Traits Linearization

During those workshops we will see how exactly we can compose multiple traits and how they this mechanism solves so called "Diamond Problem.".

Java 8 and default methods

FILE : starter.traits.JavaDefaultMethods

To have a comparison let's first check similar mechanism to traits which is available in java8. In this version of java you can have a default implementation of an interface method however when two interfaces have the same method with default implementation you are the one responsible for solving this conflict.

interface One{
    default String method(){
        return "one";
    }
}

interface Two{
    default String method(){
        return "two";
    }
}

class Example implements  One,Two{

    @Override
    public String method() {
    //you need to choose
        return  One.super.method() + " example";
//        return  Two.super.method() + " example";
    }
}

Scala multiple traits inheritance

FILE : starter.traits2.Linearization

In Scala this is solved differently. You can easily inherit from multiple traits and call super in each one. Scala will rearrange traits from right to left.

If some traits are repeated in hierarchy than linearization removes repetitions. Algorithm is not simple and visualization can be seen on image below. Generally if you need to understand this algorithm to understand your code - maybe it's time for refactoring

override abstract

If you want to attach given trait to particular hierarchy by extending the hierarchy root -> then you can not just call super because super is abstract. you need from one side to point that you are overriding a method and on the other force concrete class to implement it -> so you have modifier abstract override

abstract class DemoReader{
  def read:Char
}

trait DiagnosticRead extends DemoReader{
   override abstract def read:Char={...

Exercises

Implement web controller and security filters

FILE : traits2.exercises.LinearizationExercises TEST FILE : traits2.exercises.LinearizationExercisesTest

Part 1

Finish implementation

class ShopController extends Controller{
    override def handle(r: Request): Response = ???
}

so that it can handle requests from test

 val r=new Request("/shop","chrome","lodz",new Auth("login","password"))

and for the rest returns 404.

Part 2

Implement two filter which will be mixed into controller :

trait LocationFilter extends Controller{
    abstract override def handle(r: Request): Response = ???
  }

  trait LocationMaskingFilter extends Controller{
    abstract override def handle(r: Request): Response = ???

  }

so that request from banned country be filtered out:

val r=new Request("/shop","chrome","BANNED_COUNTRY",new Auth("login","password"))
val response=controller.handle(r)

response.status mustBe 403