Scala: getting the name of the class the trait is mixed in
Question
Given an instance of a class, we can obviously return its name:
trait MixedInClassDiscovery {
val className = this.getClass.getName
}
class AClass extends MixedInClassDiscovery {
...
this.className // returns "AClass"
...
}
But this way uses reflection, once for every instance of AClass
. Can the same be done once for every class, instead?
One solution which comes to mind is to mix it into companion objects instead of classes themselves.
Solution
I can't think of any way to do it with no extra overhead. You could do it with companion objects, however, and a couple of extra pieces of work:
object Example {
trait Discovery {
def companion: Discovered
def className: String = companion.className
}
trait Discovered extends Discovery {
override lazy val className = {
println("Getting class name!") // To see how many times we're called
this.getClass.getSuperclass.getName
}
}
class Test extends Discovery {
def companion = Test
}
object Test extends Test with Discovered {}
}
And here we see that this works:
scala> val a = new Example.Test
a: Example.Test = Example$Test@17e4c97
scala> val b = a.className
Getting class name!
b: String = Example$Test
scala> val c = a.className
c: String = Example$Test
but it comes at rather a price: you need to not only decorate the class with Discovery but also implement the companion method and write the companion object (which need not have the same name, incidentally) for every class.
OTHER TIPS
You can do it with the pimp my lib pattern. Create an implicit conversion from AnyRef
to e.g. ClassNameAdder
. But it is not recommended to do create such an implicit conversion at this level of the type hierarchy.
Anyway here comes the code:
scala> class ClassNameAdder(ref: AnyRef) { def className = ref.getClass.getName }
defined class ClassNameAdder
scala> implicit def anyref2classnameadder(ref: AnyRef) = new ClassNameAdder(ref: AnyRef)
anyref2classnameadder: (ref: AnyRef)ClassNameAdder
scala> "foo".className
res6: java.lang.String = java.lang.String
scala> new Object().className
res7: java.lang.String = java.lang.Object
scala> List(1,2,3).className
res8: java.lang.String = scala.collection.immutable.$colon$colon
scala> class MyClass
defined class MyClass
scala> val myClass = new MyClass
myClass: MyClass = MyClass@1398044
scala> myClass.className
res9: java.lang.String = MyClass