However, see below about MStack and generic collections.
Built-in Scala libraries often make heavy use of the Option type. See section about HashMap below.
Avoid scala.collections.HashMap. Use Java HashMap Instead
This is a library instance of the "avoid Option type" problem.
Scala's mutable HashMap allocates a Some\[T] object for every successful get(key) call.
This is unacceptable overhead for something done so frequently. Use Java's HashMap instead, where get(key) returns a value or null, and never allocates anything.
Avoid Generic Collections of Unboxed Types