override modifier

Scala requires override modifiers on items which override items with the same defined in the parent class, but it does not require it if these parent items are abstract:

abstract class Root {
  def x: Int = 10
  abstract def y: Int
}

class Child extends Root {
  override def x: Int = 12
  def y: Int = 42
}

However, you should always use override modifier even for implementing abstract methods. This makes the fact that the item actually "comes" from a parent class obvious and it allows accidental non-overrides, for example, as a side effect of refactoring, to be caught during compilation easily.

Note that you must use the override modifier even if you define the overriding field as a part of constructor arguments declaration:

trait Entity {
  def id: String
}

case class User(override val id: String) extends Entity