Readme.md

Design Patterns

Pattern	Type	Examples	Interview evaluation	Description
⭐ Singleton	Creational Patterns	- Redis caching and thread pool - Database connections - Config Manager - Logging - Driver objects - Networking and Database Connectivity	Thread safe? - Java - Use synchronized, static inner class, double checked locking - Go - sync.once	Singleton ensures that only one object of its kind exists and provides a single point of access to it for any other code.
⭐ Strategy	Behavioral Patterns	- Pluggable components (like cache eviction strategies - lru, lfu, recommendations, load balancing) - Payment Gateways - Sorting strategies - Compression algorithms	Design with interchangeable behaviors	Strategy lets you define a family of algorithms, put each of them into a separate class, and make their objects interchangeable.
⭐ Factory Method	Creational Patterns	- DB factory (Postgres vs MySQL) - NotificationFactory - Frameworks - Payment Processor factory in ecommerce backend	Abstraction, Decoupling of object creation from usage	Factory Method provides an interface for creating objects in a superclass, but allows subclasses to alter the type of objects that will be created.
⭐ Observer	Behavioral Patterns	- GUIs - Messaging - Microservices - Reactive systems	Tests understanding of event driven systems and pub-sub models	Observer lets you define a subscription mechanism to notify multiple objects about any events that happen to the object they’re observing.
⭐ Decorator Pattern		- Java I/O streams - UI Frameworks	Dynamically add responsibilities to objects without subclassing.	Decorator pattern attaches additional responsibilities to an object dynamically. - Decorators provide a flexible alternative to subclassing for extending functionality.
⭐ Adapter Pattern	Structural Patterns	- Integrate legacy code or external systems into new code base - API adapters to external systems (integrating with 3rd party api, legacy code)	Compatibility, design flexibility	Adapter is a structural design pattern that allows objects with incompatible interfaces to collaborate.
⭐ Command Pattern		- Transactional or queue based systems - Job commands in task queue like Celery, SQS, Kafka.	Design of undo/redo operations, request encapsulation	The command pattern encapsulates a request as an object, thereby letting us parameterize other objects with different requests, queue or log requests, and support undoable operations.
⭐ Facade	Structural Patterns	- API Gateways (expose simplified interface for complex microservices)	Simplify complex systems with clean APIs	Facade provides a simplified interface to a library, a framework, or any other complex set of classes.
⭐ Builder	Creational Patterns	- Optional parameters, immutability - Configuration heavy objects (complex request builders, query builders etc.)	Creating complex objects step-by-step without telescoping constructor	Builder is a creational design pattern that lets you construct complex objects step by step. The pattern allows you to produce different types and representations of an object using the same construction code.
Abstract Factory	Creational Patterns			Abstract Factory is a creational design pattern that lets you produce families of related objects without specifying their concrete classes.

Sample Implementations

Programming Language
Go
Java

Factory pattern

Adapter pattern

MVC Design Pattern

The Model View Controller (MVC) design pattern specifies that an application consist of a data model, presentation information, and control information.

The pattern requires that each of these be separated into different objects.

• The Model contains only the pure application data, it contains no logic describing how to present the data to a user.
• The View presents the model’s data to the user. The view knows how to access the model’s data, but it does not know what this data means or what the user can do to manipulate it.
• The Controller exists between the view and the model. It listens to events triggered by the view (or another external source) and executes the appropriate reaction to these events. In most cases, the reaction is to call a method on the model. Since the view and the model are connected through a notification mechanism, the result of this action is then automatically reflected in the view.

Template pattern

• It is part of Behavior Design Pattern type.
• The template pattern defines the skeleton of an algorithm in an operation deferring some steps to sub-classes.
• The template method lets subclasses redefine certain steps of an algorithm without changing the algorithm structure.
• Read more

Constructor Chaining

• Constructor chaining is the process of calling one constructor from another constructor with respect to current object.
• Read more

Constructor chaining can be done in two ways:

• Within same class: It can be done using this() keyword for constructors in same class
• From base class: by using super() keyword to call constructor from the base class.

Rules of constructor chaining :

• The this() expression should always be the first line of the constructor.
• There should be at-least be one constructor without the this() keyword (constructor 3 in above example).
• Constructor chaining can be achieved in any order.

Iterator Pattern

• The iterator pattern provides a way to access the elements of an aggregate object without exposing its underlying representation.

References

• Factory method for designing pattern
• Constructor Chaining In Java with Examples