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 } 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 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() // 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 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.
More resources
- Learn more about traits in the Scala Book
- Use traits to define Enum