How to Define Trait in Scala

Overview

In this article you will learn how to construct reusable parts of a program and deal with the tribulations of multiple inheritance.

A trait provides code reusability in Scala by encapsulating method and state and then offering the possibility of mixing them into classes thus allowing code reuse.

A trait is like an interface with a partial implementation. In scala, trait is a collection of abstract and non-abstract methods. You can create trait that can have all abstract methods or some abstract and some non-abstract methods.

A variable that is declared either by using val or var keyword in a trait get internally implemented in the class that implements the trait. Any variable which is declared by using val or var but not initialized is considered abstract.Let’s understand with below example.

Scala Trait Example


trait Test{  
    def print()  
}  
  
class A extends Test{  
    def print(){  
        println("Hello")  
    }  
}  
  
object MainObject{  
    def main(args:Array[String]){  
        var a = new A()  
        a.print()  
    }  
}

trait Test{

def print()

}

class A extends Test{

def print(){

println("Hello")

}

object MainObject{

def main(args:Array[String]){

var a = new A()

a.print()

}

Output

Hello

If a class extends a trait but does not implement the members declared in that trait, it must be declared abstract. Let’s see an example.


trait Test{  
    def print()  
}  
  
abstract class A extends Test{            // Must declared as abstract class  
   def printA(){  
       println("Hello, this is A Class")  
   }  
}

trait Test{

def print()

}

abstract class A extends Test{ // Must declared as abstract class

def printA(){

println("Hello, this is A Class")

}

Implementing Multiple Traits in a Class

If a class implements multiple traits, it will extend the first trait, class, abstract class. with keyword is used to extend rest of the traits.

You can achieve multiple inheritances by using trait.


trait Test{  
    def print()  
}  
  
trait Display{  
   def show()  
}  
  
class A6 extends Test with Display{  
    def print(){  
        println("This is Test Trait")  
    }  
    def show(){  
        println("This is Display Trait");  
    }  
}  
  
object MainObject{  
    def main(args:Array[String]){  
        var a = new A6()  
        a.print()  
        a.show()  
    }  
}

trait Test{

def print()

}

trait Display{

def show()

}

class A6 extends Test with Display{

def print(){

println("This is Test Trait")

}

def show(){

println("This is Display Trait");

}

object MainObject{

def main(args:Array[String]){

var a = new A6()

a.print()

a.show()

}

Output

This is Test Trait This is Display Trait

Scala Trait having abstract and non-abstract methods

You can also define method in trait as like in abstract class. I.e. you can treat trait as abstract class also. In scala, trait is almost same as abstract class except that it can’t have constructor. You can’t extend multiple abstract classes but can extend multiple traits.


trait Test{  
    def print()         // Abstract method  
    def show(){         // Non-abstract method  
        println("This is show method")  
    }  
}  
  
class A extends Test{  
    def print(){  
        println("This is print method")  
    }  
}  
  
object MainObject{  
    def main(args:Array[String]){  
        var a = new A()  
        a.print()  
        a.show()  
    }  
}