"parametric polymorphism" is fancy way to say "I'm declaring a type that is composed of other types". The canonical example is a set. In C# (that's what I'm f

"parametric polymorphism" is fancy way to say "I'm declaring a type that is composed of other types". The canonical example is a set. In C# (that's what I'm familiar with) we have IEnumerable<T>. It represents enumerating over some homogeneous set of type T. so when you write "var myList = new List<int>() { 1, 2, 3 };" the type that gets written down in the bytecode is List<int> (which implements IEnumerable<int>). So the statement "typeof(myList) == typeof(List<int>)" returns true. The type you specified at compile time doesn't loose information at runtime. (For instance you can also do typeof(IEnumerable<>).MakeGenericType(new[] { typeof(int) }), which gets the Generic Type Definition and then you can fill that in at runtime). The Generics implementation in Java 5 however was all a compiler trick. All the benefits only existed in the AST of the compiler, they never got down to the runtime. So it was easy to check that you couldn't add a int to a strongly typed list of strings, but only in source. (I think, I'm not as well read as javac / JVM internals) Hopefully that helps.