Object-Oriented Principles Dictionary

This is a list of important principles of object-oriented programming with definitions, which I will add to as I learn about additional principles.

These principles and definitions come from Head First Design Patterns by Eric Freeman and Elizabeth Freeman and Steve Smith's Pluralsight course, Solid Principles of Object-Oriented Design.

Dependency Inversion Principle - both high- and low-level modules should depend on abstractions; abstractions should not depend on details; details should depend on abstractions

Don't Repeat Yourself Principle (DRY) - self explanatory; avoid copy-pasted code, magic numbers/strings, static methods, duplicate logic, etc.

Encapsulate what Varies - self explanatory; encapsulate code that is likely to change so that these changes do not have ripple effects throughout your application

Favor Composition Over Inheritance - composition can improve flexibility

The Hollywood Principle - "Don't call us, we'll call you"; low-level components can be hooked into a system, but only the high-level components get to determine if, when, and how to call on those low-level components

Interface Segregation Principle - clients should not depend on methods they do not use; usually becomes a problem when interfaces are too large/not cohesive

Liskov Substitution Principle - subtypes should be substitutable for their base types

Loose Coupling - classes that interact should be loosely coupled, wthout interacting too much; this increases flexibility by reducing dependencies

Open/Closed Principle - classes should be open for extension and closed to modification

Principle of Least Knowledge - talk only to your friends; i.e. be careful how many other classes each class in your design needs to interact with

Program to an Interface, not an Implementation - this allows clients to depend on the interface, rather than on the concrete implementation being used

Single Responsibility Principle (SRP) - every object should have a single responsibility, no more and no less; a class should have only one reason to change

For more information on the Dependency Inversion Principle, DRY, Interface Segregation Principle, Liskov Substitution Principle, Open/Closed Principle, and SRP, check out this blog post.

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