QuestionQuestion

In this assignment, you will simulate an on-line music centre called the MusicExchangeCenter where Users log in and download Songs from other user’s computers. It is similar to the operation of the original Napster program that operated a few years ago where users shared songs. You won’t be doingany downloading or internet programming. Instead, you will just simulate what happens in real life. The assignment will give you familiarity with ArrayLists.

Here are simple Song and User classes that represent a song that is available at the Music Exchange Center and a user of the Music Exchange Center that logs in to download music:

public class Song {
private String title;
private String artist;
private int duration;

public Song() {
this("", "", 0, 0);
}

public Song(String t, String a, int m, int s) {
title = t;
artist = a;
duration = m * 60 + s;
}

public String getTitle() { return title; } public String getArtist() { return artist; } public int getDuration() { return duration; }

public int getMinutes() {
return duration / 60;
}

public int getSeconds() {
return duration % 60;
}

public String toString() {
return("\"" + title + "\" by " + artist + " " + (duration / 60) + ":" + (duration%60));
}
}

public class User {
private String userName; private boolean online;

public User() { this(""); }

public User(String u) {
userName = u;
online = false;
}

public String getUserName() { return userName; }
public boolean isOnline() { return online; }

public String toString() {
String s = "" + userName + ": XXX songs ("; if (!online) s += "not ";
return s + "online)";
}
}

Do the following in the User class:

• Add a songList attribute which is an ArrayList of Song objects. This list will contain all the songs that this user has on his/her hard drive to be made available to the Music Exchange Center community. Adjust the 2nd constructor to ensure that this attribute is initialized properly. Write a get method for the attribute as well.
• Adjust the toString() method so that the XXX is replaced by the number of songs available in his/her song list.
• Create a method called addSong(Song s) which adds a given song to the user’s song list.
• Create a method called totalSongTime() that returns an integer indicating the total duration (i.e., amount of time) (in seconds) that all of the user’s songs would require if played.

(2) The MusicExchangeCenter Class
Implement a class called MusicExchangeCenter which represents the company website that users log in and out of in order to download music. The class should have this attribute:

• users - an ArrayList of all registered Users (which may be either logged on or not logged on).

Create the following methods:
• A zero-parameter constructor that sets attributes properly.
• An onlineUsers() method that returns an ArrayList of all Users that are currently online. Note that this method creates and returns a new ArrayList each time it is called.
• An allAvailableSongs() method that returns a new ArrayList of all Songs currently available for download (i.e., all songs that are available from all logged-on users). Note that this method creates and returns a new ArrayList each time it is called.

• A toString() method that returns a string representation of the music center showing the number of users currently online as well as the number of songs currently available.
(e.g., "Music Exchange Center (3 users on line, 15 songs available)").
• A userWithName(String s) method that finds and returns the user object with the given name if it is in the list of users. If not there, null should be returned.
• A registerUser(User x) method that adds a given User to the music center’s list of users, provided that there are no other users with the same userName. If there are other users with the same userName, then this method does nothing. Use the userWithName() method above.
• An availableSongsByArtist(String artist) method that returns a new ArrayList of all Songs currently available for download by the specified artist. Note that this method creates and returns a new ArrayList each time it is called.

Go back to the User class and add these methods:

• A register(MusicExchangeCenter m) that makes use of the registerUser() method that you just wrote to register the user into the given MusicExchangeCenter m. (Note that this should be a one-line method).
• A logon(MusicExchangeCenter m) that simulates a user going online with the given music center. (Assume that users can log onto at most one music center at a time). Make use of the userWithName() method you wrote earlier. Only allow the user to log on if he/she has already registered.
• A logoff(MusicExchangeCenter m) that simulates a user going offline from the given music center. Make use of the userWithName() method you wrote earlier.

Now we will test our code. Since we will be testing our classes with the same users, you must add the following static methods to the User class (Each method creates and returns a unique user with some songs in their song list):

// Various Users for test purposes
public static User DiscoStew() {
User discoStew = new User("Disco Stew"); discoStew.addSong(new Song("Hey Jude", "The Beatles", 4, 35)); discoStew.addSong(new Song("Barbie Girl", "Aqua", 3, 54));
discoStew.addSong(new Song("Only You Can Rock Me", "UFO", 4, 59)); discoStew.addSong(new Song("Paper Soup Cats", "Jaw", 4, 18)); return discoStew;
}

public static User SleepingSam() {
User sleepingSam = new User("Sleeping Sam"); sleepingSam.addSong(new Song("Meadows", "Sleepfest", 7, 15)); sleepingSam.addSong(new Song("Calm is Good", "Waterfall", 6, 22)); return sleepingSam;
}

public static User RonnieRocker() {
User ronnieRocker = new User("Ronnie Rocker"); ronnieRocker.addSong(new Song("Rock is Cool", "Yeah", 4, 17));
ronnieRocker.addSong(new Song("My Girl is Mean to Me", "Can't Stand Up", 3, 29)); ronnieRocker.addSong(new Song("Only You Can Rock Me", "UFO", 4, 52)); ronnieRocker.addSong(new Song("We're Not Gonna Take It", "Twisted Sister", 3, 9)); return ronnieRocker;
}

public static User CountryCandy() {
User countryCandy = new User("Country Candy");

countryCandy.addSong(new Song("If I Had a Hammer", "Long Road", 4, 15)); countryCandy.addSong(new Song("My Man is a 4x4 Driver", "Ms. Lonely", 3, 7)); countryCandy.addSong(new Song("This Song is for Johnny", "Lone Wolf", 4, 22)); return countryCandy;
}

public static User PeterPunk() {
User peterPunk = new User("Peter Punk"); peterPunk.addSong(new Song("Bite My Arms Off", "Jaw", 4, 12));
peterPunk.addSong(new Song("Where's My Sweater", "The Knitters", 3, 41)); peterPunk.addSong(new Song("Is that My Toenail ?", "Clip", 4, 47)); peterPunk.addSong(new Song("Anvil Headache", "Clip", 4, 34)); peterPunk.addSong(new Song("My Hair is on Fire", "Jaw", 3, 55));
return peterPunk;
}

Now test your code with the following test program:

public class MusicExchangeTestProgram {
public static void main(String args[]) {
// Create a new music exchange center
MusicExchangeCenter mec = new MusicExchangeCenter();

// Create some users and give them some songs
User discoStew = User.DiscoStew(); User sleepingSam = User.SleepingSam();
User ronnieRocker = User.RonnieRocker(); User countryCandy = User.CountryCandy(); User peterPunk = User.PeterPunk();

// Register the users, except SleepingSam discoStew.register(mec); ronnieRocker.register(mec); countryCandy.register(mec); peterPunk.register(mec);

// Display the state of things before anyone logs on System.out.println("Status: " + mec); System.out.println("On-Line Users: " + mec.onlineUsers());
System.out.println("Available Songs: " + mec.allAvailableSongs() + "\n");

// Attempt to log on two registered users and one unregistered user
discoStew.logon(mec); sleepingSam.logon(mec); // Should not work ronnieRocker.logon(mec); System.out.println("Status: " + mec);
System.out.println("On-Line Users: " + mec.onlineUsers()); System.out.println("Available Songs: " + mec.allAvailableSongs() + "\n");

// Log on two more users countryCandy.logon(mec); peterPunk.logon(mec); System.out.println("Status: " + mec);
System.out.println("On-Line Users: " + mec.onlineUsers()); System.out.println("Available Songs: " + mec.allAvailableSongs()); System.out.println("Available Songs By Jaw: " +
mec.availableSongsByArtist("Jaw") + "\n");

// Log off three users (one is not even logged in)
countryCandy.logoff(mec); discoStew.logoff(mec); sleepingSam.logoff(mec); System.out.println("Status: " + mec);
System.out.println("On-Line Users: " + mec.onlineUsers()); System.out.println("Available Songs: " + mec.allAvailableSongs()); System.out.println("Available Songs By Jaw: " +

mec.availableSongsByArtist("Jaw") + "\n");

// Log off the last two users peterPunk.logoff(mec); ronnieRocker.logoff(mec); System.out.println("Status: " + mec);
System.out.println("On-Line Users: " + mec.onlineUsers()); System.out.println("Available Songs: " + mec.allAvailableSongs()); System.out.println("Available Songs By Jaw: " +
mec.availableSongsByArtist("Jaw") + "\n");
}
}

Here is the output that you should see:
Status: Music Exchange Center (0 users on-line, 0 songs available) On-Line Users: []
Available Songs: []

Status: Music Exchange Center (2 users on-line, 8 songs available)
On-Line Users: [Disco Stew: 4 songs (online), Ronnie Rocker: 4 songs (online)]
Available Songs: ["Hey Jude" by The Beatles 4:35, "Barbie Girl" by Aqua 3:54, "Only You Can Rock Me" by UFO 4:59, "Paper Soup Cats" by Jaw 4:18, "Rock is Cool" by Yeah 4:17, "My Girl is Mean to Me" by Can't Stand Up 3:29, "Only You Can Rock Me" by UFO 4:52, "We're Not Gonna Take It" by Twisted Sister 3:9]

Status: Music Exchange Center (4 users on-line, 16 songs available)
On-Line Users: [Disco Stew: 4 songs (online), Ronnie Rocker: 4 songs (online), Country Candy: 3 songs (online), Peter Punk: 5 songs (online)]
Available Songs: ["Hey Jude" by The Beatles 4:35, "Barbie Girl" by Aqua 3:54, "Only You Can Rock Me" by UFO 4:59, "Paper Soup Cats" by Jaw 4:18, "Rock is Cool" by Yeah 4:17, "My Girl is Mean to Me" by Can't Stand Up 3:29, "Only You Can Rock Me" by UFO 4:52, "We're Not Gonna Take It" by Twisted Sister 3:9, "If I Had a Hammer" by Long Road 4:15, "My Man is a 4x4 Driver" by Ms. Lonely 3:7, "This Song is for Johnny" by Lone Wolf 4:22, "Bite My Arms Off" by Jaw 4:12, "Where's My Sweater" by The Knitters 3:41, "Is that My Toenail ?" by Clip 4:47, "Anvil Headache" by Clip 4:34, "My Hair is on Fire" by Jaw 3:55]
Available Songs By Jaw: ["Paper Soup Cats" by Jaw 4:18, "Bite My Arms Off" by Jaw 4:12, "My Hair is on Fire" by Jaw 3:55]

Status: Music Exchange Center (2 users on-line, 9 songs available)
On-Line Users: [Ronnie Rocker: 4 songs (online), Peter Punk: 5 songs (online)]
Available Songs: ["Rock is Cool" by Yeah 4:17, "My Girl is Mean to Me" by Can't Stand Up 3:29, "Only You Can Rock Me" by UFO 4:52, "We're Not Gonna Take It" by Twisted Sister 3:9, "Bite My Arms Off" by Jaw 4:12, "Where's My Sweater" by The Knitters 3:41, "Is that My Toenail ?" by Clip 4:47, "Anvil Headache" by Clip 4:34, "My Hair is on Fire" by Jaw 3:55]
Available Songs By Jaw: ["Bite My Arms Off" by Jaw 4:12, "My Hair is on Fire" by Jaw 3:55]

Status: Music Exchange Center (0 users on-line, 0 songs available) On-Line Users: []
Available Songs: [] Available Songs By Jaw: []


(3) Downloading Music
Go back to the Song class and add to it an attribute called owner which will be a
User object representing the user who happens to own this copy of this song.
Note that a Song object may only be owned by one User and that many users can have copies of the same song. Set it to null initially. In the User class, adjust the addSong() method so that it sets the owner properly.

In the User class, add a method called requestCompleteSonglist(MusicExchangeCenter m). This method should gather the list of all available songs from all users that are online at the given music exchange center (i.e., the union of all of their local song lists), and then return an ArrayList<String> formatted as follows:

TITLE ARTIST TIME OWNER

1. Hey Jude
2. Barbie Girl The Beatles
Aqua 4:35
3:54 Disco Stew
Disco Stew
3. Only You Can Rock Me UFO 4:59 Disco Stew
4. Paper Soup Cats Jaw 4:18 Disco Stew
5. Rock is Cool Yeah 4:17 Ronnie Rocker
6. My Girl is Mean to Me Can't Stand Up 3:29 Ronnie Rocker
7. Only You Can Rock Me UFO 4:52 Ronnie Rocker
8. We're Not Gonna Take It
etc.. Twisted Sister 3:09 Ronnie Rocker

Notice that the songs are numbered and that the title, artist, time and owner are all lined up nicely. You should use the String.format() method as described in the notes. Recall that %-30s in the format string will allow you to display a left-justified 30-character string. Also, %2d and %02d will allow you to display numbers so that they take 2 places, the 0 indicating that a leading zero character is desired.

In the User class, add a method called requestSonglistByArtist(MusicExchangeCenter m, String artist). This method should gather the list of all available songs by the given artist from all users that are online at the given music exchange center (i.e., the union of all of their local song lists), and then return an ArrayList<String> formatted similar to that shown above.

In the MusicExchangeCenter class, add a method called getSong(String title, String ownerName) which returns the Song object with the given title owned by the user with the given ownerName, provided that the user is currently online and that the song exists. Return null otherwise. (Hint: you will need to search through the center’s users to find User with the matching ownerName and then search through that user’s songs to find the Song with the given title. It may be a good idea to write an extra helper method in the User class called songWithTitle() that you can make use of).

In the User class, add a downloadSong(MusicExchangeCenter m, String title, String ownerName) method that simulates the downloading of one of the songs in the catalog. It should use the getSong() method that you just wrote, and add the downloaded song to the user’s songList (if not null).

Test your code now with the following program:

import java.util.ArrayList;

public class MusicExchangeTestProgram2 {
public static void main(String args[]) { ArrayList<String> catalog;

// Create a new music exchange center
MusicExchangeCenter mec = new MusicExchangeCenter();

// Create some users and give them some songs
User discoStew = User.DiscoStew(); User sleepingSam = User.SleepingSam();
User ronnieRocker = User.RonnieRocker(); User countryCandy = User.CountryCandy(); User peterPunk = User.PeterPunk();

// Register the users discoStew.register(mec); ronnieRocker.register(mec); sleepingSam.register(mec); countryCandy.register(mec); peterPunk.register(mec);

// Log on all users

discoStew.logon(mec); sleepingSam.logon(mec); ronnieRocker.logon(mec); countryCandy.logon(mec); peterPunk.logon(mec); System.out.println("Status: " + mec);

// Simulate a user requesting a list of songs catalog = discoStew.requestCompleteSonglist(mec); System.out.println("Complete Song List: ");
for (String s: catalog) System.out.println(" " + s);

// Simulate a user downloading 3 songs from the list System.out.println("\nDisco Stew before downloading: " + discoStew); discoStew.downloadSong(mec, "Bite My Arms Off", "Peter Punk"); discoStew.downloadSong(mec, "Meadows", "Sleeping Sam"); discoStew.downloadSong(mec, "If I Had a Hammer", "Country Candy"); discoStew.downloadSong(mec, "Sandy Toes", "Country Candy"); ronnieRocker.logoff(mec); // log off Ronnie, next download should fail discoStew.downloadSong(mec, "Only You Can Rock Me", "Ronnie Rocker"); System.out.println("Disco Stew after downloading: " + discoStew);

ronnieRocker.logon(mec); // log on Ronnie, next download should work discoStew.downloadSong(mec, "Only You Can Rock Me", "Ronnie Rocker"); System.out.println("Disco Stew after downloading Ronnie’s: " + discoStew + "\n");

// Simulate a user requesting a list of songs by a specific artist catalog = discoStew.requestSonglistByArtist(mec, "Jaw"); System.out.println("Song’s by Jaw: ");
for (String s: catalog) System.out.println(" " + s);
}
}

Here is the expected output:

Status: Music Exchange Center (5 users on-line, 18 songs available) Complete Song List:
TITLE ARTIST TIME OWNER

1. Hey Jude
2. Barbie Girl The Beatles Aqua 4:35
3:54 Disco Stew Disco Stew
3. Only You Can Rock Me UFO 4:59 Disco Stew
4. Paper Soup Cats Jaw 4:18 Disco Stew
5. Rock is Cool Yeah 4:17 Ronnie Rocker
6. My Girl is Mean to Me Can't Stand Up 3:29 Ronnie Rocker
7. Only You Can Rock Me UFO 4:52 Ronnie Rocker
8. We're Not Gonna Take It Twisted Sister 3:09 Ronnie Rocker
9. Meadows Sleepfest 7:15 Sleeping Sam
10. Calm is Good Waterfall 6:22 Sleeping Sam
11. If I Had a Hammer Long Road 4:15 Country Candy
12. My Man is a 4x4 Driver Ms. Lonely 3:07 Country Candy
13. This Song is for Johnny Lone Wolf 4:22 Country Candy
14. Bite My Arms Off Jaw 4:12 Peter Punk
15. Where's My Sweater The Knitters 3:41 Peter Punk
16. Is that My Toenail ? Clip 4:47 Peter Punk
17. Anvil Headache Clip 4:34 Peter Punk
18. My Hair is on Fire Jaw 3:55 Peter Punk

Disco Stew before downloading: Disco Stew: 4 songs (online) Disco Stew after downloading: Disco Stew: 7 songs (online)
Disco Stew after downloading Ronnie’s: Disco Stew: 8 songs (online)

Song’s by Jaw:
TITLE ARTIST TIME OWNER

1. Paper Soup Cats Jaw 4:18 Disco Stew
2. Bite My Arms Off Jaw 4:12 Disco Stew

3. Bite My Arms Off Jaw 4:12 Peter Punk
4. My Hair is on Fire Jaw 3:55 Peter Punk


(4) Paying the Price
Now in order to make all of this legal, we must have a way to compensate the musical artists for their
hard work and wonderful music. The compensation depends on many factors. An artist should receive 25 cents each time one of their songs is downloaded. Hence, for each artist, we will keep track of how much money in royalties they have. Add the following attributes to the MusicExchangeCenter class:

royalties - a HashMap with the artist’s name as the keys and the values are floats representing the total amount of royalties for that artist so far. It should only contain artists who have had songs downloaded.

downloadedSongs – an ArrayList<Song> containing all of the songs that have been downloaded. This list will, in general, contain duplicate Song objects.

Write a method in the MusicExchangeCenter class called displayRoyalties() that displays the royalties for all artists who have had at least one of their songs downloaded. It should display a two-line header and then one line per artist showing the royalty amount as well as the artist name
as follows:

Amount Artist
---------------
$0.75 Sleepfest
$1.50 Clip
$0.25 Jaw
$0.50 Long Road
$0.25 Yeah
$0.25 UFO

In the getSong() method in the MusicExchangeCenter, adjust the code so that if the song is found (i.e., able to be downloaded), then it is added to the downloadedSongs list. Write the get method for the downloadedSongs attribute.

Write a method in the MusicExchangeCenter class called uniqueDownloads() that returns (i.e., not displays) a TreeSet of all downloaded Song objects such that the set is sorted alphabetically by song title. There should be no duplicates songs in this set.

Write a method in the MusicExchangeCenter class called songsByPopularity() that returns (i.e., not displays) an ArrayList of Pair objects where the key of the pair is an Integer and the value is the Song object. The integer key should represent the number of times that that song was downloaded. The list returned should be sorted in decreasing order according to the number of times the song was downloaded. To sort a list of Pair objects, you can use the following code:

Collections.sort(popular, new Comparator<Pair<Integer, Song>>() {
public int compare(Pair<Integer, Song> p1, Pair<Integer, Song> p2) {
// PUT YOUR CODE IN HERE
}
});

Just insert the missing code so that it returns an appropriate integer indicating whether pair p1 comes before or after pair p2 in the sort order (see notes page 281).


Here is a test program … make sure it works with your code:

import javafx.util.Pair;

public class MusicExchangeTestProgram3 {
public static void main(String args[]) {
// Create a new music exchange center
MusicExchangeCenter mec = new MusicExchangeCenter();

// Create some users and give them some songs
User discoStew = User.DiscoStew(); User sleepingSam = User.SleepingSam();
User ronnieRocker = User.RonnieRocker(); User countryCandy = User.CountryCandy(); User peterPunk = User.PeterPunk();

// Register the users discoStew.register(mec); ronnieRocker.register(mec); sleepingSam.register(mec); countryCandy.register(mec); peterPunk.register(mec);

// Log on all users discoStew.logon(mec); sleepingSam.logon(mec); ronnieRocker.logon(mec); countryCandy.logon(mec); peterPunk.logon(mec); System.out.println("Status: " + mec);

// Simulate users downloading various songs discoStew.downloadSong(mec, "Bite My Arms Off", "Peter Punk"); discoStew.downloadSong(mec, "Meadows", "Sleeping Sam"); discoStew.downloadSong(mec, "If I Had a Hammer", "Country Candy"); discoStew.downloadSong(mec, "Is that My Toenail ?", "Peter Punk"); sleepingSam.downloadSong(mec, "Anvil Headache", "Peter Punk"); sleepingSam.downloadSong(mec, "Is that My Toenail ?", "Disco Stew"); sleepingSam.downloadSong(mec, "If I Had a Hammer", "Country Candy"); countryCandy.downloadSong(mec, "Anvil Headache", "Peter Punk"); countryCandy.downloadSong(mec, "Meadows", "Sleeping Sam"); countryCandy.downloadSong(mec, "If I Had a Hammer", "Peter Punk");
countryCandy.downloadSong(mec, "Only You Can Rock Me", "Ronnie Rocker"); countryCandy.downloadSong(mec, "Is that My Toenail ?", "Disco Stew"); peterPunk.downloadSong(mec, "Is that My Toenail ?", "Country Candy"); peterPunk.downloadSong(mec, "Rock is Cool", "Ronnie Rocker"); peterPunk.downloadSong(mec, "What?", "Ronnie Rocker"); peterPunk.downloadSong(mec, "Meadows", "Sleeping Sam");

// Display the downloaded songs System.out.println("\nHere are the downloaded songs: "); for (Song s: mec.getDownloadedSongs())
System.out.println(s);

// Display the downloaded songs alphabetically
System.out.println("\nHere are the unique downloaded songs alphabetically: "); for (Song s: mec.uniqueDownloads())
System.out.println(s);

// Display the downloaded songs in order of popularity System.out.println("\nHere are the downloaded songs by populariry: "); for (Pair<Integer,Song> p: mec.songsByPopularity())
System.out.println("(" + p.getKey() + " downloads) " + p.getValue());

// Display the royalties System.out.println("\nHere are the royalties:\n"); mec.displayRoyalties();
}
}


Here is the expected output:

Status: Music Exchange Center (5 users on-line, 18 songs available)

Here are the downloaded songs:
"Bite My Arms Off" by Jaw 4:12 "Meadows" by Sleepfest 7:15
"If I Had a Hammer" by Long Road 4:15 "Is that My Toenail ?" by Clip 4:47 "Anvil Headache" by Clip 4:34
"Is that My Toenail ?" by Clip 4:47 "If I Had a Hammer" by Long Road 4:15 "Anvil Headache" by Clip 4:34 "Meadows" by Sleepfest 7:15
"Only You Can Rock Me" by UFO 4:52 "Is that My Toenail ?" by Clip 4:47 "Is that My Toenail ?" by Clip 4:47 "Rock is Cool" by Yeah 4:17 "Meadows" by Sleepfest 7:15

Here are the unique downloaded songs alphabetically: "Anvil Headache" by Clip 4:34
"Bite My Arms Off" by Jaw 4:12
"If I Had a Hammer" by Long Road 4:15 "Is that My Toenail ?" by Clip 4:47 "Meadows" by Sleepfest 7:15
"Only You Can Rock Me" by UFO 4:52 "Rock is Cool" by Yeah 4:17

Here are the downloaded songs by populariry:
(4 downloads) "Is that My Toenail ?" by Clip 4:47 (3 downloads) "Meadows" by Sleepfest 7:15
(2 downloads) "Anvil Headache" by Clip 4:34
(2 downloads) "If I Had a Hammer" by Long Road 4:15 (1 downloads) "Bite My Arms Off" by Jaw 4:12
(1 downloads) "Only You Can Rock Me" by UFO 4:52 (1 downloads) "Rock is Cool" by Yeah 4:17

Here are the royalties:

Amount Artist
---------------
$0.75 Sleepfest
$1.50 Clip
$0.25 Jaw
$0.50 Long Road
$0.25 Yeah
$0.25 UFO

Process finished with exit code 0

Solution PreviewSolution Preview

These solutions may offer step-by-step problem-solving explanations or good writing examples that include modern styles of formatting and construction of bibliographies out of text citations and references. Students may use these solutions for personal skill-building and practice. Unethical use is strictly forbidden.

public class MusicExchangeCenter {

    /*an ArrayList of all registered Users (which may be
    either logged on or not logged on).
    */
    private ArrayList<User> users;

    /**
    * a HashMap with the artist’s name as the keys and the values are floats
    * representing the total amount of royalties for that artist so far. It
    * should only contain artists who have had songs downloaded.
    */
    private HashMap<String, Double> royalties;

    /**
    * an ArrayList<Song> containing all of the songs that have been downloaded.
    * This list will, in general, contain duplicate Song objects.
    */
    private ArrayList<Song> downloadedSongs;

    /**
    * A zero-parameter constructor that sets attributes properly
    */
    public MusicExchangeCenter() {
       users = new ArrayList<>();
       royalties = new HashMap<>();
       downloadedSongs = new ArrayList<>();
    }

    /**
    *
    * @return returns an ArrayList of all Users that are currently online. Note
    * that this method creates and returns a new ArrayList each time it is
    * called.
    */
    public ArrayList<User> onlineUsers() {
       ArrayList<User> new_users = new ArrayList<>();
       for (User user : users) {
            if (user.isOnline()) {
                new_users.add(user);
            }
       }
       return new_users;
    }

    public ArrayList<Song> allAvailableSongs() {
       ArrayList<Song> available = new ArrayList<>();
       for (User user : users) {
            if (user.isOnline()) {
                for (Song song : user.getSongList()) {
                   available.add(song);
                }
            }
       }
       return available;
    }...

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