4. 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 then
			val t = current
			current += 1
			t
		else
			0
end IntIterator
 
val iterator = IntIterator(10)
iterator.next() // 0
iterator.next() // 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 = Dog("Harry")
val cat = 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 that 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.