Scala Cookbook

8.3. Using a Trait Like an Abstract Class

Problem

You want to use a trait as something like an abstract class in Java.

Solution

Define methods in your trait just like regular Scala methods. In the class that extends the trait, you can override those methods or use them as they are defined in the trait.

In the following example, an implementation is provided for the speak method in the Pet trait, so implementing classes don’t have to override it. The Dog class chooses not to override it, whereas the Cat class does:

trait Pet {
  def speak { println("Yo") }   // concrete implementation
  def comeToMaster              // abstract method
}

class Dog extends Pet {
  // don't need to implement 'speak' if you don't need to
  def comeToMaster { ("I'm coming!") }
}

class Cat extends Pet {
  // override the speak method
  override def speak { ("meow") }
  def comeToMaster { ("That's not gonna happen.") }
}

If a class extends a trait without implementing its abstract methods, it must be defined as abstract. Because FlyingPet does not implement comeToMaster, it must be declared as abstract:

abstract class FlyingPet extends Pet {
  def fly { ("I'm flying!") }
}

Discussion

Although Scala has abstract classes, it’s much more common to use traits than abstract classes to implement base behavior. A class can extend only one abstract class, but it can implement multiple traits, so using traits is more flexible.

See Also

• Like Java, you use super.foo to call a method named foo in an immediate superclass. When a class mixes in multiple traits—and ...