Design Patterns: A Comprehensive Tutorial
Tags: java, designpatterns, systemdesign
Published: 2023-05-01 15:37:14

---
title: Design Patterns: A Comprehensive Tutorial
published: true
date: 2023-05-01 07:17:30 UTC
tags: Java, DesignPatterns, SystemDesign
canonical_url: https://blog.ashutoshkrris.in/design-patterns-a-comprehensive-tutorial
---

## Introduction

Design patterns are proven solutions to commonly occurring software design problems. In other words, they are reusable templates that help to solve recurring design problems in software development. Design patterns were first introduced in the book [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.javier8a.com/itc/bd1/articulo.pdf) by the _Gang of Four_ (Gamma, Helm, Johnson, and Vlissides) in 1994.

Design patterns are essential in software development because they provide a common language and framework for communicating design ideas and solutions. They help developers to design robust, flexible, and maintainable software systems. Using design patterns ensures that the software is more reusable, testable, and scalable. Design patterns also promote code reuse and reduce the likelihood of errors and bugs in the software.

In this tutorial, we will explore different types of design patterns, their importance, and the benefits of using them in software development. We will also discuss how to choose the right design pattern for a particular situation and provide examples of design patterns used in real-world applications.

## Types of Design Patterns

Design patterns are categorized into three main categories: **Creational** , **Structural** , and **Behavioral** patterns.

### Creational Design Patterns

Creational patterns are concerned with object creation mechanisms, trying to create objects in a manner suitable to the situation. There are five types of creational patterns:

- **Abstract Factory Pattern** : Provides an interface for creating families of related or dependent objects without specifying their concrete classes.

- **Builder Pattern** : Separates the construction of a complex object from its representation, allowing the same construction process to create various representations.

- **Factory Method Pattern** : Defines an interface for creating an object, but lets subclasses decide which class to instantiate.

- **Object Pool Pattern** : Reuses and shares objects that are expensive to create.

- **Prototype Pattern** : Creates new objects by copying an existing object, which serves as a prototype.

- **Singleton Pattern** : Ensures that a class has only one instance, and provides a global point of access to it.

### Structural Design Patterns

Structural patterns deal with object composition to form larger structures and provide ways to realize relationships between objects. There are seven types of structural patterns:

- **Adapter Pattern** : Allows objects with incompatible interfaces to work together by creating an intermediary object that translates one interface to another.

- **Bridge Pattern** : Decouples an abstraction from its implementation so that the two can vary independently.

- **Composite Pattern** : Composes objects into tree structures to represent whole-part hierarchies.

- **Decorator Pattern** : Dynamically adds responsibilities to an object by wrapping it in an object of a decorator class.

- **Facade Pattern** : Provides a unified interface to a set of interfaces in a subsystem, simplifying the interaction between the client and the subsystem.

- **Flyweight Pattern** : Uses sharing to support large numbers of fine-grained objects efficiently.

- **Private Class Data Pattern** : Restricts access to an object's data by encapsulating it within a separate object.

- **Proxy Pattern** : Provides a surrogate or placeholder for another object to control access to it.

### Behavioral Design Patterns

Behavioral patterns deal with communication between objects, focusing on how objects collaborate and fulfill responsibilities. There are eleven types of behavioral patterns:

- **Chain of Responsibility Pattern** : Avoids coupling the sender of a request to its receiver by giving more than one object a chance to handle the request.

- **Command Pattern** : Encapsulates a request as an object, allowing you to parameterize clients with different requests, queue or log requests, and support undoable operations.

- **Interpreter Pattern** : Defines a grammatical representation of a language and provides an interpreter to parse sentences in the language.

- **Iterator Pattern** : Provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

- **Mediator Pattern** : Defines an object that encapsulates how a set of objects interact, promoting loose coupling by keeping objects from referring to each other explicitly.

- **Memento Pattern** : Captures and externalizes an object's internal state so that it can be restored later, without violating encapsulation.

- **Null Object Pattern** : Provides a default value for an object, which can be used to avoid null checks in client code

- **Observer Pattern** : Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

- **State Pattern** : Allows an object to alter its behavior when its internal state changes so that it appears to change its class.

- **Strategy Pattern** : Defines a family of algorithms, encapsulates each one and makes them interchangeable.

- **Template Method Pattern** : Defines the skeleton of an algorithm in a method, deferring some steps to subclasses.

- **Visitor Pattern** : Separates an algorithm from an object structure by moving the hierarchy of methods into one object.

## Choosing the Right Design Pattern

Choosing the right design pattern for a particular situation requires a good understanding of the problem at hand and the available design patterns. It's important to choose a pattern that fits the problem, rather than trying to fit the problem into a preconceived pattern. When choosing a design pattern, there are several factors to consider, including:

1. **Complexity** : Consider the complexity of the problem and the proposed solution. Some patterns are simple and can be implemented quickly, while others are more complex and may require more time and resources.

2. **Flexibility** : Consider the flexibility of the pattern and how well it will adapt to changes in the problem or requirements. Some patterns are more flexible than others and may be better suited to situations where requirements are likely to change.

3. **Reusability** : Consider the reusability of the pattern and how it can be applied to other problems. Some patterns are more reusable than others and may be better suited to situations where similar problems are likely to occur.

4. **Familiarity** : Consider the familiarity of the pattern and whether it is well-known and widely used. Choosing a well-known pattern can make it easier for other developers to understand and maintain the code.

Examples of when to use certain design patterns include:

1. Use the Singleton pattern when you need to ensure that only one instance of a class exists and it needs to be globally accessible.

2. Use the Factory Method pattern when you need to create objects without specifying the exact class of object that will be created.

3. Use the Observer pattern when you need to notify a set of objects about changes to another object.

4. Use the Strategy pattern when you need to dynamically change the behavior of an object at runtime.

5. Use the Adapter pattern when you need to adapt the interface of an existing class to meet the needs of a new interface.

In summary, choosing the right design pattern requires careful consideration of the problem and the available patterns, taking into account factors such as complexity, flexibility, reusability, and familiarity. Choosing the right pattern can help simplify the code and make it easier to maintain over time.

## Wrapping Up

In this tutorial, we have covered the basics of design patterns, including the different types of patterns, how to choose the right pattern for a particular situation, and examples of when to use certain patterns.

Key points to remember include the importance of understanding the problem at hand before choosing a design pattern, the need to consider factors such as complexity, flexibility, reusability, and familiarity when choosing a pattern, and the importance of using patterns to simplify code and make it easier to maintain.

Overall, design patterns are an essential tool in any developer's toolkit. By using patterns effectively, developers can build more robust, maintainable, and scalable software systems that meet the needs of users and businesses alike. We recommend that developers continue to learn and explore design patterns to improve their skills and knowledge of software development.