Sam Stites

Trait Fields in Scala

07 April 2015 - Reykjavik, Iceland

I mentioned in an earlier post that methods declared, but not defined, are implicitly defined as abstract:

trait Foo {
  def bar(x: String)     // abstract definition
  def baz(y: String) {}  // concrete definition

The same goes with fields:

trait Foo {
  val bar:Int   // abstract field
  val baz = 10  // concrete field

When a class inherits from a trait, these fields are placed into the subclass - not inherited. Note that the JVM itself only allows for one superclass, so traits can’t be inherited in the same way - hence this distinction.

It’s also worth noting that, when you are dealing with traits which contain abstract fields, you must override the field in the class itself - just like you must with methods. Some syntactic sugar includes that you do not need to add an override modifier in your class in order to initiate abstract fields.