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Design Patterns | Set 1 (Introduction)
  • Difficulty Level : Easy
  • Last Updated : 23 Dec, 2020

A design pattern provides a general reusable solution for the common problems that occur in software design. The pattern typically shows relationships and interactions between classes or objects. The idea is to speed up the development process by providing well tested, proven development/design paradigm. Design patterns are programming language independent strategies for solving a common problem. That means a design pattern represents an idea, not a particular implementation. By using the design patterns you can make your code more flexible, reusable, and maintainable.

It’s not mandatory to implement design patterns in your project always. Design patterns are not meant for project development. Design patterns are meant for common problem-solving. Whenever there is a need, you have to implement a suitable pattern to avoid such problems in the future. To find out which pattern to use. You just have to try to understand the design patterns and their purposes. Only by then, you will be able to pick the right one. 

Understand the purpose and usage of each design pattern. So, you will be able to pick and implement the correct pattern as needed. 

For example, in many real-world situations, we want to create only one instance of a class. For example, there can be only one active president of the country at a time regardless of personal identity. This pattern is called a Singleton pattern. Other software examples could be a single DB connection shared by multiple objects as creating a separate DB connection for every object will be costly. Similarly, there can be a single configuration manager or error manager in an application that handles all problems instead of creating multiple managers. 
Types of Design Patterns
There are mainly three types of design patterns: 

  1. Creational 
    These design patterns are all about class instantiation or object creation. These patterns can be further categorized into Class-creational patterns and object-creational patterns. While class-creation patterns use inheritance effectively in the instantiation process, object-creation patterns use delegation effectively to get the job done. 

    Creational design patterns are the Factory Method, Abstract Factory, Builder, Singleton, Object Pool, and Prototype. 

    Use case of creational design pattern- 
    1) Suppose a developer wants to create a simple DBConnection class to connect to a database and wants to access the database at multiple locations from code, generally what developer will do is create an instance of DBConnection class and use it for doing database operations wherever required. Which results in creating multiple connections from the database as each instance of DBConnection class will have a separate connection to the database. In order to deal with it, we create DBConnection class as a singleton class, so that only one instance of DBConnection is created and a single connection is established. Because we can manage DB Connection via one instance so we can control load balance, unnecessary connections, etc. 

    2) Suppose you want to create multiple instances of similar kind and want to achieve loose coupling then you can go for Factory pattern. A class implementing factory design pattern works as a bridge between multiple classes. Consider an example of using multiple database servers like SQL Server and Oracle. If you are developing an application using SQL Server database as back end, but in future need to change database to oracle, you will need to modify all your code, so as factory design patterns maintain loose coupling and easy implementation we should go for factory for achieving loose coupling and creation of similar kind of object.

  2. Structural 
    These design patterns are about organizing different classes and objects to form larger structures and provide new functionality. 

    Structural design patterns are Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Private Class Data, and Proxy. 

    Use Case Of Structural Design Pattern- 

    1) When 2 interfaces are not compatible with each other and want to establish a relationship between them through an adapter it’s called an adapter design pattern. Adapter pattern converts the interface of a class into another interface or class that the client expects, i.e adapter lets classes works together that could not otherwise because of incompatibility. so in these type of incompatible scenarios, we can go for the adapter pattern.

  3. Behavioral 
    Behavioral patterns are about identifying common communication patterns between objects and realize these patterns. 

    Behavioral patterns are Chain of responsibility, Command, Interpreter, Iterator, Mediator, Memento, Null Object, Observer, State, Strategy, Template method, Visitor 

    Use Case of Behavioral Design Pattern- 

    1) Template pattern defines the skeleton of an algorithm in an operation deferring some steps to sub-classes, Template method lets subclasses redefine certain steps of an algorithm without changing the algorithm structure. For example in your project you want the behavior of the module to be able to extend, such that we can make the module behave in new and different ways as the requirements of the application change, or to meet the needs of new applications. However, no one is allowed to make source code changes to it, i.e you can add but can’t modify the structure in those scenarios a developer can approach template design pattern.


This article is contributed by Abhijit Saha and Tanuja Praharaj. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.


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