Scala Cookbook

8.1. Using a Trait as an Interface

Problem

You’re used to creating interfaces in other languages like Java and want to create something like that in Scala.

Solution

You can use a trait just like a Java interface. As with interfaces, just declare the methods in your trait that you want extending classes to implement:

trait BaseSoundPlayer {
  def play
  def close
  def pause
  def stop
  def resume
}

If the methods don’t take any argument, you only need to declare the names of the methods after the def keyword, as shown. If a method should require parameters, list them as usual:

trait Dog {
  def speak(whatToSay: String)
  def wagTail(enabled: Boolean)
}

When a class extends a trait, it uses the extends and with keywords. When extending one trait, use extends:

class Mp3SoundPlayer extends BaseSoundPlayer { ...

When extending a class and one or more traits, use extends for the class, and with for subsequent traits:

class Foo extends BaseClass with Trait1 with Trait2 { ...

When a class extends multiple traits, use extends for the first trait, and with for subsequent traits:

class Foo extends Trait1 with Trait2 with Trait3 with Trait4 { ...

Unless the class implementing a trait is abstract, it must implement all of the abstract trait methods:

class Mp3SoundPlayer extends BaseSoundPlayer {
  def play   { // code here ... }
  def close  { // code here ... }
  def pause  { // code here ... }
  def stop   { // code here ... }
  def resume { // code here ... }
}

If a class extends a trait but does not implement the abstract methods ...