withDefaultValue in case class - scala

Question

simple case class:
case class country(name: String, townPopulation: Map[String,Int])
with simple example:
scala> val germany = country("Germany",Map("Berlin" -> 100000, "Saale" -> 4000))
germany: country = country(Germany,Map(Berlin -> 100000, Saale -> 4000))
scala> germany.townPopulation("Berlin")
res77: Int = 100000
scala> germany.townPopulation("blergh")
java.util.NoSuchElementException: key not found: blergh
at scala.collection.MapLike$class.default(MapLike.scala:228)
at scala.collection.AbstractMap.default(Map.scala:59)
at scala.collection.MapLike$class.apply(MapLike.scala:141)
at scala.collection.AbstractMap.apply(Map.scala:59)
... 42 elided
I would like to return 0 for towns that dont exist, this can be done when declaring a val:
scala> val germany = country("Germany",Map("Berlin" -> 100000, "Saale" -> 4000).withDefaultValue(0))
germany: country = country(Germany,Map(Berlin -> 100000, Saale -> 4000))
scala> germany.townPopulation("fdhjkjhkhjdfg")
res79: Int = 0
but I can not figure out how to do it in one place, at least not when it is a case class, I would like something simple as the following, but I am obviously doing it wrong:
scala> case class country(name: String, townPopulation: Map[String,Int].withDefaultValue(0))
<console>:1: error: ')' expected but '.' found.
case class country(name: String, townPopulation: Map[String,Int].withDefaultValue(0))
^
<console>:1: error: ';' expected but ')' found.
case class country(name: String, townPopulation: Map[String,Int].withDefaultValue(0))
Is there a short simple path to a solution that has 0 as defaultValue always?

Answer 1

I see few possible ways:
add auxiliary method which encapsulate logic of default value
def population(town : String) : Int = townPopulation.getOrElse(town, 0)
add method to companion object with same purpose
def withDefault(name: String, townPopulation: Map[String, Int]) : country =
country(name, townPopulation.withDefaultValue(0))

Answer 2

Use map.get(), which returns an Option:
println germany.townPopulation.get("blergh").getOrElse(0)
// or, more concisely:
println germany.townPopulation.getOrElse("blergh", 0)
Ah, on re-reading your question, you want to hard-code the default value in the case class. I guess you'll have to mess with the apply() method.

Answer 3

val germany = country("Germany",
Map("Berlin" -> 100000, "Saale" -> 4000)
.withDefaultValue(0))
Edit (after OP's answer):
My bad! Should have read more thoroughly your question.
As stated in this SO question: You do not have the option of changing the way the default constructor stores its parameters (e.g. by modifying the parameters before they are stored as vals) […].
An alternate solution would be to declare a regular class along with its companion object:
class Country(val name: String, val townPopulation: Map[String, Int])
case object Country {
def apply(name: String, townPopulation: Map[String, Int]) =
new Country(name, townPopulation.withDefaultValue(0))
}
This would allow you to declare countries using the nice syntax:
val germany = Country("Germany", Map("Berlin" -> 100000, "Saale" -> 4000))
assert(germany.townPopulation("Berlin") == 100000)
assert(germany.townPopulation("Blergh") == 0)
But note that, as it is not a case class you won't get the usual case class perks such as:
// Compiler will give you
// "object Country is not a case class,
// nor does it have an unapply/unapplySeq member"
//germany match {
// case Country(a, b) => println("It won't compile! Not a case class")
//}
Depending on your use case, you could go the long hard road and implement methods unapply and unapplySeq to retrieve such behavior if needed!