How do I use java reflection to instantiate a class that cannot be imported?

I'm writing JUnit tests to test code in another package which must match a pre-defined specification. The code under test must contains a nested class which I want to instantiate inside the test package -- it's not possible to import this class because it's nested (and so not visible outside the original package).

E.g. package structure:

packageA
-> ClassA
     -> packageA.ClassA$ClassB

tests
-> ClassATest

I need to test a method in ClassA that takes an instance of ClassB as an argument.

I'm currently using reflection to get from ClassA (which I can import) to ClassB [cls.getDeclaredClasses]. I can then get a constructor for ClassB [cls.getDeclaredConstructor] and thus create a new instance of ClassB using the found constructor [constructor.newInstance]. This call to constructor.newInstance returns an Object.

How do I get from the returned Object type to something of the correct ClassB type so that I can pass it into the ClassA method under test?