🎵 Advanced Java 21 Streams & Lambdas Mastery

"In learning you will teach, and in teaching you will learn." — Phil Collins

📘 Project Overview

This is an advanced academic project designed to help Java developers deeply practice and master key features introduced and enhanced in Java 21, including:

• ✅ Streams
• ✅ Lambdas
• ✅ Method References
• ✅ Records

Unlike the many beginner-level examples that can be found online, this repository is built with seasoned professionals in mind.

The exercises were originally created for a corporate training program delivered to a top-tier international tech company, where the participants were highly experienced software engineers preparing for the official Oracle Java 21 certification.

Each of the 15 exercises includes:

• A progressively increasing level of complexity.
• A real-world inspired domain (Spotify-style music streaming logic).
• Clean, idiomatic use of modern Java 21 features — using only the standard JDK, no external libraries.
• Detailed JUnit 5 test cases with clear documentation of the desired functionality.

🎯 Purpose

This repository is made public to support other professionals who:

• Are preparing for the Java 21 certification.
• Want to sharpen their skills with realistic, professionally relevant exercises.
• Are looking for examples that go beyond the basics, using functional programming patterns in Java at a senior engineering level.

🙌 Who Is This For?

• Senior backend developers
• Java educators and trainers
• Certification candidates
• Teams preparing for interviews or internal upskilling in modern Java

📋 Exercises Overview

The project contains 15 increasingly complex exercises in the MusicAnalyticsService class:

1. Average Genre Popularity: Calculate average popularity ratings for each music genre
2. Popular Songs by Genre: Find the most popular song in each music genre
3. Playlist Duration: Calculate the total playing time of all songs in a user's playlists
4. User Genre Overlaps: Find users who share musical taste based on genre preferences
5. Top Songs Ranking: Generate a ranked list of the most played songs across the platform
6. Complex Album Filtering: Find albums matching multiple complex criteria for content curation
7. Album Summary Generation: Generate rich album summaries for the platform's user interface
8. User Statistics: Generate detailed user profile statistics for personalization
9. Premium vs. Free Analysis: Generate listening statistics comparing premium and free user behavior
10. Personalized Recommendations: Generate personalized song recommendations based on user preferences
11. Catalog Analysis: Analyze music catalog distribution by decade and genre
12. Artist Collaborations: Discover artist collaboration networks within the music catalog
13. Genre Affinity Scoring: Calculate users' affinity scores for different music genres
14. Dynamic Playlist Generation: Generate smart playlists tailored to specific activities and moods
15. Track Transitions: Analyze track transition patterns to improve shuffle and radio algorithms

🚀 Getting Started

1. Prerequisites

   • Java 21 JDK installed on your machine
   • Maven installed

2. Clone the repository

   git clone https://github.com/yourusername/java21-streams-lambdas.git
   cd java21-streams-lambdas

3. Build with Maven

   mvn clean install

4. Run the tests

   mvn test

5. Study the code

   • Start by examining the model classes in com.streamexercises.model
   • Review each exercise in MusicAnalyticsService.java
   • Understand the test cases in MusicAnalyticsServiceTest.java

🏗️ Project Structure

src/
├── main/java/com/streamexercises/
│   ├── model/
│   │   ├── Album.java          # Music album representation
│   │   ├── Genre.java          # Music genre enum
│   │   ├── Playlist.java       # User playlist implementation
│   │   ├── Song.java           # Song with metadata
│   │   └── User.java           # User profile with preferences
│   └── service/
│       └── MusicAnalyticsService.java  # Contains all 15 exercises
└── test/java/com/streamexercises/
    └── service/
        └── MusicAnalyticsServiceTest.java  # Comprehensive tests

🛠️ Technologies Used

• Java 21 (standard JDK only)
• JUnit 5
• Maven

💡 Learning Path Recommendation

1. Start with the simplest exercises (1-5)
2. Move to intermediate complexity (6-10)
3. Challenge yourself with advanced exercises (11-15)
4. Try implementing variations of each solution

📄 License

MIT License

✨ Credits

This project was designed as part of a high-level corporate training initiative for senior software engineers preparing for Java 21 certification.

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"The function of good software is to make the complex appear to be simple." — Grady Booch