Topic 6. Inheritance, polymorphism, and the Object class

Goal: learn to build correct hierarchies, override methods, handle subtypes through a shared contract, and check object equality.

Lecture contents

1. Subclasses and their constructors — Shared behavior of domain types · The subclass constructor and base portion · Example 1. Employee payroll
2. Overriding and type casts — Protected access does not mean public · Overriding, overloading, and hiding · Example 2. Shapes and safe casts
3. Object, equality, and copying — Object and logical equality · Example 3. An immutable point as a value · Copying: references, fields, and nested objects
4. Composition and the substitution principle — Composition and the substitution principle · Example 4. A stack through composition · Inspecting the hierarchy in IntelliJ IDEA

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