Scala asInstanceOf generic type does not fail in Try

Question

Trying to cast a String to a Double obviously should fail:

scala> Try("abc".asInstanceOf[Double])
res11: scala.util.Try[Double] = Failure(java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Double)

However, if I define the above as a function:

scala> def convertAsTypeOf[T](anyValue: Any): Try[T] = Try(anyValue.asInstanceOf[T])
convertAsTypeOf: [T](anyValue: Any)scala.util.Try[T]

Strangely it returns Success when I try to cast a String to a Double :

scala> convertAsTypeOf[Double]("abc")
res10: scala.util.Try[Double] = Success(abc)

If I try to get the value out of Success, I get the following exception:

scala> convertAsTypeOf[Double]("abc").get
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Double
        at scala.runtime.BoxesRunTime.unboxToDouble(BoxesRunTime.java:119)

Why does this happen? Is there any way to fix this and make asInstanceOf[T] when it's in a generic function?

Answer 1

Because of type erasure, the type T is unknown at runtime, so asInstanceOf can't actually check the cast that it does. In fact, it compiles as a no-op. However, when the .get is finally done a cast is made to Double , and because the type is known here we can get a ClassCastException .

If you change convertAsTypeOf to use ClassTag :

def convertAsTypeOf[T](anyValue: Any)(implicit tag: ClassTag[T]): Try[T] =
  Try(tag.runtimeClass.cast(anyValue).asInstanceOf[T])

You will then get errors when you expect:

scala> convertAsTypeOf[Double]("abc")
res1: scala.util.Try[Double] = Failure(java.lang.ClassCastException: Cannot cast java.lang.String to double)

The ClassTag represents the type T at runtime, allowing the value to be checked against it.

Answer 2

The reason this happens is type erasure, which makes asInstanceOf[T] for generic T useless. I don't know why the compiler doesn't issue any warning.

You can force the check to happen by using a ClassTag. Something like this:

import reflect.ClassTag

def convertAsTypeOf[T : ClassTag](anyValue: Any): Try[T] = Try {
    anyValue match { case t: T => t }
}

This works because the compiler will use the implicit ClassTag in the case match if it is available (and generate a warning if it is not).