Polymorphism in Java

Quick Answer

The detail that makes an interface reference calling a method the clearest demonstration of polymorphism is that the decision about which actual implementation runs is made at runtime, based on the real class of the object behind the reference, rather than at compile time based on the reference's declared type. Declaring a variable with an interface type and assigning it any object that implements that interface lets the same line of calling code — reference.method() — invoke a completely different method body depending on which concrete class was actually assigned, without the calling code needing to know or care which implementation it's dealing with. This is the essence of polymorphism: one interface, many possible forms of behavior, resolved dynamically rather than fixed in advance. It's a more direct illustration of the concept than something like method overloading, which is resolved at compile time based on argument types and doesn't involve this runtime dispatch at all — a distinction worth holding onto, since overloading and polymorphism get conflated but work through entirely different mechanisms. Recognizing runtime method resolution through a supertype or interface reference, as opposed to compile-time resolution, is the specific skill these polymorphism questions test.