# Design Patterns

### Introduction

The Gang of Four (GoF) Design Patterns are a set of software design patterns that were introduced by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides in their book "Design Patterns: Elements of Reusable Object-Oriented Software". The GoF patterns are divided into three categories: creational, structural, and behavioral.

### Creational Patterns

Creational patterns are concerned with object creation mechanisms, trying to create objects in a manner suitable to the situation. Creational patterns are:

* **Singleton Pattern**: Ensures that a class has only one instance, and provides a global point of access to it.
* **Factory Method Pattern**: Defines an interface for creating objects, but lets subclasses decide which class to instantiate.
* **Abstract Factory Pattern**: Provides an interface for creating families of related or dependent objects without specifying their concrete classes.
* **Builder Pattern**: Separates the construction of a complex object from its representation, allowing the same construction process to create various representations.
* **Prototype Pattern**: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

### Structural Patterns

Structural patterns are concerned with object composition, providing ways to associate objects to form larger structures. Structural patterns are:

* **Adapter Pattern**: Converts the interface of a class into another interface that clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
* **Bridge Pattern**: Decouples an abstraction from its implementation so that the two can vary independently.
* **Composite Pattern**: Composes objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
* **Decorator Pattern**: Attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
* **Facade Pattern**: Provides a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
* **Flyweight Pattern**: Uses sharing to support large numbers of fine-grained objects efficiently.
* **Proxy Pattern**: Provides a surrogate or placeholder for another object to control access to it.

### Behavioral Patterns

Behavioral patterns are concerned with object interactions, distributing responsibilities among objects, and controlling the flow of control between objects. Behavioral patterns are:

* **Chain of Responsibility Pattern**: Avoids 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.
* **Command Pattern**: Encapsulates a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
* **Interpreter Pattern**: Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
* **Iterator Pattern**: Provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
* **Mediator Pattern**: Defines 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.
* **Memento Pattern**: Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.
* **Observer Pattern**: Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
* **State Pattern**: Allows an object to alter its behavior when its internal state changes. The object will appear to change its class.
* **Strategy Pattern**: Defines a family of algorithms, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently
* **Template Method Pattern**: Defines the skeleton of an algorithm in a method, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.
* **Visitor Pattern**: Represents 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.

### Conclusion

The GoF Design Patterns provide a standardized way of solving common software design problems. They are widely used in object-oriented programming and provide a common vocabulary for discussing design patterns. By understanding these patterns, software developers can improve the quality of their code and create more maintainable, flexible, and scalable software systems.