Scala Cookbook

10.5. Declaring a Type When Creating a Collection

Problem

You want to create a collection of mixed types, and Scala isn’t automatically assigning the type you want.

Solution

In the following example, if you don’t specify a type, Scala automatically assigns a type of Double to the list:

scala> val x = List(1, 2.0, 33D, 400L)
x: List[Double] = List(1.0, 2.0, 33.0, 400.0)

If you’d rather have the collection be of type AnyVal or Number, specify the type in brackets before your collection declaration:

scala> val x = List[Number](1, 2.0, 33D, 400L)
x: List[java.lang.Number] = List(1, 2.0, 33.0, 400)

scala> val x = List[AnyVal](1, 2.0, 33D, 400L)
x: List[AnyVal] = List(1, 2.0, 33.0, 400)

Discussion

By manually specifying a type, in this case Number, you control the collection type. This is useful any time a list contains mixed types or multiple levels of inheritance. For instance, given this type hierarchy:

trait Animal
trait FurryAnimal extends Animal
case class Dog(name: String) extends Animal
case class Cat(name: String) extends Animal

create a sequence with a Dog and a Cat:

scala> val x = Array(Dog("Fido"), Cat("Felix"))
x: Array[Product with Serializable with Animal] = Array(Dog(Fido), Cat(Felix))

As shown, Scala assigns a type of Product with Serializable with Animal. If you just want an Array[Animal], manually specify the desired type:

scala> val x = Array[Animal](Dog("Fido"), Cat("Felix"))
x: Array[Animal] = Array(Dog(Fido), Cat(Felix))

This may not seem like a big deal, but imagine declaring ...