Introduction

There are so many books and articles on the internet that already explain these topics in depth. But sometimes what we look at is brief but up to date description of topics. In the coming days, I will be writing some articles which will explain different design patterns as short as possible. anyone can use this as Quick Reference material.

In day to day software development practice, many problems are repeated over and over again. Design patterns provide a reusable general solution to such problems. Patterns are just the blueprint of the overall code or program and we cannot transform design patterns into the finished code. They are only a formal description or template of how to solve a specific problem. Most of the patterns only describe interactions between classes and objects rather than large-scale problems of overall software architecture.

Design patterns are documented tried and tested solutions for recurring problems in a given context. Design patterns are a very powerful tool for software developers, but you must keep in mind, they are not silver bullets so do not overdo design patterns.

Design Patterns and their types

There are 3 main types of design patterns

• Creational  Patterns
• Structural Patterns
• Behavioral Patterns

Creational patterns

• One of the main problems is how to create an object.
• Centralized and delegate the object creation to a different class for better management.
• They’re particularly useful when you are taking advantage of polymorphism and need to choose between different classes at run-time rather than compile time.
• Following are some most used Creational patterns
  • Abstract factory: Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
  • Builder: Separate the construction of a complex object from its representation allowing the same construction process to create various representations.
  • Factory method: Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses  (dependency injection)
  • Lazy initialization: Tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed.
  • Multiton: Ensure a class has only named instances, and provide a global point of access to them.
  • Object Pool: Avoid expensive acquisition and release of resources by recycling objects that are no longer in use. It can be considered a generalization of the connection pool and thread pool patterns.
  • Prototype: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
  • Resource acquisition is initialization: Ensure that resources are properly released by tying them to the lifespan of suitable objects.
  • Singleton: Ensure a class has only one instance, and provide a global point of access to it.

Structural patterns

• Where we are changing the structure of existing-working stuff.
• Structural patterns form larger structures from individual parts, generally of different classes.
• Following are some most used Structural patterns
  • Adapter or Wrapper or Translator: Convert the interface of a class into another interface clients expect. An adapter lets classes work together that could not otherwise because of incompatible interfaces. The enterprise integration pattern equivalent is the translator.
  • Bridge: Decouple an abstraction from its implementation allowing the two to vary independently.
  • Composite: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
  • Decorator: Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality.
  • Facade: Provide 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: Use sharing to support large numbers of similar objects efficiently.
  • Front Controller: The pattern relates to the design of Web applications. It provides a centralized entry point for handling requests.
  • Module: Group several related elements, such as classes, singletons, methods, globally used, into a single conceptual entity.
  • Proxy: Provide a surrogate or placeholder for another object to control access to it.

Behavioral patterns

• Behavioral patterns describe interactions between objects.
• They focus on how objects communicate with each other.
• They can reduce complex flow charts to mere interconnections between objects of various classes.
• Here we change the behavior of the class.
• Following are some most used Behavioral patterns
  • Blackboard: Generalized observer, which allows multiple readers and writers. Communicates information system-wide.
  • Chain of responsibility: 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.
  • Command: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
  • Interpreter: Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
  • Iterator: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
  • Mediator: 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.
  • Memento: Without violating encapsulation, capture, and externalize an object’s internal state allowing the object to be restored to this state later.
  • Null object: Avoid null references by providing a default object.
  • Observer or Publish/subscribe: Define a one-to-many dependency between objects where a state change in one object results in all its dependents being notified and updated automatically.
  • Servant: Define common functionality for a group of classes
  • Specification: Recombinable business logic in a Boolean fashion
  • State: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
  • Strategy: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
  • Template method: Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. The template method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.
  • Visitor: Represent 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.