Why defining class as final improves JVM performance?

Question

Quoting from http://sites.google.com/site/gson/gson-design-document:

Why are most classes in Gson marked as final?

While Gson provides a

Answer 1

Marking classes as final allows further optimizations to be applied during the JIT stage.

If you are calling a virtual method on a non-final class, you don't know whether the proper implementation is the one defined in that class, or some sub-class that you don't know about.

However, if you have a reference to a final class, you know the specific implementation that is required.

Consider:

A extends B
B extends C

B myInstance = null;
if(someCondition)
   myInstance = new B();
else
   myInstance = new C();
myInstance.toString();

In this case, the JIT can't know whether C's implementation of toString() or B's implementation of toString() will be called. However, if B is marked as final, it is impossible for any implementation other than B's to be the proper implementation