OOP foundations

In this module

• A class is a blueprint; an object is a concrete instance created from it, with its own state.
• Bundle data with the methods that guard it, hide the internals, and expose only a safe public interface, so invariants can’t be broken.
• Expose WHAT something does through a simple interface, and hide HOW it does it. Abstraction fights complexity; encapsulation enforces it.
• A subclass IS-A superclass, it inherits the parent’s fields and methods for free, and may add to or override them.
• One call, many forms: the SAME method name resolves to different behaviour depending on the actual object, decided at runtime.
• Inheritance locks behaviour in at compile time and explodes combinatorially. Composition, “has-a”, keeps it swappable. Prefer it.
• The arrows ARE the design. Dependency, association, aggregation, composition and inheritance say exactly how tightly two classes are bound, and who owns whose lifecycle.
• An abstract class is a half-built base you EXTEND (one parent, shared code + state); an interface is a pure contract you IMPLEMENT (many of them, no state). Knowing which to reach for is a classic interview question.