Traits

Traits are used to share interfaces and fields between classes. They are similar to Java 8’s interfaces. Classes and objects can extend traits but traits cannot be instantiated and therefore have no parameters.

Defining a trait

A minimal trait is simply the keyword trait and an identifier:

trait HairColor

Traits become especially useful as generic types and with abstract methods.

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

Extending the trait Iterator[A] requires a type A and implementations of the methods hasNext and next.

Using traits

Use the extends keyword 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
  }
}


val iterator = new IntIterator(10)
iterator.next()  // returns 0
iterator.next()  // returns 1

This IntIterator class takes a parameter to as an upper bound. It extends Iterator[Int] which means that the next method must return an Int.

Subtyping

Where a given trait is required, a subtype of the trait can be used instead.

import scala.collection.mutable.ArrayBuffer

trait Pet {
  val name: String
}

class Cat(val name: String) extends Pet
class Dog(val name: String) extends Pet

val dog = new Dog("Harry")
val cat = new Cat("Sally")

val animals = ArrayBuffer.empty[Pet]
animals.append(dog)
animals.append(cat)
animals.foreach(pet => println(pet.name))  // Prints Harry Sally

The trait Pet has an abstract field name which gets implemented by Cat and Dog in their constructors. On the last line, we call pet.name which must be implemented in any subtype of the trait Pet.