Clean exception handling in Scala

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.