Martin revisits the three pillars of OOP — encapsulation, inheritance, and polymorphism — and argues that OO's true architectural gift is safe, convenient polymorphism that enables plugin architecture and dependency inversion.
Deep dive into Java inheritance, abstract classes, polymorphism, method overriding rules, final keyword, immutable class design, and Object methods — with focus on the subtle OCP exam traps around field hiding and initialization order.
If S is a subtype of T, objects of type S must be substitutable for objects of type T without altering the correctness of the program. LSP governs not just inheritance but any interface contract — and its violation breaks architectural boundaries.