A map from keys of type A to values of type B. To implement a concrete map, you need to provide implementations of the following methods (where This is the type of the map in question):

    def get(key: A): Option[B]
    def iterator: Iterator[(A, B)]
    def + [B1 >: B](kv: (A, B1)): This
    def -(key: A): This

If you wish that methods like, take, drop, filter return the same kind of map, you should also override:

    def empty: This

It might also be a good idea to override methods foreach and size for efficiency.