Topic 6. Inheritance, abstract classes, interfaces, and polymorphism

Goal: create class hierarchies and interface contracts; use overriding, polymorphic algorithms, and consistent object equality.

Lecture contents

1. A common type and open classes — Why a program needs a common type · Classes are closed by default · Dynamic dispatch
2. Abstract classes — An abstract class and a complete hierarchy
3. Interfaces and implementation conflicts — An interface: a capability without a shared field · Conflicting default implementations
4. Type casts, Any, and initialization — Type checks and safe casts · Any: equality, hashing, and string representation · Initialization and unsafe open calls
5. The template method and hierarchy testing — The template method and composition · UML and hierarchy testing · Designing a contract test

Practice and review

• Practice — 3 worked examples with complete code
• Tasks — 30 variants, each with tasks at three difficulty levels
• Summary — conclusions and self-check questions