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Lessons from Head First Design Pattern - 1

OO Basics
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- Abstraction
- Encapsulation
- Polymorphism
- Inheritance


OO Principles
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- Encapsulate what varies
- Favor composition over inheritance
- Program to interface, not implementations.
- Strive for loosely coupled designs between objects that interact
- Classes should be open for extension but closed for modification.
- Depend on abstractions. Do not depend on concrete classes.

OO Patterns
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Strategy - Defines a family of algorithms, encapsulate each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

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

Decorator - Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Factory Method - Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to the subclasses.

Singleton - Ensures a class only has one instance and provide a global point of access to it.

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