Question

say your given an instance of List[Class[_ <: Base]], and say that many classes extends Base:

class A extends Base
class B extends Base
class C extends A

etc’…
now, the given list may contain only some classes. e.g. : val classes = classOf[A] :: Nil, well, how can i test when i get an instantiated val if it’s class is of type that is found in the list, or if it a subclass for a class in the list? i.e. how would you implement:
def testClass(class : List[Class[_ <: Base]], instance : Base) : Boolean
when:

val classes = classOf[A] :: Nil
testClass(classes, new A) // should return true
testClass(classes, new B) // should return false
testClass(classes, new C) // should return true

use case:
i’m trying to write a generic retry pattern, i.e. getting some code that is very error prone, and i want to retry executing it for some number of maximum tries, when every time it fails, it should execute some “wait” method.
e.g.

retryRequest({
    //Some code that throws exceptions
}, classOf[SomeException] :: classOf[SomeOtherException] :: Nil,
100, {Thread.sleep(5000)})

well this works OK, but it won’t test for subclasses of a given exception:

def retryRequest(req : => Unit, validExceptions : List[Class[_ <: java.lang.Throwable]], tries : Int, waitMethod : => Unit) {
    var keepTrying = false
    var tryCount = 0
    do{
        try{
            logger.debug("retryRequest, try #" + tryCount)
            keepTrying = false
            req
        }catch{
            case ex if(tryCount >= tries && validExceptions.contains(ex.getClass)) => {
                throw new MaxTriesReachedException("tried for " + tryCount + "times, but no luck. " +
                                                   "you may want to try ommitting generic exceptions types from the given list.") 
            }
            case ex if (validExceptions.contains(ex.getClass)) => {
                logger.debug("intercepted " + ex.toString)
                tryCount += 1
                keepTrying = true
                waitMethod
            }
        }
    }while(keepTrying)
}

i would realy want to replace:

validExceptions.contains(ex.getClass)

with something like:

validExceptions.exists(exClass => ex.isInstanceOf[exClass]) //won't compile

is it possible? how?

Answer 1

I think this should work:

validExceptions.exists(exClass => exClass.isAssignableFrom(ex.getClass))

http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#isAssignableFrom%28java.lang.Class%29