A design pattern is a reusable solution to a commonly occurring problem in software design, which has been proven to be effective through experience and is typically presented in a standardized format. Design patterns help developers to create software that is more modular, flexible, and maintainable, by providing a common vocabulary and a set of best practices for solving common problems.
I. Types of Design Pattern
I.1. Creational Patterns
Creational patterns are design patterns that deal with the process of object creation. They provide various ways to create objects and decouple the object creation process from their usage.
There are several creational patterns, including:
Singleton Pattern: Ensures that a class has only one instance and provides a global point of access to it.
Factory Method Pattern: Defines an interface for creating objects, but allows subclasses to decide which class to instantiate.
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 different representations.
Prototype Pattern: Creates new objects by copying existing ones, providing an efficient way to create new objects with minimal overhead.
These patterns are often used in software systems to create objects in a flexible and modular way, making it easier to modify and extend the system over time.
Video: Creational Design Patterns
I.2. Structural Patterns
Structural patterns are design patterns that deal with the composition of classes and objects to form larger structures or systems. They provide ways to organize and structure classes and objects in a flexible and maintainable way.
There are several structural patterns, including:
Adapter Pattern: Converts the interface of a class into another interface that clients expect, allowing classes with incompatible interfaces to work together.
Bridge Pattern: Separates an object's interface from its implementation, allowing the two to vary independently.
Composite Pattern: Allows you to compose objects into tree structures to represent part-whole hierarchies, making it easier to work with complex hierarchies of objects.
Decorator Pattern: Attaches additional responsibilities to an object dynamically, providing a flexible way to add functionality to objects without subclassing.
Facade Pattern: Provides a simplified interface to a larger and more complex body of code, making it easier to use and understand.
Flyweight Pattern: Shares common parts of objects between multiple objects to reduce memory usage and improve performance.
Proxy Pattern: Provides a surrogate or placeholder for another object to control access to it, providing a way to add security or additional functionality to an object.
These patterns are often used in software systems to organize and structure code in a way that is flexible, maintainable, and easy to modify over time.
Video: Structural Patterns
I.3. Behavioral Patterns
Behavioral patterns are design patterns that deal with the communication between objects and classes and their behavior. They provide ways to organize and structure the interactions between objects in a flexible and maintainable way.
There are several behavioral patterns, including:
Chain of Responsibility Pattern: Allows you to pass requests between objects in a chain, with each object in the chain either handling the request or passing it on to the next object.
Command Pattern: Encapsulates a request as an object, allowing you to parameterize clients with different requests and queue or log requests.
Interpreter Pattern: Defines a grammar for a language and provides a way to interpret and execute 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 the communication between other objects, allowing them to communicate in a more flexible and decoupled way.
Memento Pattern: Allows you to capture and externalize an object's internal state, providing a way to undo or restore previous states.
Observer Pattern: Defines a one-to-many dependency between objects, so that when one object changes state, all its dependents are notified and updated.
State Pattern: Allows an object to alter its behavior when its internal state changes, providing a way to manage complex state transitions in a flexible and maintainable way.
Strategy Pattern: Defines a family of algorithms, encapsulates each one, and makes them interchangeable, providing a way to select an algorithm at runtime.
Template Method Pattern: Defines the skeleton of an algorithm in a base class, allowing subclasses to override specific steps of the algorithm.
Visitor Pattern: Separates an algorithm from an object structure by moving the algorithm into a separate object, providing a way to add new operations to an object structure without modifying its classes.
These patterns are often used in software systems to organize and structure the behavior of objects and classes in a way that is flexible, maintainable, and easy to modify over time.
Video: Behavioral Patterns
I.4. Architectural Patterns
Architectural patterns are high-level design patterns that deal with the overall organization and structure of software systems. They provide a blueprint for organizing and structuring code at the system level, helping to ensure that the system is flexible, maintainable, and scalable.
There are several architectural patterns, including:
Layered Architecture Pattern: Organizes the system into layers, with each layer responsible for a specific set of tasks, making it easier to manage and maintain the system.
Client-Server Architecture Pattern: Separates the system into a client (user interface) and server (application logic), allowing the system to scale and distribute tasks across multiple machines.
Model-View-Controller (MVC) Architecture Pattern: Separates the system into three components - model (data and business logic), view (user interface), and controller (processes user input), providing a way to manage complex systems and allowing each component to change independently.
Microservices Architecture Pattern: Decomposes the system into small, independent services, each with its own data storage and communication mechanisms, providing a way to build large, complex systems that are easy to modify and scale.
Event-Driven Architecture Pattern: Separates the system into components that communicate through events, allowing the system to be more scalable and flexible.
Service-Oriented Architecture (SOA) Pattern: Decomposes the system into a set of services that communicate with each other through well-defined interfaces, providing a way to build large, complex systems that can be easily modified and maintained.
These patterns are often used in software systems to organize and structure code at the system level, making it easier to manage and maintain large, complex systems.
Video: Architectural Pattern
I.5. Concurrency Patterns
Concurrency patterns are design patterns that deal with coordinating multiple threads or processes in a concurrent system. They provide a way to manage concurrency in a way that is safe, efficient, and maintainable.
There are several concurrency patterns, including:
Active Object Pattern: Encapsulates a task as an object, allowing it to be executed asynchronously and in parallel with other tasks, while hiding the implementation details of the thread management.
Barrier Pattern: Provides a synchronization point for a set of threads, so that they can all reach the same point in their execution and continue together.
Read-Write Lock Pattern: Provides a way to allow multiple readers or a single writer to access a shared resource, providing a way to manage concurrency without sacrificing performance.
Monitor Object Pattern: Provides a way to synchronize access to a shared resource by allowing threads to wait on a monitor object, ensuring that only one thread can access the resource at a time.
Producer-Consumer Pattern: Provides a way to coordinate the flow of data between threads, with one thread producing data and another thread consuming it.
Thread Pool Pattern: Provides a way to manage a pool of threads, allowing tasks to be executed in parallel without creating too many threads and causing performance issues.
These patterns are often used in concurrent systems to manage multiple threads or processes, ensuring that they execute safely and efficiently.
Video: Concurrency Patterns
II. Design Pattern Principles
Design pattern principles are general guidelines that help developers create software that is more modular, flexible, and maintainable. These principles are based on best practices that have been identified through experience, and they provide a set of guidelines for solving common problems in software design.
II.1. Single Responsibility Principle (SRP)
The Single Responsibility Principle (SRP) is a design principle that states that a class or module should have only one reason to change. In other words, a class should have only one responsibility, and that responsibility should be entirely encapsulated by the class.
The SRP is important because it helps ensure that classes are focused on a single task and do not become bloated or difficult to maintain. It also helps promote the creation of modular code, since a class with a single responsibility can be easily reused and combined with other classes to create more complex systems.
For example, if a class is responsible for both processing data and writing it to a file, it violates the SRP. Instead, the class should be split into two separate classes, one responsible for processing data and another responsible for writing it to a file, with each class having a single responsibility. This makes the code easier to understand, test, and modify.
II.2. Open-Closed Principle (OCP)
The Open-Closed Principle (OCP) is a design principle that states that software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. In other words, you should be able to add new functionality to a system without having to modify existing code.
The OCP is important because it helps ensure that software is more maintainable, flexible, and scalable. By designing software in a way that is open for extension, you can add new functionality to the system without having to modify the existing code, which can reduce the risk of introducing bugs and make the system easier to maintain.
To implement the OCP, you can use techniques like abstraction, inheritance, and composition to create a modular and extensible system. For example, you can define interfaces and abstract classes that describe the behavior of a system, and then create concrete implementations of those interfaces and classes that can be swapped in and out as needed. You can also use design patterns like the Strategy pattern or the Decorator pattern to create flexible and extensible systems that can be easily modified and extended.
II.3. Liskov Substitution Principle (LSP)
The Liskov Substitution Principle (LSP) is a design principle that states that objects of a superclass should be able to be replaced with objects of a subclass without affecting the correctness of the program. In other words, a subclass should be able to be used wherever its superclass is expected, without causing any problems or errors.
The LSP is important because it helps ensure that software is more modular, flexible, and maintainable. By designing software in a way that adheres to the LSP, you can create a system that is easier to modify and extend, and that is less prone to bugs and errors.
To implement the LSP, you should make sure that a subclass does not change the behavior or assumptions of its superclass. For example, if a superclass has a method that accepts a certain type of argument, the subclass should also accept that same type of argument. You should also make sure that a subclass does not add any new preconditions or postconditions that are not present in the superclass.
The LSP is closely related to other design principles like the Single Responsibility Principle (SRP) and the Dependency Inversion Principle (DIP), and is often used in conjunction with other principles and design patterns to create flexible, maintainable, and extensible software systems.
II.4. Interface Segregation Principle (ISP)
The Interface Segregation Principle (ISP) is a design principle that states that clients should not be forced to depend on interfaces they do not use. In other words, you should avoid creating large, monolithic interfaces that force clients to implement methods they don't need.
The ISP is important because it helps ensure that software is more modular, flexible, and maintainable. By designing software in a way that adheres to the ISP, you can create a system that is easier to modify and extend, and that is less prone to bugs and errors.
To implement the ISP, you should break large interfaces into smaller, more focused interfaces that are tailored to specific clients or use cases. By doing so, you can minimize the impact of changes to the system and make it easier to add or remove functionality as needed.
The ISP is closely related to other design principles like the Single Responsibility Principle (SRP) and the Open-Closed Principle (OCP), and is often used in conjunction with other principles and design patterns to create flexible, maintainable, and extensible software systems.
II.5. Dependency Inversion Principle (DIP)
The Dependency Inversion Principle (DIP) is a design principle that states that high-level modules or classes should not depend on low-level modules or classes, but rather on abstractions. In other words, you should depend on interfaces or abstract classes instead of concrete implementations.
The DIP is important because it helps ensure that software is more modular, flexible, and maintainable. By designing software in a way that adheres to the DIP, you can create a system that is easier to modify and extend, and that is less prone to bugs and errors.
To implement the DIP, you should use abstractions like interfaces or abstract classes to define the behavior of a system, and then use dependency injection or inversion of control (IoC) to provide concrete implementations of those abstractions at runtime. By doing so, you can decouple the high-level modules or classes from the low-level modules or classes, and make it easier to modify or replace those low-level components without affecting the rest of the system.
The DIP is closely related to other design principles like the Single Responsibility Principle (SRP), the Open-Closed Principle (OCP), and the Liskov Substitution Principle (LSP), and is often used in conjunction with other principles and design patterns to create flexible, maintainable, and extensible software systems.
