Implementing a Java interface method in Scala

Question

My implementation of a getHandler method, which is defined on a Java interface, has failed, and I don't know why. Here is the method signature:

<H extends EventHandler> H getHandler( Type<H> type, int index );

My implementation in Scala is:

def getHandler[H <: com.google.gwt.event.shared.EventHandler]
    (aType: Type[H], index: Int): H 
    = new com.google.gwt.event.shared.EventHandler() {}

...but the compiler gives me this message:

type mismatch;  
    found: java.lang.Object with com.google.gwt.event.shared.EventHandler
    required: H

Where have I erred?

Answer 1

I think it's because your implementation doesn't uphold the contract of the interface. H could be any subtype of EventHandler , determined by the type of the aType argument. But your implementation always returns the same anonymous subtype of EventHandler , regardless of what is passed as the aType argument.

I don't know what a correct implementation would be, but I can't see how this could be implemented without somehow making use of the aType parameter.

Answer 2

Adding to Lachlan's answer I'd like to point out, that simply casting the return type to the expected type may have desastrous effects.

Consider the following implementation where I have used a named class instead of the anonymous inner class from the question:

class MyHandlerA extends EventHandler
class MyHandlerB extends EventHandler

object BadImplementation extends I {
  def getHandler[H <: EventHandler](typ : Type[H], index: Int) = {
    (new MyHandlerA).asInstanceOf[H] // BAD: This asInstanceOf is a lie!
  }
}

The following line will cause a ClassCastException without warning.

val b: MyHandlerB = BadImplementation.getHandler(new Type[MyHandlerB] {} , 0)

So in addition to the cast an implementation will have to ensure that the returned handler is actually assignable to H or return null or throw an exception.

Answer 3

I think this may make it compile:

def getHandler[H <: com.google.gwt.event.shared.EventHandler]
    (aType: Type[H], index: Int): H = {
  val h = new com.google.gwt.event.shared.EventHandler() {}
  h.asInstanceOf[H]
}

It's expecting an H . As Lachlan says, aType probably is needed somewhere.