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scalacovariance

calling a Covariant method in Scala

I have a class

class Getable [+T] (val data: T)

And I create a method for the class to print the Number

def printNumber (in: Getable[Number]) = { println("It's " + in.data)}

So it works for

get(new Getable(10.0))

But does not work for 

val g = new Getable(10.0)
get(g)

I get this error message

scala> get(g)
<console>:16: error: type mismatch;
found   : Getable[Double]
required: Getable[Number]
   get(g)
       ^

But overcoming the error message is the reason why we use covariance.

Solution

  • You have to hint a compiler with a type of Getable.

    val g: Getable[Number] = new Getable(10.0)

    or

    val g = new Getable[Number](10.0)