week 4.3 4.4 polymorphysm

type of polymorphism ==> bounds

def assertAllPos[S<:IntSet](r:S):S

S is a subtype of IntSet

Notes:

• In java Arrays are co-variant ==> we allow to have subtypes of the defined types
• In scala List are covatiant
• In scala Arrays are not ==> new Array[NonEmpty]
• class is covariant C[+A]
• methods are covariant for results, and are not covariant for parameters

Object vs class:

• You can think of the object keyword as creating a singleton object of a class that is defined implicitly.

Abstract classes vs traits:

• Abstract classes can have constructor parameters as well as type parameters. Traits can have only type parameters. There was some discussion that in future even traits can have constructor parameters
• Abstract classes are fully interoperable with Java. You can call them from Java code without any wrappers. Traits are fully interoperable only if they do not contain any implementation code