scala.Null and scala.Nothing
excerpted from : An overview of the Scala Programming Language
Type Null is the subtype of all reference type; its only instance is the null reference. since Null is not a subtype of value types, null is not a member of any such type. For instance, it is not possible to assign null to a variable of type int.
Type Nothing is a subtype of every other type; there exist no instances of this type. Even though type Nothing is empty, it is nevertheless useful as a type parameter.
eg., the scala library defines a value Nil of type List[Nothing]. Because lists are covariant in Scala, this makes Nil an instance of List[T], for any element type T.