Exercise 1

Design Patterns

1. Creational Patterns

1.1 Factory Pattern

Definition: Creates objects without specifying the exact class to create.

Use Case: "Flexible Payment Processing System"

Description:

• Defines a PaymentGateway interface with methods like processPayment() and getPaymentMethod().
• Concrete classes: CreditCardPayment and BankTransferPayment implement PaymentGateway.
• PaymentGatewayFactory class with a createPaymentGateway() method to instantiate payment objects.

How it works: User selects a payment method, PaymentGatewayFactory creates the corresponding object, which processes the payment.

1.2 Singleton Pattern

Definition: Ensures a class has only one instance with global access.

Use Case: "Global In-Memory Cache Management"

Description:

• Cache class with private constructor and private static Cache instance.
• Public getInstance() method for accessing the single instance.
• Methods like put(), get(), and clear() for cache operations.

How it works: getInstance() method returns the single instance, allowing global access to the cache.

2. Behavioral Patterns

2.1 Observer Pattern

Definition: Defines a subscription mechanism to notify multiple objects about events.

Use Case: "Real-time YouTube Video Upload Notification System"

Description:

• Observer interface with update() method.
• Subject interface with registerObserver(), removeObserver(), and notifyObservers() methods.
• YouTubeChannel class implements Subject.
• Subscriber class implements Observer.

How it works: Subscribers register with a channel. When a new video is uploaded, all subscribers are notified via their update() method.

2.2 State Pattern

Definition: Allows an object to alter its behavior when its internal state changes.

Use Case: "Dynamic Vending Machine Operation Management"

Description:

• State interface with methods like selectProduct(), insertMoney(), dispense(), and cancel().
• Concrete state classes: NoSelectionState, HasSelectionState, HasMoneyState, DispensedState.
• VendingMachine class that maintains current state and delegates operations to it.

How it works: VendingMachine delegates operations to its current state object, which changes based on actions performed.

3. Structural Patterns

3.1 Adapter Pattern

Definition: Allows objects with incompatible interfaces to collaborate.

Use Case: "Unified Music Playback System for Multiple Audio Formats"

Description:

• StandardTrack interface with methods like getTitle(), getArtist(), play().
• VinylRecord and DigitalAudioFile classes with incompatible interfaces.
• Adapter classes: VinylAdapter and DigitalAudioAdapter implementing StandardTrack.

How it works: Adapters wrap specific music sources, translating method calls to the common StandardTrack interface.

3.2 Composite Pattern

Definition: Composes objects into tree structures to represent part-whole hierarchies.

Use Case: "Hierarchical File System Structure Management"

Description:

• FileSystemComponent interface with methods like print() and getSize().
• File class implements FileSystemComponent (leaf node).
• Directory class implements FileSystemComponent and can contain other FileSystemComponent objects (composite node).

How it works: Directories can contain files and other directories, allowing for a nested structure. Both files and directories are treated uniformly through the FileSystemComponent interface.

These patterns provide solutions to common software design problems, promoting code reusability, flexibility, and maintainability. Each pattern addresses specific challenges in software design and can be applied in various contexts beyond the examples provided here.

Exercise 2

Virtual Classroom Manager

A simple command-line application for managing virtual classrooms, students, and assignments.

Table of Contents

• Overview
• Features
• Classes
  • Main
  • ClassroomManager
  • Classroom
  • Student
  • Assignment
• Commands
• Logging

Overview

The Virtual Classroom Manager is a Java-based application designed to simplify the management of virtual classrooms, students, and assignments. It provides a command-line interface for users to interact with the system, performing actions such as adding classrooms, enrolling students, scheduling assignments, and submitting assignments.

Features

• Add classrooms with unique names
• Enroll students in specific classrooms
• Schedule assignments for classrooms
• Submit assignments for students in specific classrooms
• Log all system activities to a file

Classes

Main

• main(String[] args): The entry point of the program. Sets up the logger, prints welcome messages, provides a list of available commands, and enters a loop to prompt the user for commands and execute them.
• setupLogger(): Sets up the logger for the application. Creates a FileHandler to log messages to a file named virtual_classroom.log, sets up a SimpleFormatter to format the log messages, and adds the logger to the FileHandler.
• addClassroom(String name): Adds a classroom with the given name using the ClassroomManager and prints a message to the console.
• addStudent(String studentId, String className): Adds a student to a classroom using the ClassroomManager and prints a message to the console.
• scheduleAssignment(String className, String assignmentDetails): Schedules an assignment for a classroom using the ClassroomManager and prints a message to the console.
• submitAssignment(String studentId, String className, String assignmentDetails): Submits an assignment for a student in a classroom using the ClassroomManager and prints a message to the console.

ClassroomManager

• getInstance(): Returns the singleton instance of ClassroomManager.
• addClassroom(String name): Adds a new Classroom with the given name. Throws an IllegalArgumentException if the name is empty or already exists.
• enrollStudent(String studentId, String studentName, String classroomName): Enrolls a Student in a Classroom. Throws an IllegalArgumentException if the classroom does not exist.
• scheduleAssignment(String classroomName, String assignmentName, String assignmentDetails): Schedules an Assignment for a given classroom. Throws an IllegalArgumentException if the classroom does not exist.
• submitAssignment(String studentId, String classroomName, String assignmentName): Submits an Assignment for a student in a classroom. Throws an IllegalArgumentException if the classroom or assignment does not exist, or if the student is not enrolled in the classroom.

Classroom

• Classroom(String name): Constructor that initializes the name of the classroom and empty lists for students and assignments.
• getName(): Returns the name of the classroom.
• addStudent(Student student): Adds a student to the classroom. Throws an exception if the student with the same ID is already enrolled.
• listStudents(): Returns an unmodifiable list of all students in the classroom.
• addAssignment(Assignment assignment): Adds an assignment to the classroom if it does not already exist. Throws an exception if the assignment already exists.
• listAssignments(): Returns an unmodifiable list of all assignments in the classroom.

Student

• Student(String id, String name): Constructor that initializes the student's ID and name.
• getId(): Returns the ID of the student.
• getName(): Returns the name of the student.

Assignment

• Assignment(String name, String details): Constructor that initializes the assignment's name and details.
• getName(): Returns the name of the assignment.
• getDetails(): Returns the details of the assignment.

Commands

The following commands are available:

• add_classroom <class_name>: Add a new classroom with the specified name
• add_student <student_id> <class_name>: Add a new student to the specified classroom
• schedule_assignment <class_name> <assignment_details>: Schedule a new assignment for the specified classroom
• submit_assignment <student_id> <class_name> <assignment_details>: Submit an assignment for the specified student and classroom
• exit: Exit the application

Logging

All system activities are logged to a file named virtual_classroom.log in the project directory. The log file is created automatically when the application starts. The logger logs messages at different levels, such as info, warning, and severe.