using trait as generic interface in scala

Question

I've got a pseudo-generic class factory that I'm using for DAO purposes. The operational model is pretty straightforward:
1) the factory is instantiated with knowledge of a specific DAO-object "T"ype
2) it then creates an internal instance of T
3) it should call a trait-contracted function of T
4) finally, return the instance of T (assuming all goes well)

It looks like this:

class DAOFactory[T](implicit m: Manifest[T]) {
   def obj = m.runtimeClass.asInstanceOf[Class[T]]
   def doFoo(): Option[Any] = {
       val dao = obj.asInstanceOf[DAO]     // (A) this line will crash
       println(obj.toString())    // some.object.called.DAOTodo
       // val bar = dao.doSomethingSmart(now)   <--- ALL DAO-derived classes implement this function
       return Some(obj) // error-catching excluded for brevity 
   }
}

It is used like this:
val f = new DAOFactory[DAOTodo]
val shinyNewObject = f.doFoo() // never gets here.

Where 'DAOTodo' is, in fact, a class that implements the DAO trait:

class DAOTodo extends DAO {
   def doSomethingSmart(when: Whenever) = {...}
}

Q: what needs to be done at point "A" to use DAO as the interface to the "doSomethingSmart" function of "obj"?

Thanks in advance.

ANSWER: Aside from the code modification as outlined in the answer below, this was not running as predicted due to the fact that the lineage for the classes being created (within the factory) had a primary constructor that was not being fulfilled. Creating an additional zero-param constructor resolved the issue.

cmbaxter · Accepted Answer · 2013-05-04 00:19:43Z

5

Try changing this line:

val dao = obj.asInstanceOf[DAO]

To this:

val dao = obj.newInstance().asInstanceOf[DAO]

You can't cast the Class[T] to an instance of the DAO trait, so I'm assuming what you instead wanted to do was instantiate a new instance from the class type.

But to make it even better, you can define your factory like so:

class DAOFactory[T <: DAO](implicit m: Manifest[T]) {

And then change that line to be like this:

val dao = obj.newInstance()

Removing the cast there is possible because you are constraining the type of T to contain the DAO trait.

Putting it all together (with a couple of small mods), this code runs successfully for me using Scala 2.10:

import java.util.Date

object DAOTest{
  def main(args: Array[String]) {
    val fact = new DAOFactory[DAOTodo]
    fact.doFoo
  }
}

class DAOFactory[T <: DAO](implicit m: Manifest[T]) {
   def obj = m.runtimeClass.asInstanceOf[Class[T]]
   def doFoo(): Option[Any] = {
     val dao = obj.newInstance()
     println(obj.toString())
     val bar = dao.doSomethingSmart(new Date) 
     return Some(obj) 
   }
}

class DAOTodo extends DAO {
   def doSomethingSmart(date:Date) = {}
}

trait DAO{
  def doSomethingSmart(date:Date)
}

edited May 4, 2013 at 0:19

answered May 3, 2013 at 23:53

cmbaxter

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4 Comments

mjk Over a year ago

That compiles with the warning, "fruitless type test: a value of type Class[T] cannot also be a some.library.DAO" and then throws a "java.lang.InstantiationException." I'll have to read up on that one. Any ideas? OOC, how are scala traits used for generics? I've seen a lot of usage for calling traits of known classes, but not generics. Does Scala steer the user towards a different model? (Or maybe i've got the terminology wrong for scala-ese."

cmbaxter Over a year ago

Funny, I'm not getting any errors or warnings. Posted my full code example.

mjk Over a year ago

hmmm. I'm on 2.10.0 as well. I'm running this is scalatra, but I don't think that would effect it, do you? One slight oversight...the DAOTodo class I'm using is actually a "case class." Would that matter?

mjk Over a year ago

cmbaxter, thank you. I still cannot get it to execute in the framework - but I did do an extraction such as your example and it worked fined. So, there's something else at the heart of the problem & I'm closing this issue out. cheers -mjk

Harshal Pandya · Accepted Answer · 2013-05-03 23:52:51Z

0

Seems to me like you're using a Manifest for the wrong reasons. You should be using the Runtime Reflection from reflection Api. Manifest is for a getting around type erasure for parameterized types. What you need is explained here: http://docs.scala-lang.org/overviews/reflection/overview.html

answered May 3, 2013 at 23:52

Harshal Pandya

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1 Comment

mjk Over a year ago

I've used the reflection in cases where the classname was passed in as a string, but thought this type would be functional -- and it appears to be, to some extent: the class type vocalized at the println is the right class instance, but I just just can seem to access it via an interface/trait.

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