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Scala – Traits

  • Post category:Scala
  • Reading time:5 mins read

What is Scala Trait ?

Traits are similar to Java 8’s interfaces. Classes and objects can extend traits, but traits cannot be instantiated and therefore have no parameters. Traits become especially useful as generic types and with abstract methods.

A trait encapsulates method and field definitions, which can then be reused by mixing them into classes. Unlike class inheritance, in which each class must inherit from just one super class, a class can mix in any number of traits.

Scala traits are a great feature of the language. You can use them just like a Java interface, and you can also use them like abstract classes that have real methods. Scala classes can also extend and “mix in” multiple traits.

A trait definition looks just like a class definition except that it uses the keyword trait. The following is the basic example syntax of trait.

Defining a Trait

trait Iterator[A] {
  def hasNext: Boolean
  def next(): A
}

Using trait

extends keyword is used to extend a trait. Then implement any abstract members of the trait using the override keyword.

trait Iterator[A] {
   def hasNext: Boolean
   def next(): A
}

class IntIterator(to: Int) extends Iterator[Int] {
    private var current = 0
    override def hasNext: Boolean = current < to
    override def next(): Int = {
       if (hasNext) {
          val t = current
          current += 1
          t
       } else 0
    }
}

object MyTest extends App {
   val iterator = new IntIterator(10)
   while(iterator.hasNext){
      println(iterator.next())
   }
}

Save the above program in MyTest.scala. Compile and run as mentioned below

scalac MyTest.scala
scala MyTest

Output

0
1
2
3
4
5
6
7
8
9

Lets take another simple example

We have a Trait Animal and there are two concrete classes that extends the Trait.

The trait Animal has an abstract field name that gets implemented by Cat and Dog in their constructors.
Animal also overrides the toString() method of Super class Any.

Example

trait Animal {
   val name: String
   override def toString()= { name }
}

class Cat(val name: String) extends Animal

class Dog(val name: String) extends Animal

object MyTest extends App {
   val dog = new Dog("Dog")
   val cat = new Cat("Cat")
   println(dog)
   println(dog)
}

Save the above program in MyTest.scala. The following commands are used to compile and execute this program.

scalac MyTest.scala
scala MyTest

Output

Dog
Cat

Example – Mixing classes and traits.

trait Logger{
   def log(msg: String){}
}

trait ConsoleLogger extends Logger{
   override def log(msg: String) { println(msg) }
}

class Account {
   protected var balance = 0.0
}

class SavingsAccount extends Account with ConsoleLogger{
   def withdraw(amount: Double) {
      if (amount > balance) log("Insufficient funds")
      else balance -= amount
   }
}  

object MyTest extends App {
   val acct1 = new SavingsAccount
   acct1.withdraw(100)
}

Save the above program in MyTest.scala. The following commands are used to compile and execute this program.

scalac MyTest.scala
scala MyTest

Output

Insufficient funds

References : https://docs.scala-lang.org/tour/traits.html

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