GOF / Gang of Four Design Patterns in Java

This tutorial provides an overview of GOF/Gang of Four Design Patterns. It starts with the basic definition of a design pattern. Next it introduces Gang of Four's design patterns, followed by the three categories of GOF Design Patterns. It then lists out all the GOF design patterns category-wise, provides a brief description of each pattern, and a link to the detailed tutorial for each of the design patterns' implementation and usage in Java.
What is a design pattern A design pattern is a general reusable solution to a commonly occurring problem in software design. It is a template for how to solve a problem that can be used in many different situations. Patterns are formalized best practices that the programmer can use to solve common problems when designing an application or system. Gang of Four Design Patterns These are design patterns which were defined by four authors - Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides in their book Design Patterns: Elements of Reusable Object-Oriented Software. A lot has evolved in the field of software design since this book came out in 1994. This book and its patterns however make the foundation of the field of object oriented design patterns. Categories of GOF Design Patterns Gang Of Four design patterns are grouped into 3 categories:

Creational Design Patterns: Creational patterns deal with object creation i.e they look at ways to solve design issues arising out of creation of objects.
Structural Design Patterns: Structural patterns ease the design by identifying a simple way to realize relationships between entities.
Behavioral Design Patterns: Behavioral patterns identify common communication patterns between objects and realize these patterns.

Having looked at the 3 categories of GOF design patterns, now let us take a look at the individual patterns in each of these categories.

Creational Design Patterns
Pattern Name	Tutorial Link	Pattern Description
Abstract Factory		Provides an interface for creating families of related or dependent objects without specifying their concrete classes.
Factory Method	read tutorialClick to Read Factory Method Design Pattern Tutorial	Deals with the problem of creating related objects without specifying the exact class of object that will be created.
Singleton		This pattern ensures a class has only one instance and provides a global(app-level) point of access to it.
Prototype	read tutorialClick to Read Prototype Design Pattern Tutorial	Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
Builder	read tutorialRead Builder Pattern Tutorial	Separates the construction of a complex object from its representation, thus enabling the same construction process to create various representations.

Structural Design Patterns
Pattern Name	Tutorial Link	Pattern Description
Adapter	read tutorialAdapter Design Pattern tutorial	This pattern lets classes work together that could not otherwise because of incompatible interfaces.
Bridge		This pattern decouples an abstraction from its implementation so that they become loosely coupled.
Composite	read tutorialLink to Composite Design Pattern Tutorial	This pattern allows aggregating objects such that individual objects and composition of objects can be treated uniformly.
Decorator		This pattern attaches additional responsibilities to an object dynamically while keeping the interface same
Facade	read tutorialLink to Facade Design Pattern Tutorial	This pattern provides a simpler interface to a larger and more complex system such as a class library or a complex API.
Flyweight		This pattern minimizes memory usage by sharing common data between objects.
Proxy	read tutorial Click to Read to Proxy Design Pattern Tutorial	Proxy is a surrogate or placeholder class for another class mostly done with an intention of intercepting access to the said class.

Behavioral Design Patterns
Pattern Name	Tutorial Link	Pattern Description
Chain of Responsibility	read tutorialRead Chain of Responsibility Pattern Tutorial	This pattern defines a chain of processing objects in a chain in such a way that the incoming request is processed by each processing objects in sequence.
Command		In this pattern an object is used to represent and encapsulate all the information needed to call a method at a later time.
Interpreter		This pattern defines a representation for a given language's grammar along with an interpreter that uses the representation to interpret sentences in the language.
Iterator	read tutorialRead Iterator Pattern Tutorial	This pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
Mediator		In this pattern communication between objects is encapsulated with a mediator object. Objects no longer communicate directly with each other, but instead communicate through the mediator.
Memento	read tutorialLink to Memento Design Pattern Tutorial	Mementos capture and externalize an object's internal state allowing the object to be restored to this state later.
Observer	read tutorialClick to Read Observer Design Pattern Tutorial	An observable object called 'Subject' maintains a list of objects called 'Observers'. Subject notifies the observers of any state changes.
State	read tutorialClick to Read State Design Pattern Tutorial	State pattern allows an object to alter its behavior when its internal state changes.
Strategy	read tutorialStrategy Design Pattern Tutorial	This pattern defines a family of algorithms, encapsulate each one, and make them interchangeable.
Template Method	read tutorialLink to tutorial on Template Method Pattern	Pattern defines steps of an algorithm as methods in an interface while allowing subclasses to override some steps in the overall algorithm.
Visitor	read tutorialLink to Visitor Design Pattern Tutorial	Pattern separates an algorithm from the object structure on which it operates, which provides the ability to add new operations to existing object structures without modifying those structures.

Summary In the above tutorial we understood the Gang of Four design patterns, their categories and the individual patterns making up each of the categories. The next logical step will be start going through the individual design patterns' tutorials and understanding their working in-depth. Please use the 'read tutorial' links in the patterns' tables to go to each individual design pattern.

Gang of Four (GOF) Design Patterns on JavaBrahman
Creational Patterns: Factory method Pattern Builder Pattern Prototype Pattern
Behavioral Patterns: Chain of Responsibility Pattern Observer Pattern Iterator Pattern State Pattern Memento Pattern Visitor Pattern Strategy Pattern Template Method Pattern
Structural Patterns: Adapter Pattern Composite Pattern Facade Pattern Proxy Pattern Java
Analysis of Patterns: Strategy vs State Pattern

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