Features Required:

1. User Authentication and Registration: Users should be able to create accounts, log in, and manage their profiles.

2. Music Catalog: The service should have a catalog of songs, albums, artists, and playlists that users can browse and search.

3. Music Streaming: Users should be able to stream music and listen to songs, albums, playlists, and personalized recommendations.

4. Personalized Recommendations: The service should provide personalized recommendations based on user preferences, listening history, and user interactions.

5. Playlist Management: Users should be able to create, modify, and manage their own playlists.

6. Social Features: Users should be able to follow other users, view their public playlists, and share music with friends.

7. Offline Mode: Users should have the option to download songs and listen to them offline.

8. Music Licensing: The service should comply with music licensing regulations and obtain necessary licenses to stream music legally.

Design Patterns Involved or Used:

1. Model-View-Controller (MVC): The MVC pattern can be used to separate the application into three components: the model (data layer), the view (user interface), and the controller (logic and interaction between the model and view).

2. Observer Pattern: The observer pattern can be used to notify users about updates, such as new releases, playlist changes, or social activities.

3. Factory Pattern: The factory pattern can be used to create different types of music objects, such as songs, albums, and playlists, based on user requests.

4. Decorator Pattern: The decorator pattern can be used to add additional features or behaviors dynamically to music objects, such as adding lyrics, related artists, or album artwork.

5. Singleton Pattern: The singleton pattern can be used to manage centralized components, such as the music player, user authentication, and the recommendation engine.

Code/Solutions:

Implementing a full-fledged music streaming service like Spotify requires a significant amount of code and infrastructure. It involves designing and implementing various components, such as user management, music catalog, streaming infrastructure, recommendation system, and social features.

import java.util.ArrayList;
import java.util.List;

// Music interface
interface Music {
    void play();
}

// Song class implementing the Music interface
class Song implements Music {
    private String title;
    private String artist;
    private String album;

    public Song(String title, String artist, String album) {
        this.title = title;
        this.artist = artist;
        this.album = album;
    }

    @Override
    public void play() {
        System.out.println("Playing song: " + title);
    }

    // Getters and setters
}

// Playlist class implementing the Music interface
class Playlist implements Music {
    private String name;
    private List<Music> songs;

    public Playlist(String name) {
        this.name = name;
        this.songs = new ArrayList<>();
    }

    public void addSong(Music song) {
        songs.add(song);
    }

    public void removeSong(Music song) {
        songs.remove(song);
    }

    @Override
    public void play() {
        System.out.println("Playing playlist: " + name);
        for (Music song : songs) {
            song.play();
        }
    }

    // Getters and setters
}

// User class
class User {
    private String username;
    private String password;
    private List<Playlist> playlists;

    public User(String username, String password) {
        this.username = username;
        this.password = password;
        this.playlists = new ArrayList<>();
    }

    public void createPlaylist(String name) {
        Playlist playlist = new Playlist(name);
        playlists.add(playlist);
    }

    public void deletePlaylist(Playlist playlist) {
        playlists.remove(playlist);
    }

    // Getters and setters
}

// Singleton MusicPlayer class
class MusicPlayer {
    private static MusicPlayer instance;
    private boolean isPlaying;

    private MusicPlayer() {
        this.isPlaying = false;
    }

    public static MusicPlayer getInstance() {
        if (instance == null) {
            instance = new MusicPlayer();
        }
        return instance;
    }

    public void playMusic(Music music) {
        System.out.println("Starting music playback.");
        music.play();
        isPlaying = true;
    }

    public void stopMusic() {
        System.out.println("Stopping music playback.");
        isPlaying = false;
    }

    public boolean isPlaying() {
        return isPlaying;
    }
}

// Main Class
public class MusicStreamingService {
    public static void main(String[] args) {
        // Create users
        User user1 = new User("user1", "password1");
        User user2 = new User("user2", "password2");

        // Create songs
        Music song1 = new Song("Song 1", "Artist 1", "Album 1");
        Music song2 = new Song("Song 2", "Artist 2", "Album 2");

        // Create playlists
        user1.createPlaylist("My Playlist");
        user1.createPlaylist("Favorites");

        user1.getPlaylists().get(0).addSong(song1);
        user1.getPlaylists().get(0).addSong(song2);
        user1.getPlaylists().get(1).addSong(song1);

        // Play a song and a playlist
        MusicPlayer musicPlayer = MusicPlayer.getInstance();
        musicPlayer.playMusic(song1);

        System.out.println("Is music playing? " + musicPlayer.isPlaying());

        Playlist playlist = user1.getPlaylists().get(0);
        musicPlayer.playMusic(playlist);

        System.out.println("Is music playing? " + musicPlayer.isPlaying());

        musicPlayer.stopMusic();

        System.out.println("Is music playing? " + musicPlayer.isPlaying());
    }
}

In this above code, the following design patterns are used:

1. Factory Pattern: The Song and Playlist classes implement the Music interface, acting as product classes in a factory pattern. The User class acts as the creator, creating and managing playlists.

2. Singleton Pattern: The MusicPlayer class implements the singleton pattern, ensuring that only one instance of the music player is created and accessed throughout the application.

3. Composite Pattern: The Playlist class uses the composite pattern to treat both songs and playlists uniformly as music objects. It can contain a mix of songs and other playlists, enabling nested and hierarchical structures.

The code demonstrates the creation of users, songs, and playlists, along with the usage of the MusicPlayer singleton to play music. Playlists can contain both individual songs and other playlists, allowing for a flexible and composite music structure.

Please note that this is a simplified example, and a complete implementation of a music streaming service like Spotify requires additional features and more sophisticated design considerations.