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Abstract Members

A member of a class or trait is abstract if the member does not have a complete definition in the class. Abstract members are intended to be implemented in subclasses of the class in which they are declared. Unlike Java, besides methods, you can also declare abstract fields and even abstract types as members of classes and traits.
  • Traits by definition are abstract
  • Types: one reason to use a type member is to define a short, descriptive alias for a type whose real name is more verbose, or less obvious in meaning, than the alias. Such type members can help clarify the code of a class or trait. The other main use of type members is to declare abstract types that must be defined in subclasses
  • It is OK to override a ‘def’ with a ‘val’ in the child class, however you cannot override a ‘val’ with a ‘def’
  • vars declared as members of classes come equipped with getter and setter methods. This holds for abstract vars as well. If you declare an abstract var named hour, for example, you implicitly declare an abstract getter method, hour, and an abstract setter method, hour_=
  • Pre-initialized fields let you initialize a field of a subclass before the superclass is called:
    • Pre-initialized fields in an anonymous class expression:
new {
     val numerArg = 1 * x
     val denomArg = 2 * x
} with RationalTrait
  • Pre-initialized fields in an object definition:
object twoThirds extends {
    val numerArg = 2
    val denomArg = 3
} with RationalTrait
  • Lazy vals: sometimes you might prefer to let the system itself sort out how things should be initialized. This can be achieved by making your val definitions lazy. If you prefix a val definition with a lazy modifier, the initialising expression on the right-hand side will only be evaluated the first time the val is used. This is similar to the situation where x is defined as a parameterless method, using a def. However, unlike a def a lazy val is never evaluated more than once. In fact, after the first evaluation of a lazy val the result of the evaluation is stored, to be reused when the same val is used subsequently