Clean exception handling in Scala - ruby

Question

In Ruby I often like to handle null values from collections with the following function:
def nilstuff(a,stuff="")
if(a.nil?)
return stuff
else
return a
end
end
In Scala there is an annoyance that empty values in collections throw exceptions, not nil:
val myMap = Map[String, String]()
myMap += ("Apple" -> "Plant")
myMap += ("Walrus" -> "Animal")
println(myMap("Elephant"))
//Exception in thread "main" java.lang.ExceptionInInitializerError
// at MyProgram.main(MyProgram.scala)
//Caused by: java.util.NoSuchElementException: key not found: Elephant
Is there a way to make a similar function in Scala that handles exceptions and returns the "stuff" instead?
println(missing_stuff(myMap("Elephant"),"Unknown"))

Answer 1

You can add a default value to your Map:
scala> import scala.collection.mutable.Map
import scala.collection.mutable.Map
scala> val myMap = Map[String, String]().withDefaultValue("Unknown")
myMap: scala.collection.mutable.Map[String,String] = Map()
scala> myMap("foo")
res0: String = Unknown
Another option is the getOrElse method of Map.
Or apply a pattern match to the result of get:
myMap.get("foo") match {
case Some(value) => useAsDesired(value)
case None => useAsDesired("Unknown")
}
The last might be the most general solution to what your title calls "Clean exception handling."

Answer 2

There are several ways built in.
(1) Don't get the value, get an option.
myMap.get("Elephant")
Now that you have an Option, you can do all sorts of things with it (including get either its contents or a default value if there is none):
myMap.get("Elephant").getOrElse("")
(2) Get the value, or a default value if it's not there
myMap.getOrElse("Elephant", "")
(3) Create the map with a default value (warning--this will not survive filtering, mapping, or any other handy collections operations). Immutably you'd add this after you were done building the map:
val defaultMap = myMap.withDefault(_ => "")
defaultMap("Elephant")
With a mutable map, you might add it at the beginning:
val myMap = new collection.mutable.HashMap[String,String].withDefaultValue("")
(4) Add the item that you're missing when you find it not there (mutable maps only):
myMap.getOrElseUpdate("Elephant", "Dumbo")
Probably not what you want in this particular case, but often useful.