JIT'ers tend to be aggressive and disregard language semantics. As far as I know, the java hotspot compiler goes ahead and inlines implementations of calls on generic interfaces, as long as a couple of hundred calls to into the same implementation method. For large methods, it might not inline, but it'll remove the conditional jump based on the type with a direct jump.

That's obviously wrong, examples to show that are trivial. But that's why the direct call also contains a trap to check the type to perform de-optimization if the assertion appears to be wrong. But a simple 'assert type byte sequence is a known value' is still faster than a dynamic jump. A lot faster.