Any definition of polymorphism should at least mention something about the method that is invoked depends on runtime data, and is not resolved (or known) at com

Any definition of polymorphism should at least mention something about the method that is invoked depends on runtime data, and is not resolved (or known) at compile time. Yes class A { ... f() ... } class B extends A { ... f() ... } class C extends A { ... f() ... } A x = ... // a B object or a C object x.f() is polymorphism if anything.