How can I match classes in a Scala “match” statement?

Question

How can I use a \"match\" statement to identify the value of a class variable? The following is invalid, and I can\'t find an acceptable variant -- other than if ... else i

Answer 1

You can match on class values if you create a stable identifier (ie. a val) for them,

scala> val c: Class[_] = classOf[Int]
c: Class[_] = int

scala> val ClassOfInt = classOf[Int]
ClassOfInt: java.lang.Class[Int] = int

scala> val ClassOfFloat = classOf[Float]
ClassOfFloat: java.lang.Class[Float] = float

scala> val what = c match {
     |     case ClassOfInt => "int!"
     |     case ClassOfFloat => "float!"
     | }
what: String = int!

Note that you can't match on type (ie. Class[Int]) because erasure means that the different type instantiations of Class[T] are indistinguishable at runtime ... hence the warning below

scala> val what = c match {
     |     case _: Class[Int] => "int!"
     |     case _: Class[Float] => "float!"
     | }
warning: there were 2 unchecked warnings; re-run with -unchecked for details
what: java.lang.String = int!