Object Oriented Design Principles Cheat Sheet

Concepts and Lists of Principles

Photo by Ronan Furuta on Unsplash

TL;DR

My aim here is not to go into detail but create a cheatsheet to quickly review before interviews to serve as a reminder.

OOP Concepts

• Abstraction (hide complexity — blackbox, show only essential details. interfaces, abstract classes)
• Encapsulation (private fields, public getter/setter. code update in one place)
• Inheritance
• Polymorphism (method overloading, method overriding)

Cohesion & Coupling

Cohesion = how related the functions within a single module are

Coupling = interdependencies between modules

High Cohesion, Low Coupling 👍🏻

SOLID Principles

• Single Responsibility
• Open-Closed Principle (open for extension, closed for modification)
• Liskov’s Substitution (Rabbit extends Animal. classes should be substitutable)
• Interface Segregation (instead of a single interface, prefer multiple smaller interfaces)
• Dependency Inversion (Decoupling. depend on abstractions, interfaces)

GOF => interface > implementation, object decomposition > inheritance

Design Patterns
(Creational — Structural — Behavioral)

Best examples can be found here:

Design Pattern - Overview

Creational

• Factory Method
• Abstract Factory (factory of factories)
• Builder (complex objects using smaller objects)
• Prototype (cloning)
• Singleton (single instance)

Structural

• Adapter (incompatible interfaces)
• Bridge (decouple an abstraction from its implementation)
• Composite (a class that contains group of its own objects)
• Decorator (wrapper / no modifications on existing class, new functionality added)
• Facade (hides complexity)
• Flyweight (reduce weight of object creation, create if needed or reuse)
• Proxy (a class represents functionality of another class — create a proxy class having original object)

Behavioral

• Chain of Responsibility
• Command (stock.buy returns BuyStock, stock.sell returns SellStock — both using Stock class)
• Interpreter (language grammer, sql parsing)
• Iterator (.next)
• Mediator (communication between different classes)
• Memento (restores to a previous state)
• Observer (register, notify)
• State (status, context)
• Null Object (for do-nothing relationship, instead of returning null value)
• Strategy (a class behavior or its algorithm can change at runtime)
• Template (abstract class, method flow)
• Visitor (execution algorithm of element can vary as and when visitor varies)
• MVC

Happy Coding!