What are the differences between asInstanceOf[T] and (o: T) in Scala?

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  • foo.asInstanceOf[Bar] is a type cast, which is primarily a runtime operation. It says that the compiler should be coerced into believing that foo is a Bar. This may result in an error (a ClassCastException) if and when foo is evaluated to be something other than a Bar at runtime.

  • foo:Bar is a type ascription, which is entirely a compile-time operation. This is giving the compiler assistance in understanding the meaning of your code, without forcing it to believe anything that could possibly be untrue; no runtime failures can result from the use of type ascriptions.

Type ascriptions can also be used to trigger implicit conversions. For instance, you could define the following implicit conversion:

implicit def foo(s:String):Int = s.length

and then ensure its use like so:

scala> "hi":Int                                 
res29: Int = 2

Ascribing type Int to a String would normally be a compile-time type error, but before giving up the compiler will search for available implicit conversions to make the problem go away. The particular implicit conversion that will be used in a given context is known at compile time.

Needless to say, runtime errors are undesirable, so the extent to which you can specify things in a type-safe manner (without using asInstanceof), the better! If you find yourself using asInstanceOf, you should probably be using match instead.

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