Scala Cookbook

4.13. Defining Properties in an Abstract Base Class (or Trait)

Problem

You want to define abstract or concrete properties in an abstract base class (or trait) that can be referenced in all child classes.

Solution

You can declare both val and var fields in an abstract class (or trait), and those fields can be abstract or have concrete implementations. All of these variations are shown in this recipe.

Abstract val and var fields

The following example demonstrates an Animal trait with abstract val and var fields, along with a simple concrete method named sayHello, and an override of the toString method:

abstract class Pet (name: String) {
  val greeting: String
  var age: Int
  def sayHello { println(greeting) }
  override def toString = s"I say $greeting, and I'm $age"
}

The following Dog and Cat classes extend the Animal class and provide values for the greeting and age fields. Notice that the fields are again specified as val or var:

class Dog (name: String) extends Pet (name) {
  val greeting = "Woof"
  var age = 2
}

class Cat (name: String) extends Pet (name) {
  val greeting = "Meow"
  var age = 5
}

The functionality can be demonstrated with a simple driver object:

object AbstractFieldsDemo extends App {
  val dog = new Dog("Fido")
  val cat = new Cat("Morris")

  dog.sayHello
  cat.sayHello

  println(dog)
  println(cat)

  // verify that the age can be changed
  cat.age = 10
  println(cat)
}

The resulting output looks like this:

Woof
Meow
I say Woof, and I'm 2
I say Meow, and I'm 5
I say Meow, and I'm 10

Concrete field implementations ...