Java does not have parametric polymorphism because instanceof exists in the language. That reduces what you can say about something from the type alone to "maybe parametric, maybe bounded by runtime checks that aren't visible in the type."
How many languages provide you a way to guarantee at the type level that the behavior of a function cannot be influenced by instantiation of type variables? I don't believe that to be true of anything that runs in the JVM or in .NET, for instance, as runtime type information is always available. C++ has dynamic_cast. What common language actually has parametricity expressed in the type system?
That’s true..but Haskell has bottom and Scala has bottom + instanceOf checks and JVM reflection as well. And they clearly have usable parametric polymorphism despite things being “broken.”
Saying Java doesn’t have parametric polymorphism because you can cheat is like saying Haskell isn’t pure because of unsafePerformIO.
Agreed. There is always a point where it's the developer's responsibility to write good code, even in languages where it's very difficult to shoot yourself in the foot.
How many languages provide you a way to guarantee at the type level that the behavior of a function cannot be influenced by instantiation of type variables? I don't believe that to be true of anything that runs in the JVM or in .NET, for instance, as runtime type information is always available. C++ has dynamic_cast. What common language actually has parametricity expressed in the type system?