design_patterns.md

Introduction

The best way to retain design patterns is to memorize, understand how they work and finally identify places where they are applicable while designing applications.

CHANGE is the One Constant in Software Development.

Design Patterns

1. Strategy Pattern

Strategy lets the algorithm vary independently from clients that use it.

Design Principles

i. Identify the aspects of your application that vary and seperate them from what stays the same

• Take the parts that vary and encapsulate them, so that later you can alter or extend the parts that vary without affecting those that don’t.

ii. Program to interface and not the implmentation

• Depending only the interface allows for loose coupling of inter dependent classes
• It prevents a class from getting locked with a concrete implementation, instead it will be relying on an interface

iii. Favor composition over inheritance

• There are three types of relationships between classes: IS-A (inheritance), HAS-A (composition), Implements
• Using compostion allows for more flexibilty by encapsulating a family of algorithms into logical classes

“Knowing basic OO concepts doesn't automatically make you a good OO designer”

2. Observer Pattern

It defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically.

Design Principles

i. Strive for loosely coupled designs between objects that interact.

Allows us to build flexible OO systems that minimize interdependency and allow for extensibility

example

3. Decorator Pattern

It involves a set of decorator classes that are used to wrap concrete components.

Design Principles

i. Classes should be open for extension, but closed for modification.

4. Factory Pattern

The Factory Method Pattern defines an interface for creating an object, but lets subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.