how to use asInstanceOf properly in Scala

Question

I have been playing with basic Scala data types. I noticed that the scala.Any class defines the method asInstanceOf[T0]: T0 from here The API has i

Answer 1

Java (and Scala) allows you to cast a primitive double to int (in Scala's case, Double to Int). On the other hand, you cannot cast java.lang.Double to java.lang.Int.

When you declare the Double as an Any, you're explicitly asking the compiler to forget that you gave it a Double. Therefore, in order to support the Any interface, the compiler is storing the value as a boxed double (i.e., java.lang.Double).

The behavior does seem confusing, but it's not a bug. According §12.1 of the Scala Language Spec:

The test x.asInstanceOf[T] is treated specially if T is a numeric value type (§12.2). In this case the cast will be translated to an application of a conversion method x.toT (§12.2.1).