Generics were only added in Java 1.5, they were absolutely not part of the original design, and type erasure is very much a considerable restriction in some cases. I would wager that a greenfield design would look quite different.

Whether you call that an afterthought? I'm sure the designers had this in mind, but at the same time they painted themselves into a corner somewhat with earlier decisions and a huge dedication to backward compatibility. Their work on threads on the other hand shows some real forethought and they were able to avoid most of the pitfalls that a few other language impl's fell into.

Generics in C# were also added after launch but have a significantly better implementation. While they are a key part of the language they aren't a key part of the platform.