Trait Linearization in Scala

Scala Linearization is a deterministic process which comes into play when an object of a class is created which is defined using inheritance of different traits and classes. linearization helps to resolve the diamond problem which occurs when a class or trait inherits a same property from 2 different concrete classes or traits.

Syntax :

trait C{}
trait B{}
class A{}
object a_obj= new class A extends B with C

The linearization will look like as follows :-

C-> AnyRef-> Any   
B-> AnyRef-> Any   
A-> AnyRef-> Any    
a_obj-> A-> C-> B-> AnyRef-> Any     

Here Any is the superclass of all classes, also called the top class. It defines certain universal methods such as equals, hashCode, and toString. AnyRef represents reference classes. All non-value types are defined as reference types. AnyRef corresponds to java.lang.object . Every Scala trait and class implicitly extend these Scala objects at the end of linearization hierarchy.

Examples :

// Scala program defining trait A
trait A
{
    def name: String
}

// defining trait B inheriting A
trait B extends A
{
    override def name: String ="class b"
}

// defining trait C inheriting A
trait C extends A
{
    override def name: String ="class c"
}

// defining class D inheriting B and C both
class D extends B with C
{
    override def name: String = super.name
}

// Creating object
object GFG
{
    // Main method
    def main(args: Array[String])
    {
        var class_d = new D

        // whose property will be inherited
        println(class_d.name)
    }
}

Output :

class c

Linearization for class D follows dark bold arrow. Inheritance for class D follows light arrow.

Trait linearization and inheritance diagram

As we can see in above diagram linearization will not be as same as inherited structure. Scala traits/classes are dynamically placed in linear order in which will the linearization will be applied as below.

D-> C-> B-> A-> AnyRef-> Any

Following rules are followed for the determining the linearization:

1. Take the first extended trait/class and write its complete inherited hierarchy in vertical form, store this hierarchy as X.
2. Take the next trait/class after the with clause, write its complete hierarchy and cancel the classes or traits that are repeated in hierarchy X. Add the remaining traits/classes to the front of the hierarchy X.
3. Go to step 2 and repeat the process, until no trait/class is left out.
4. Place the class itself in front of hierarchy as head for which the hierarchy is being written.

Let’s understand some examples.
Example :

// Scala program for linearization
// defining old_car class
class old_Car
{
    def method: String= "old car "
}

// defining new_Car_Designs trait
trait new_Car_Designs extends old_Car
{
    override def method: String ="Designing-> "+ super.method
}

// defining new_Car_Part trait
trait new_Car_Part extends old_Car
{
    override def method: String = "Add new part-> "+ super.method
}

// defining new_Car_Paint trait
trait new_Car_Paint extends old_Car
{
    override def method: String = "Repainting-> "+ super.method
}

// defining new_Car class
class new_Car extends new_Car_Paint with
new_Car_Part with new_Car_Designs
{
    override def method: String = "new car-> "+ super.method
}

// Creating object
object geekforgeeks
{
    // Main method
    def main(args: Array[String])
    {
        // new_Car object
        var car1 = new new_Car
        println(car1.method)
    }
}

Output :

new car-> Designing-> Add new part-> Repainting-> old car 

Example :

// Scala program for trait linearization
// defining classes and traits
class flavour
{
    def make (flavour: String): Unit =
    {
        println(flavour)
    }
}

// defining texture trait
trait texture extends flavour
{
    abstract override def make (flavour : String)
    {
        super.make(flavour + "texture ")
    }
}

// defining cream trait
trait cream extends texture
{
    abstract override def make (flavour : String)
    {
        super.make(flavour + "with cream ")
    }
}

// defining jelly trait
trait jelly extends texture
{
    abstract override def make (flavour : String)
    {
        super.make(flavour + "with jelly ")
    }
}
// defining cone trait
trait cone extends flavour
{
    abstract override def make (flavour : String)
    {
        super.make(flavour + "in cone ")
    }
}

// creating new ice-cream flovours
// with above traits and classes
// inheriting different traits and classes
class Myflavour extends flavour with jelly
{
    override def make (flavour : String)
    {
        super.make(flavour)
    }
}
class Myflavour2 extends flavour with cream with cone
{
    override def make (flavour : String)
    {
        super.make(flavour)
    }
}

// Creating object
object GFG
{
    // Main method
    def main(args: Array[String])
    {
        // creating new objects
        var icecream1 = new Myflavour
        var icecream2 = new Myflavour2 with jelly
        println(icecream1.make("chocolate "))
        println(icecream2.make("vanilla "))
    }
}

Output :

chocolate with jelly texture 
()
vanilla with jelly in cone with cream texture 
()

Important Points About Linearization

• Scala solves ambiguity of traits/classes by linearization process.
• Scala uses linearization whenever a new class has been instantiated. Taking all the traits/classes and forming a linear order which points to corresponding super classes/traits thus super method knows its parent method.
• These super method calling is done in a stackable manner.
• Linearization may or not be the same as the inherited mixins as they are written.
• We cannot explicitly add a class to inheritance when it is already been implicitly inherited in a linearization otherwise it will result in error as inheritance twice.
• No trait/class is ever repeated in linearization.