在Scala中$()的含义是什么？

Question

当我从apache-spark读取代码时遇到这种情况:

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

我已经在网上搜索了很长时间但是仍然无法找到$(alpha)这里意味着什么的好解释？它如何为自定义的类DoubleParam变量赋值Double？谢谢!!

仅供参考,该DoubleParam课程定义为:

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

$()是特征中定义的Spark函数Params.它只是调用getOrDefault上的Params对象.

它的定义是

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