使用多个隐式参数调用主构造函数：从Java调用

Question

我有一个 Scala 类，它使用 Apache Spark 中的 MR 作业返回输入类型 IN 的最大值。如果我从Scala调用它，这个类工作正常，它的工作原理如下：

// Class
class TypedMax[IN, T](val f: IN => T)(implicit ev$1: T => Ordered[T], ev$2: TypeTag[T])
  extends Aggregator[IN, T, T] {

  override def zero: T = null.asInstanceOf[T]
  override def reduce(b: T, a: IN): T = if (b > f(a)) b else f(a)
  override def merge(b1: T, b2: T): T = if (b1 > b2) b1 else b2
  override def finish(reduction: T): T = reduction
}

// Call function
def max[IN, T](f: IN => T)(implicit ev$1: T => Ordered[T], ev$2: TypeTag[T]): TypedColumn[IN, T] =
    new TypedMax[IN, T](f).toColumn

现在我想让它也可以从java中调用，但是我在传递隐式参数时遇到了一些困难。我知道隐式参数可以通过在Java中将它们追加到参数列表中来传递，但是隐式参数是在Scala中。因此，我正在努力做到以下几点:

class TypedMax[IN, T](val f: IN => T)(implicit ev$1: T => Ordered[T], ev$2: TypeTag[T])
  extends Aggregator[IN, T, T] {

  override def zero: T = null.asInstanceOf[T]
  override def reduce(b: T, a: IN): T = if (b > f(a)) b else f(a)
  override def merge(b1: T, b2: T): T = if (b1 > b2) b1 else b2
  override def finish(reduction: T): T = reduction

  // Java api support
  def this(f: MapFunction[IN, java.lang.Double]) =
    this(x => f.call(x).asInstanceOf[T])(ev$1: T => Ordered[T], ev$2: TypeTag[T])

然后可以从java调用：

public static <T> TypedColumn<T, Double> max(MapFunction<T, Double> f) {
  return new TypedMax<T, Double>(f).toColumn();
}

我尝试了许多从辅助构造函数传入隐式的排列，使用隐式隐式并摆弄逗号和括号。但是，它总是抱怨找不到ev$1和ev$2。但是，我必须传入参数，否则它将出错：

Error:(135, 5) No implicit view available from T => Ordered[T].
    this(x => f.call(x).asInstanceOf[T])
Error:(135, 5) not enough arguments for constructor TypedMax: (implicit ev$1: T => Ordered[T], implicit ev$2: reflect.runtime.universe.TypeTag[T])org.apache.spark.sql.execution.aggregate.TypedMax[IN,T].
Unspecified value parameters ev$1, ev$2.
    this(x => f.call(x).asInstanceOf[T])

如果我尝试：

def this(f: MapFunction[IN, T]) =
  this(x => f.call(x))(T => Ordered[T], TypeTag[T])

结果是：

Error:(135, 38) value Ordered of type scala.math.Ordered.type does not take type parameters.
    this(x => f.call(x))(T => Ordered[T], TypeTag[T])
Error:(135, 50) missing argument list for method apply in object TypeTag
Unapplied methods are only converted to functions when a function type is expected.
You can make this conversion explicit by writing `apply _` or `apply(_,_)` instead of `apply`.
    this(x => f.call(x))(T => Ordered[T], TypeTag[T])

我错过了什么/误会了什么？谢谢！

Answer 1

<代码>T=

def this(f: MapFunction[IN, T], e: MapFunction[T, Ordered[T]], tt: TypeTag[T]) =
  this(x => f.call(x))(x => e.call(x), tt)

(我不确定您是否能够从java中具体化一个< code>TypeTag[T])