Scala diamond prob, find class name from instance of class D

Question

I have four traits A, B, C and D in the typical diamond problem hierarchy. I have implemented a method calculate in trait A which checks the instance of callerObject, performs some calculation and returns trait A type object. calculate method is successfully able to check the instanceType of callerObject when callerObject belongs to trait B or C but doesn't work for object of trait D and I get the following class cast exception:

java.lang.ClassCastException: packageName.B$$anon$1 cannot be cast to packageName.D

Can you please suggest way forward, how can I check the Type of the object from trait D in method of trait A.

PS: I am new to Scala.

Answer 1

With pattern matching the order of the case statements matters. Make sure that the most specific class is always matched at the top. For example if B extends A and C extends B, it means that objects of C will always match anything that looks for either B or A, etc.

Here is a toy example that can better explain the solution:

sealed trait A {
   def calculate(i: A) = i match {
      case _:D => "d" // Make sure the D is checked first!
      case _:B => "b"
      case _:C => "c"
      // If you want to match for A make sure it is added last
   }
}
trait B extends A
trait C extends A
trait D extends B with C

Here is an example in the REPL:

val b = new B{}
val c = new C{}
val d = new D{}

b.calculate(b)
res> "b"
b.calculate(c)
res> "c"
b.calculate(d)
res> "d"