Cheat Sheet

Creational Patterns

Design Pattern	Main Purpose
Abstract Factory (GoF)	Designed to create families of objects without specifying their concrete classes
Builder Pattern (GoF)	Designed to build complex objects with a lot of optional fields or when the input order of the values is not clear
Factory Method (GoF)	Designed to create objects without specifying their concrete classes. Subclasses should decide which concrete class to instantiate.
Prototype (GoF)	Designed to create objects faster, when creating a new object is an expensive operation
Singleton (GoF)	Designed to control the number of instances (only 1) and the access to that instance in serial way via the provided entry/ access point
Factory (Pattern?)	Designed to encapsulate the instantiation logic and to create objects without specifying their concrete classes. Some people argue if this is a pattern or not, but we will mention it, because it’s widely used.
Object Pool	Designed to improve performance by reusing the instances of stateless objects, that are otherwise expensive to create.
Telescoping Constructor	When you need a mechanism to provide default values or behavior when constructing objects.

Structural Patterns

Design Pattern	Main Purpose
Adapter (GoF)	Designed to make two incompatible types compatible without changing their existing code
Bridge (GoF)	Designed to separate object abstraction from the implementation, so that you can change implementation without affecting the rest of the code
Composite (GoF)	Designed to build a class hierarchy from primitive and composite objects, that may represent complex structures, but have similar behavior
Decorator (GoF)	Designed to to add/ remove responsibilities to objects dynamically (even at runtime), without changing their implementation
Facade (GoF)	Designed to hide an entire subsystem behind a facade and present it is as a single object/ interface
Flyweight (GoF)	Designed to share objects (to reuse instances) for better efficiency and consistency
Proxy (GoF)	Designed to provide a level of indirection to object members and may add additional logic (i.e. to control access or to provide a wrapper implementation for better performance)
Private Class Data	Designed to protect the object data (to restrict write access) even from it’s own methods.

Behavioral Patterns

Design Pattern	Main Purpose
Chain Of Responsibility (GoF)	Designed to to decouple sender and receiver and to allow multiple handler implementations, that can handle the request, especially when the concrete handler isn’t known a priori
Command (GoF)	Designed to encapsulate the data, needed to perform the action as an object, and execute the action. Sometimes action can be stored in a queue and executed later.
Interpreter (GoF)	Designed to define a representation of the grammar of a given language und use that representation to interpret sentences in that language
Iterator (GoF)	Designed to give access to the elements of an aggregate object sequentially without exposing their underlying representation
Mediator (GoF)	Designed to encapsulate the communication between multiple objects in a single object in order to reduce the direct relations between them
Memento (GoF)	Designed to externalize the internal state of the object in order to be stored or to restore an object to some previous version of its state
Observer (GoF)	Designed to define one-to-many dependency between objects, so that when one object changes its state, all its dependents get notified and updated automatically
State (GoF)	Designed to change the object class (type), when object state changes, because its behavior also changes
Strategy (GoF)	Designed to separate the behavior from it’s host class, by encapsulating the algorithm in a separate class. This allows you to implement many algorithms and make them interchangeable.
Template Method (GoF)	Designed to define the skeleton of an algorithm, but lets subclasses to implement or override separate parts of it
Visitor (GoF)	Designed to separate the algorithm from the object and to apply it to a group of similar types
Null Object	Designed to provide a default behavior if the object is not found

Visit https://iretha.github.io/design-patterns for detailed explanation and code examples.