @group getParam */
    def getAlpha: Double = $(alpha)

我在网上搜索了很长时间，但仍然没有找到很好的解释 $(alpha)这里的意思？如何分配自定义类 DoubleParam变量为 Double ?谢谢!!

仅供引用， DoubleParam类定义为:

class DoubleParam(parent: String, name: String, doc: String, isValid: Double => Boolean)
  extends Param[Double](parent, name, doc, isValid) {

  def this(parent: String, name: String, doc: String) =
    this(parent, name, doc, ParamValidators.alwaysTrue)

  def this(parent: Identifiable, name: String, doc: String, isValid: Double => Boolean) =
    this(parent.uid, name, doc, isValid)

  def this(parent: Identifiable, name: String, doc: String) = this(parent.uid, name, doc)

  /** Creates a param pair with the given value (for Java). */
  override def w(value: Double): ParamPair[Double] = super.w(value)
}

Answer 1

$()是在特征 Params 中定义的 Spark 函数.它只是调用 getOrDefault在 Params目的。

它的定义是

/** An alias for [[getOrDefault()]]. */
protected final def $[T](param: Param[T]): T = getOrDefault(param)