How to use a Scala trait like an abstract class

By Alvin Alexander. Last updated: November 20, 2020

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is a short recipe, Recipe 8.3, “How to use a Scala trait like an abstract class.”

Problem

You want to use a Scala 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.

Like Java, you use super.foo to call a method named foo in an immediate superclass. When a class mixes in multiple traits—and those traits implement a method declared by a common ancestor — you can be more specific, and specify which trait you’d like to invoke a method on. See Recipe 5.2, “Calling a Method on a Superclass”, for more information.
See Recipe 4.12, “When to Use an Abstract Class”, for information on when to use an abstract class instead of a trait. (Spoiler: Use an abstract class (a) when you want to define a base behavior, and that behavior requires a constructor with parameters, and (b) in some situations when you need to interact with Java.)