OO Basics
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Components can be objects, variables, methods etc...

1. Abstraction - group together common components.
2. Inheritance - extend and gain components from a parent.
3. Polymorphism - entities that can take on multiple forms.
4. Encapsulation - restrict access to components that logically sit together.

OO Design Principles
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1. Identify what varies and keep it from what stays the same.
2. Program to interfaces not implementations.
3. Favour composition over inheritance.
4. Aim for loosely coupled objects.

OO Patterns
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1. Strategy - interchangeable algorithms.
2. Observer - publish/subscribe model.