Scala Cookbook

4.10. Handling Constructor Parameters When Extending a Class

Problem

You want to extend a base class, and need to work with the constructor parameters declared in the base class, as well as new parameters in the subclass.

Solution

Declare your base class as usual with val or var constructor parameters. When defining a subclass constructor, leave the val or var declaration off of the fields that are common to both classes. Then define new constructor parameters in the subclass as val or var fields, as usual.

For example, first define a Person base class:

class Person (var name: String, var address: Address) {
  override def toString = if (address == null) name else s"$name @ $address"
}

Next define Employee as a subclass of Person, so that it takes the constructor parameters name, address, and age. The name and address parameters are common to the parent Person class, so leave the var declaration off of those fields, but age is new, so declare it as a var:

class Employee (name: String, address: Address, var age: Int)
extends Person (name, address) {
  // rest of the class
}

With this Employee class and an Address case class:

case class Address (city: String, state: String)

you can create a new Employee as follows:

val teresa = new Employee("Teresa", Address("Louisville", "KY"), 25)

By placing all that code in the REPL, you can see that all of the fields work as expected:

scala> teresa.name
res0: String = Teresa

scala> teresa.address
res1: Address = Address(Louisville,KY)

scala> teresa.age res2: Int ...