Scala在运行时检查对象是否与方法调用的给定参数化类型具有相同的类型

Question

I have a type called T and some subclasses (forgive me if this is not the correct terminology) of T called T1, T2, T3, etc.

I want to write a method that

• takes in a collection of Ts
• takes (preferably as a type parameter of the method) a type S <: T
• returns the elements of the input collection that are of type S

The first approach that comes to mind is as follows, but it is defeated by runtime type erasure.

    elements.filter(_.isInstanceOf[S])

Can someone please provide some insight on what would be the simplest way to achieve this? Looking into this on Google suggests that it is doable by having my method take a java.lang.Class as an argument and doing runtime reflection with java.lang.reflect related classes. That seems like it is too generic, though.

That is to say, I know what type I want to filter for at the call site. I would ideally want to be able to call, say, something like getInstancesOf[S] because I statically know what SI want at the call site.

Thanks in advance!

Answer 1

I ended up finding the answer in another StackOverflow question . I initially missed it because of how I was framing the question.

The outline of the answer is:

• The method takes an implicit ClassTag
• The compiler substitutes the appropriate ClassTag into the value of the implicitly call
• At runtime, the runtime class of the objects in question are checked against the ClassTag