Question

This is a trivial question, I was just wondering if I can use something Scala-ish here. I have 2 different classes (Server1 and Server2) both implement a common interface/trait (with methods .A() and .B()).

This obviously confuses the compiler:

var server = null
if(cond) server=new Server1 else server=new Server2
server.A() //or server.B()

I was mainly curious if I could use Scala’s Option to get around this in a neat way. Thanks.

Answer 1

Type annotations can always be added:

var server: TraitOrInterface = null

However, since vars are somewhat icky…

val server = if (cond) {
   new Server1()
} else {
   new Server2()
}

In the second example, Scala should be able to unify types. (I am fairly certain there are some situations it can’t unify — or where it doesn’t unify quite as desired, but give it a shot before falling back to type annotations, which can be added as per the first example.)

REPL demonstration:

class X
trait Y
class A extends X with Y {}
class B extends X with Y {}
val uni = if (true) new A() else new B()
uni
>> res3: X with Y = A@17b8cf0

Happy coding.

---

Structural typing example, the type alias is for convenience but technically not required.

class Cat { def speak() = "meow" }
class Dog { def speak() = "woof" }
type ThingThatSpeaks = { def speak(): String }

val speaker : ThingThatSpeaks = if (true /* smart */) new Cat() else new Dog()
speaker
>> res4: ThingThatSpeaks = Cat@2893fc
speaker.speak()
>> res5: String = meow

Note that the type annotation was required, otherwise…

val speaker = if (true /* smart */) new Cat() else new Dog()
speaker
>> res6: ScalaObject = Cat@f0ac6e
speaker.speak()
>> error: value speak is not a member of ScalaObject