Is there a way to "program to interface" in scala

Question

Is there a way to "program to interface" in scala?

I am new in Scala. I have a "trait" in scala and many classes extending that trait. I am looking for a way on how to use the interface instead of the class directly.

Currently, I am doing :

val clazz = new MyClass()

where MyClass is extended from a trait.

I am looking for a way to return the interface as an instance from the factory mathods for generating classes. Is there a way?

Widening conversions in Scala work just like they would in Java. If MyClass implements a trait Foobar and your method has return type Foobar, you can return clazz just fine. — Silvio Mayolo
– Silvio Mayolo, Commented Aug 17, 2018 at 15:31
calling asInstanceOf[MyClass] breaks type safety and defeats the purpose of "program to interface" — Alvaro Carrasco
– Alvaro Carrasco, Commented Aug 17, 2018 at 16:03
Your question is unclear. You are talking about "programming to an interface", but in your example, you cast the value to a concrete subclass type using asInstanceOf, which is the exact opposite of programming to an interface. — Jörg W Mittag
– Jörg W Mittag, Commented Aug 17, 2018 at 16:03
Also, it is not clear what you mean by "I am looking for a way to return the interface as an instance from the factory mathods for generating classes." You can only return objects from methods. But traits aren't objects, therefore you cannot return a trait from a factory method. (Well, you can obtain a reflective proxy for a trait using the Scala reflection API and return that …) — Jörg W Mittag
– Jörg W Mittag, Commented Aug 17, 2018 at 16:06

Alvaro Carrasco · Accepted Answer · 2018-08-17 16:01:45Z

1

You can always just request the type of the interface:

val clazz: MyInterf = new MyClass()

If you really want to hide the implementation, one approach is to create a companion for the trait:

trait MyInterf {
  ...
}

class MyClass extends MyInterf {
  ...
}

object MyInterf {
  def create: MyIntef = new MyClass()
}

// calling code
val impl = MyInterf.create

answered Aug 17, 2018 at 16:01

Alvaro Carrasco

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2 Comments

VictorGram Over a year ago

If I have 2 classes implementing this interface, how to change the create method to return the right instance based on the requirement? I am trying to understand how to implement the facetory mathods

Alvaro Carrasco Over a year ago

based on what requirement? If it's just during creation, you can pass a parameter to the create method. Beyond creation, if your calling code needs to be aware of what class is providing the implementation, then you're not really "programming to the interface"

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