如何重载超类方法

Question

I have an abstract class with a method that takes a List of Floats as a parameter:

abstract class a {
    def m(p: List[Float]) {
        println("Passed List[Float] to p()")
    }
}

I want to overload m in a concrete extending class:

class b extends a {
    def m(p: List[Double]) {
        println("Passed List[Double] to p()")
    }
}

Is there a way to accomplish this? When I try it I get this error:

error: name clash between defined and inherited member:
method m:(p: List[Double])Unit and
method m:(p: List[Float])Unit in class a
have same type after erasure: (p: List)Unit
   def m(p: List[Double]) {

Answer 1

There is an override keyword for that, however in this case the problem is TypeErasure at compile time. The first method takes a List[Float] and the other one a List[Double] .

These types will be erased, so you get two methods that take a List and return Unit . That is the clash.

Change the name

Now the simple resolution is to give them different names.

Use generics

abstract class a[T] {
    def m(p: List[T]) {
        println("Passed List[Float] to p()")
    }
}

class b extends a[Double] {
}

Answer 2

You have to change the signature somehow, so you need a dummy argument.

abstract class A {
  def m(p: List[Float]) { println(p.sum) }
}

class B extends A {
  def m(p: List[Double])(implicit dummy: Unit = ()) { println(p.product) }
}

and now

scala> val b = new B
b: B = B@6275a43c

scala> { b.m(List(3.0f,3.0f)); b.m(List(3.0, 3.0)) }
6.0
9.0

Yay!

On the other hand, this can actually make things less convenient.

scala> val a = new A {}
a: A = $anon$1@1283bbd2

scala> a.m(List(3,3))
6.0

scala> b.m(List(3,3))
<console>:11: error: overloaded method value m with alternatives:
  (p: List[Double])(implicit dummy: Unit)Unit <and>
  (p: List[Float])Unit
 cannot be applied to (List[Int])
              b.m(List(3,3))
                ^

Not that you'd know which one was the right one anyway, with b . So this does kind of protect you from errors, but at the cost of not inferring the correct type (since which is correct is ambiguous).

Answer 3

I'm not sure what exactly you are trying to do here but I have another suggestion for you

abstract class A {
  protected def _m(p: List[Float]) {
  }
}

class B extends A {
  def m(p: List[Double]){
    _m(p.map(_.toFloat))
  }
}

class C extends A {
  def m(p: List[Float]) {
    println("Passed List[Float] to p()")
    _m(p)
  }
}