The main thing to keep in mind is that polymorphism requires an inheritance or an interface implementation. You can see this in the example below, featuring Duke and Juggy: ...

A method might accept a specific type and its subclasses, but you want to restrict usage to a select few implementations of your own design. Java's open inheritance model can feel like an open ...

Compare inheritance and composition, the two fundamental ways to relate Java classes, then practice debugging ClassCastExceptions in Java inheritance.

This looks like the Liskov substitution principle that Uncle Bob Martin described: Substitute a derived class (Square) for a base class (Rectangle). From the standpoint of the compiler, the above code ...