如何从Java调用具有数值参数的Scala方法

Question

I have a Scala method that takes a Numeric[T] object:

def needNumeric[T](value: T)(implicit n: Numeric[T]) = {
  // do something
}

How do I call this method from Java? The best way I've come up with is this:

needNumeric(0, scala.math.Numeric.IntIsIntegral$.MODULE$);

But the code looks ugly and is not very general. Is there a better way to do it?

Answer 1

Java supports polymorphic methods, so how about something like this:

object original {
  def needNumeric[T](value: T)(implicit n: Numeric[T]) = {
    // do something
  }
}

object NeedNumeric {
  def needNumeric(value: Int) = original.needNumeric(value)
  def needNumeric(value: Long) = original.needNumeric(value)
  def needNumeric(value: Float) = original.needNumeric(value)
  def needNumeric(value: Double) = original.needNumeric(value)
  def needNumeric(value: BigInt) = original.needNumeric(value)
  ...
}

import NeedNumeric._

It is tedious to have to enumerate the types (which is why the Scala uses a type class) but it should be OK for numerical values as there aren't very many numeric types.

---

If this is your own needNumeric method then note that the signature can be simplfied to this:

def needNumeric[T: Numeric](value: T) = {

Answer 2

A slight workaround for the ugliness issue: define Java-convenient access like

class Numerics {
    public static final Numeric<Integer> INTEGER = Numeric.IntIsIntegral$.MODULE$;

    public static final Numeric<Double> DOUBLE = Numeric.DoubleIsFractional$.MODULE$;

    ...
}

The tradeoff is that it allows calling any method requiring Numeric s without modifying it.