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This repository contains examples of all the design patterns listed in the

"Design patterns - Elements of Reusable Object-oriented Software" book

by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides

popularly known as Gang of Four (GoF).

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Table of contents

• What is a design pattern
• Why you should learn design patterns
• How to approach
• Creational design patterns (5)
  1. Singleton
  2. Builder
  3. Prototype
  4. Factory Method
  5. Abstract Factory
• Structural design patterns (7)
  1. Adapter
  2. Bridge
  3. Composite
  4. Decorator
  5. Facade
  6. Flyweight
  7. Proxy
• Behavioral design patterns (11)
  1. Chain of Responsibility
  2. Command
  3. Interpreter
  4. Iterator
  5. Mediator
  6. Memento
  7. Observer
  8. State
  9. Strategy
  10. Template method
  11. Visitor

What is a design pattern?

From Wiki:-

• A general reusable solution to a commonly occurring problem within a given context in software design.
• Not a finished design that can be transformed directly into source or machine code.
• Rather, it is a description or template for how to solve a problem that can be used in many different situations.
• Design patterns are formalized best practices that the programmer can use to solve common problems when designing an application or system.

Why you should learn design patterns?

• Easy to communicate a proble among fellow developers
• It provides a common vocabulary to explain about problem
• It is an abstract topic
• Revisit materails about patterns will alway give you an new perspective everytime.

How to approach?

For each pattern you will see below points covered:

• Overview of the pattern
• Concepts involved
• Design considerations
• Demo / Live example from an API / Steps to create
• Drawbacks (Pitfalls)
• Contrast to another patterns
• Summary

Creational design patterns (5)

Sr. no	Pattern name	GoF book description
1	Singleton	Ensure a class only has one instance, and provide a global point of access to it
2	Builder	Saperate the construction of complex object from its representation so that the same construction process can create different representations
3	Prototype	Specify the kinds of objects to create using a protypical instance, and create new objects by copying this prototype
4	Factory Method	Define an interface for creating an object, but let sub-classess decide which class to instantiate. Factory method lets a class defer instantiation to subclasses
5	Abstract Factory	Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

Structural design patterns (7)

Sr. no	Pattern name	GoF book description
1	Adapter	Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
2	Bridge	Decouple an abstraction from its implementation so that the two can vary independently
3	Composite	Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
4	Decorator	Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
5	Facade	Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
6	Flyweight	Use sharing to support large numbers of fine-grained objects efficiently.
7	Proxy	Provide a surrogate or placeholder for another object to control access to it.

Behavioral design patterns (11)

Sr. no	Pattern name	GoF book description
1	Chain of Responsibility	Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
2	Command	Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
3	Interpreter	Given a language, define a represention for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
4	Iterator	Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
5	Mediator	Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
6	[Memento]	Without violating encapsulation, capture and externali ze an object's internal state so that the object can be restored to this st ate later.
7	[Observer]	Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
8	[State]	Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
9	[Strategy]	Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
10	[Template method]	Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.
11	[Visitor]	Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.