behavioral-design-patterns.md

Behavioral Design Patterns

Problem Statement:

A company that provides online courses wants to create a new platform that allows students to enroll in courses, track their progress, and receive personalized feedback. The platform needs to be scalable, reliable, and easy to use, and it needs to support different types of courses, such as video lectures, interactive exercises, and quizzes.

Story:

The company has been offering online courses for some time, but they have been facing challenges in keeping students engaged and motivated. They decide to create a new platform that provides students with personalized feedback and support, to help them stay on track and achieve their learning goals.

The lead developer on the project decides to use a combination of design patterns in order to create a platform that is scalable, reliable, and easy to use. Here are the requirements and the design patterns used to fulfill them:

Requirements:

1. The platform needs to be able to handle a large number of users and courses.

• Classes: CourseManager, UserManager

• Design Patterns: Observer pattern, Chain of Responsibility pattern

• Explanation: The CourseManager and UserManager classes can be used to manage the course and user data, respectively. The Observer pattern can be used to notify users of errors or system failures in a timely manner, allowing them to take corrective action. The Chain of Responsibility pattern can be used to handle different types of errors or exceptions in a systematic and organized way, reducing the risk of system failures.

2. The platform needs to be able to handle different types of courses, such as video lectures, interactive exercises, and quizzes.

• Classes: Course, LectureCourse, ExerciseCourse, QuizCourse

• Design Patterns: Strategy pattern, Template Method pattern

• Explanation: The Course class can be used as a base class for different types of courses, such as LectureCourse, ExerciseCourse, and QuizCourse. The Strategy pattern can be used to define different strategies for delivering course content, such as video lectures or interactive exercises. The Template Method pattern can be used to provide a standardized user interface and workflow for different types of courses, making the platform easier to navigate and use.

3. The platform needs to be able to provide personalized feedback to students.

• Classes: Feedback, Student

• Design Patterns: Observer pattern, Command pattern

• Explanation: The Feedback class can be used to encapsulate personalized feedback for students, and the Student class can be used to manage student data. The Observer pattern can be used to notify students of their progress and provide personalized feedback based on their performance. The Command pattern can be used to encapsulate feedback as a command object, allowing it to be easily modified or extended.

4. The platform needs to be able to handle different types of assessments, such as quizzes and exams.

• Classes: Assessment, QuizAssessment, ExamAssessment

• Design Patterns: Strategy pattern, Template Method pattern

• Explanation: The Assessment class can be used as a base class for different types of assessments, such as QuizAssessment and ExamAssessment. The Strategy pattern can be used to define different strategies for delivering assessments, such as quizzes or exams. The Template Method pattern can be used to provide a standardized user interface and workflow for different types of assessments, making the platform easier to navigate and use.

5. The platform needs to be able to handle different types of users, such as students, instructors, and administrators.

• Classes: User, Student, Instructor, Administrator

• Design Patterns: Chain of Responsibility pattern, Command pattern

• Explanation: The User class can be used as a base class for different types of users, such as Student, Instructor, and Administrator. The Chain of Responsibility pattern can be used to handle different types of user requests or actions in a systematic and organized way, reducing the risk of errors or security issues. The Command pattern can be used to encapsulate user requests as command objects, allowing them to be easily modified or extended.

6. The platform needs to be able to track student progress and performance.

• Classes: Progress, Student

• Design Patterns: Observer pattern, Command pattern

• Explanation: The Progress class can be used to encapsulate progress and performance data for students, and the Student class can be used to manage student data. The Observer pattern can be used to notify students of their progress and provide personalized feedback based on their performance. The Command pattern can be used to encapsulate progress and performance data as a command object, allowing it to be easily modified or extended.

7. The platform needs to be able to provide support for different types of devices, such as desktops, tablets, and smartphones.

• Classes: Device, Desktop, Tablet, Smartphone

• Design Patterns: Strategy pattern, Template Method pattern

• Explanation: The Device class can be used as a base class for different types of devices, such as Desktop, Tablet, and Smartphone. The Strategy pattern can be used to define different strategies for delivering course content and assessments to different types of devices. The Template Method pattern can be used to provide a standardized user interface and workflow for different types of devices, making the platform easier to navigate and use.

8. The platform needs to be able to handle different types of languages and localizations.

• Classes: Language, Localization

• Design Patterns: Strategy pattern, Template Method pattern

• Explanation: The Language class can be used to encapsulate different languages supported by the platform, and the Localization class can be used to manage localization data. The Strategy pattern can be used to define different strategies for delivering course content and assessments in different languages and localizations. The Template Method pattern can be used to provide a standardized user interface and workflow for different types of languages and localizations, making the platform easier to navigate and use.

9. The platform needs to be secure and protect sensitive data.

• Classes: SecurityManager, User

• Design Patterns: Chain of Responsibility pattern, Command pattern

• Explanation: The SecurityManager class can be used to handle different types of security requests or actions in a systematic and organized way, reducing the risk of errors or security issues. The User class can be used to manage user data, including sensitive data. The Chain of Responsibility pattern can be used to handle different types of security requests or actions in a systematic and organized way, reducing the risk of errors or security issues. The Command pattern can be used to encapsulate security data as a command object, allowing it to be easily modified or extended.

10. The platform needs to be able to handle different types of payment methods.

• Classes: PaymentManager, PaymentMethod

• Design Patterns: Strategy pattern, Template Method pattern

• Explanation: The PaymentManager class can be used to handle different types of payment requests or actions in a systematic and organized way, reducing the risk of errors or security issues. The PaymentMethod class can be used to encapsulate different types of payment methods, such as credit cards or online wallets. The Strategy pattern can be used to define different strategies for handling different types of payment methods. The Template Method pattern can be used to provide a standardized user interface and workflow for different types of payment methods, making the platform easier to navigate and use.

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.

Example: A web application that needs to notify users of updates to a particular resource (e.g. blog post, news article, etc.).

public interface Subject {
    public void attach(Observer observer);
    public void detach(Observer observer);
    public void notifyObservers();
}

public class Resource implements Subject {
    private List<Observer> observers = new ArrayList<>();
    private String content;
    
    public void attach(Observer observer) {
        observers.add(observer);
    }
    
    public void detach(Observer observer) {
        observers.remove(observer);
    }
    
    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.update(content);
        }
    }
    
    public void setContent(String content) {
        this.content = content;
        notifyObservers();
    }
}

public interface Observer {
    public void update(String content);
}

public class User implements Observer {
    private String name;
    
    public User(String name) {
        this.name = name;
    }
    
    public void update(String content) {
        // Implement user notification logic
    }
}

Chain of Responsibility Pattern

• Allows you to pass requests along a chain of handlers, where each handler decides whether to handle the request or pass it to the next handler in the chain.

Example: A web application that needs to handle different types of requests (e.g. HTTP requests, database requests, etc.) using different handlers.

public interface RequestHandler {
    public void handleRequest(Request request);
}
public class HTTPRequestHandler implements RequestHandler {
    private RequestHandler nextHandler;
    public void setNextHandler(RequestHandler nextHandler) {
        this.nextHandler = nextHandler;
    }
    public void handleRequest(Request request) {
        if (request.getType() == Request.Type.HTTP) {
            // Implement HTTP request handling logic
        } else if (nextHandler != null) {
            nextHandler.handleRequest(request);
        } else {
            // No more handlers in the chain, cannot handle request
        }
    }
}
public class DatabaseRequestHandler implements RequestHandler {
    private RequestHandler nextHandler;
    public void setNextHandler(RequestHandler nextHandler) {
        this.nextHandler = nextHandler;
    }
    public void handleRequest(Request request) {
        if (request.getType() == Request.Type.DATABASE) {
            // Implement database request handling logic
        } else if (nextHandler != null) {
            nextHandler.handleRequest(request);
        } else {
            // No more handlers in the chain, cannot handle request
        }
    }
}

public class Request { 
    public enum Type {
        HTTP, 
        DATABASE
    }
    
    private Type type;
    
    public Request(Type type) {
        this.type = type;
    }
    
    public Type getType() {
        return type;
    }
}

Command Pattern

• Encapsulates a request as an object, thereby allowing you to parameterize clients with different requests, queue or log requests, and support undoable operations.

Example: A web application that needs to implement an undo/redo feature for user actions.

public interface Command {
    public void execute();
    public void undo();
}

public class CreateCommand implements Command {
    private Object object;
    
    public CreateCommand(Object object) {
        this.object = object;
    }
    
    public void execute() {
        // Implement create object logic
    }
    
    public void undo() {
        // Implement undo create object logic
    }
}

public class UpdateCommand implements Command {
    private Object object;
    
    public UpdateCommand(Object object) {
        this.object = object;
    }
    
    public void execute() {
        // Implement update object logic
    }
    
    public void undo() {
        // Implement undo update object logic
    }
}

public class DeleteCommand implements Command {
    private Object object;
    
    public DeleteCommand(Object object) {
        this.object = object;
    }
    
    public void execute() {
        // Implement delete object logic
    }
    
    public void undo() {
        // Implement undo delete object logic
    }
}

public class CommandInvoker {
    private Stack<Command> undoStack = new Stack<>();
    private Stack<Command> redoStack = new Stack<>();
    
    public void executeCommand(Command command) {
        command.execute();
        undoStack.push(command);
        redoStack.clear();
    }
    
    public void undoCommand() {
        if (!undoStack.isEmpty()) {
            Command command = undoStack.pop();
            command.undo();
            redoStack.push(command);
        }
    }
    
    public void redoCommand() {
        if (!redoStack.isEmpty()) {
            Command command = redoStack.pop();
            command.execute();
            undoStack.push(command);
        }
    }
}

Strategy Pattern

• Allows you to define a family of algorithms, encapsulate each one as an object, and make them interchangeable at runtime.

Example: A web application that needs to implement different types of search algorithms (e.g. linear search, binary search, etc.).

public interface SearchStrategy {
    public int search(int[] array, int key);
}

public class LinearSearchStrategy implements SearchStrategy {
    public int search(int[] array, int key) {
        // Implement linear search algorithm
    }
}

public class BinarySearchStrategy implements SearchStrategy {
    public int search(int[] array, int key) {
        // Implement binary search algorithm
    }
}

public class SearchAlgorithm {
    private SearchStrategy strategy;
    
    public SearchAlgorithm(SearchStrategy strategy) {
        this.strategy = strategy;
    }
    
    public int search(int[] array, int key) {
        return strategy.search(array, key);
    }
    
    public void setStrategy(SearchStrategy strategy) {
        this.strategy = strategy;
    }
}

Template Method Pattern

• Defines the skeleton of an algorithm in a method, allowing subclasses to provide specific behavior for certain steps.

Example: A web application that needs to implement a login process that varies depending on the type of user (e.g. admin, regular user, etc.).

public abstract class LoginTemplate {
    public final void login() {
        enterCredentials();
        authenticateCredentials();
        if (validateSession()) {
            redirect();
        } else {
            displayError();
        }
    }
    
    protected abstract void enterCredentials();
    protected abstract void authenticateCredentials();
    
    protected boolean validateSession() {
        // Implement default session validation logic
        return true;
    }
    
    protected abstract void redirect();
    protected abstract void displayError();
}

public class AdminLogin extends LoginTemplate {
    protected void enterCredentials() {
        // Implement admin login credentials input
    }
    
    protected void authenticateCredentials() {
        // Implement admin login authentication logic
    }
    
    protected void redirect() {
        // Implement admin login redirect logic
    }
    
    protected void displayError() {
        // Implement admin login error display logic
    }
}
public class UserLogin extends LoginTemplate {
    protected void enterCredentials() {
        // Implement user login credentials input
    }
    
    protected void authenticateCredentials() {
        // Implement user login authentication logic
    }
    
    protected boolean validateSession() {
        // Implement user session validation logic
        return false;
    }
    
    protected void redirect() {
        // Implement user login redirect logic
    }
    
    protected void displayError() {
        // Implement user login error display logic
    }
}