How to call function from hashmap in Scala - list

Question

I'm pretty new to scala and basically I want to have a couple of functions coupled to a string in a hashmap.
However I get an error at subscribers.get(e.key)(e.EventArgs); stating Option[EventArgs => Unit] does not take parameters...
Example code:
object Monitor {
val subscribers = HashMap.empty[String, (EventArgs) => Unit ]
def trigger(e : Event){
subscribers.get(e.key)(e.EventArgs);
}
def subscribe(key: String, e: (EventArgs) => Unit) {
subscribers += key -> e;
}
}

Answer 1

The get method of a Map gives you an Option of the value, not the value. Thus, if the key if found in the map, you get Some(value), if not, you get None. So you need to first "unroll" that option to make sure there is actually a value of a function which you can invoke (call apply on):
def trigger(e: Event): Unit =
subscribers.get(e.key).foreach(_.apply(e.EventArgs))
or
def trigger(e: Event): Unit =
subscribers.get(e.key) match {
case Some(value) => value(e.EventArgs)
case None =>
}
There are many posts around explaining Scala's Option type. For example this one or this one.
Also note Luigi's remark about using an immutable map (the default Map) with a var instead.

Answer 2

Since the get method returns Option, you can use 'map' on that:
subscribers.get(e.key).map(f => f(e.EventArgs))
or even shorter:
subscribers.get(e.key) map (_(e.EventArgs))

Answer 3

get only takes one argument. So subscribers.get(e.key) returns an Option, and you're trying to feed (e.EventArgs) to that Option's apply method (which doesn't exist).
Also, try making the subscribers a var (or choosing a mutable collection type). At the moment you have an immutable collection and an immutable variable, so your map cannot change. A more idiomatic way to declare it would be
var subscribers = Map[String, EventArgs => Unit]()

Answer 4

HashMap.get() in Scala works in a bit different way, than in Java. Instead of returning value itself, get() returns Option. Option is a special type, that can have 2 values - Some(x) and None. In first case it tells "there's some value with such a key in a map". In second case it tells "nope, there's nothing (none) for this key in a map". This is done to force programmers check whether map actually has an object or not and avoid NullPointerException, which appears so frequently in Java code.
So you need something like this:
def trigger(e: Event) {
val value = subscribers.get(e.key)
value match {
case None => throw new Exception("Oops, no such subscriber...")
case Some(f) => f(e.EventArgs)
}
}
You can find more info about Option type and pattern matching in Scala here.