Gang of Four

• Design pattern
• Gangs of Four Design Patterns is the collection of 23 design patterns from the book “Design Patterns: Elements of Reusable Object-Oriented Software

GoF Design Patterns are divided into three categories:

1. Creational: The design patterns that deal with the creation of an object.
2. Structural: The design patterns in this category deals with the class structure such as Inheritance and Composition.
3. Behavioral: This type of design patterns provide solution for the better interaction between objects, how to provide lose coupling, and flexibility to extend easily in future.

Creational Design Patterns

There are 5 design patterns in the creational design patterns category.

Pattern Name	Description
Singleton	The singleton pattern restricts the initialization of a class to ensure that only one instance of the class can be created.
Factory	The factory pattern takes out the responsibility of instantiating a object from the class to a Factory class.
Abstract Factory	Allows us to create a Factory for factory classes.
Builder	Creating an object step by step and a method to finally get the object instance.
Prototype	Creating a new object instance from another similar instance and then modify according to our requirements.

Structural Design Patterns

There are 7 structural design patterns defined in the Gangs of Four design patterns book.

Pattern Name	Description
Adapter	Provides an interface between two unrelated entities so that they can work together.
Composite	Used when we have to implement a part-whole hierarchy. For example, a diagram made of other pieces such as circle, square, triangle, etc.
Proxy	Provide a surrogate or placeholder for another object to control access to it.
Flyweight	Caching and reusing object instances, used with immutable objects. For example, string pool.
Facade	Creating a wrapper interfaces on top of existing interfaces to help client applications.
Bridge	The bridge design pattern is used to decouple the interfaces from implementation and hiding the implementation details from the client program.
Decorator	The decorator design pattern is used to modify the functionality of an object at runtime.

Behavioral Design Patterns

There are 11 behavioral design patterns defined in the GoF design patterns.

Pattern Name	Description
Template Method	used to create a template method stub and defer some of the steps of implementation to the subclasses.
Mediator	used to provide a centralized communication medium between different objects in a system.
Chain of Responsibility	used to achieve loose coupling in software design where a request from the client is passed to a chain of objects to process them.
Observer	useful when you are interested in the state of an object and want to get notified whenever there is any change.
Strategy	Strategy pattern is used when we have multiple algorithm for a specific task and client decides the actual implementation to be used at runtime.
Command	Command Pattern is used to implement lose coupling in a request-response model.
State	State design pattern is used when an Object change it’s behavior based on it’s internal state.
Visitor	Visitor pattern is used when we have to perform an operation on a group of similar kind of Objects.
Interpreter	defines a grammatical representation for a language and provides an interpreter to deal with this grammar.
Iterator	used to provide a standard way to traverse through a group of Objects.
Memento	The memento design pattern is used when we want to save the state of an object so that we can restore later on.

Conclusion

Gangs of Four design patterns lay the foundation of core design patterns in programming. There are many other design patterns built on top of these patterns for specific requirements.