Design Patterns commonly used in OOP

A design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. It is a framework based on which applications can be made. They are widely used in Object Oriented Programming/Design. The intent of this post is to give general awareness on the various design patterns that are commonly used. 

Strategy Pattern

According to the GoF book, Strategy design pattern should “Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.” You encapsulate code in external algorithms for easy use rather than spreading it around inside your core code and modifying it throughout that code.

Decorator Pattern

According to the GoF book, Decorator Pattern should “Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.”

Factory Pattern

According to the GoF book, the Factory Method design pattern should “Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory method lets a class defer instantiation to subclasses.” The idea is that you have an object that creates other objects.

Observer Pattern

The Gang of Four book says that the Observer design pattern should “Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.”

Chain of Responsibility Pattern

The GoF book says that the Chain of Responsibility design pattern should “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.”

Singleton design pattern

The Gang of Four (GoF) book (Design Patterns: Elements of Reusable Object-Oriented Software, 1995, Pearson Education, Inc. Publishing as Pearson Addison Wesley) says that the Singleton design pattern must: “Ensure a class only has oneinstance, and provide a global point of access to it.”

Flyweight Design pattern

The GoF book says the Flyweight pattern must, “Use sharing to support large numbers of fine-grained objects efficiently.” They go on to say, “A flyweight is a shared object that can be used in multiple contexts simultaneously. The flyweight acts as an independent object in each context — it’s indistinguishable from an instance of the object that’s not shared.”

Adapter Design Pattern

The Gang of Four (GoF) book (Design Patterns: Elements of Reusable Object- Oriented Software, 1995, Pearson Education, Inc. Publishing as Pearson Addison Wesley) says the Adapter pattern lets you “Convert the interface of a class into another interface the client expects. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.”

Facade Design pattern

The GoF book says the Facade pattern should “Provide a unified interface to a set of interfaces in a system. Facade defines a higher-level interface that makes the subsystem easier to use.”

Template Method Pattern

According to the official Gang of Four (GoF) phrasing, the Template Method will “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.”

Builder Design Pattern

The GoF says that the Builder design patterns let you “Separate the construction of a complex object from its representation so that the same construction processes can create different representations.”

Iterator Design Pattern

According to the Gang of Four (GoF), you can use the Iterator design pattern to “Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.” (Design Patterns: Elements of Reusable Object-Oriented Software, 1995, Pearson Education, Inc. Publishing as Pearson Addison Wesley).

Composite Design Patterns

The GoF says you use the Composite design pattern to “Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.”

State Design Pattern

The Gang of Four (GoF) book (Design Patterns: Elements of Reusable Object- Oriented Software, 1995, Pearson Education, Inc. Publishing as Pearson Addison Wesley) says that the State design pattern will “Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.”

Proxy Design Pattern

The GoF book says you can use the Proxy design pattern to, “Provide a surrogate or placeholder for another object to control access to it.”

Command Design Pattern

The Gang of Four (GoF) book (Design Patterns: Elements of Reusable Object- Oriented Software, 1995, Pearson Education, Inc. Publishing as Pearson Addison Wesley) says you use the Command pattern to, “Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.”

Mediator Design Pattern

The Gang of Four book says you can use the Mediator pattern to, “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.”

More Patterns and more details of the patterns will be done shortly

The contents are borrowed from Design Patterns for Dummies written by Steve Holzner. In depth details of the patterns are given in the book. This is a just a snapshot for those who want only definitions.