How to erase a current object using a function | Java

Question

I am trying to make an erase function to delete the teams of the tournament using the team code (value c in the constructor). Firstly I want to check if that team exists in the objects I made in the main method. Is that possible to do that using an if statement?

Exercise:

Create a java application that stores data for various football teams. Each team has a name, a code and the current points in the championship. In your class create methods for registering new teams, erasing existing teams using their code and showing a list for all the teams currently active in the championship

package assignment.exercise4;

public class Data {
    private String  name = "";
    private  int code = 0;
    private static int register;
    private int erase;
    private int currentpoints = 0;

    public Data(int c, int points, String n) { //constructor
        code = c;
        this.currentpoints = points;
        name = n;
    }

    public void Erase(int c)
    {
        code = c;
        if(code != 0)
            System.out.println("Team  with Code: "+code+" has been erased" );
        else
            System.out.print("Team with code "+code+" does not exist!");
    }

    public void Register(String newTeam,int code)
    {
        name = newTeam;
        this.code = code;
        System.out.println("New Team " + name + " registered with code " + code);
    }

    public void print()
    {
        System.out.println("Team name: " + name + "\nTeam code: " + code + "\nTeam points: " + currentpoints + "\n");
    }

}

/*
    public static void main(String[] args) {
        System.out.println("\nList of Teams: \n");
        Data t1 = new Data(110,42,"Juventus");
        Data t2= new Data(105,45,"Manchester City");
        Data t3= new Data(240,50,"Barcelona");
        Data t4= new Data(122,36,"Arsenal");
        Data Team = new Data(0,0,""); //use for erase

        t1.print();
        t2.print();
        t3.print();
        t4.print();

        System.out.println("Teams erased: \n");

        Team.Erase(110);
        Team.Erase(122);
        Team.Erase(0);

        System.out.println("\n\nTeams Registered: \n");

        t1.Register("Real madrid", 11);
        t1.Register("Atletico Madric", 112);
    }

}
*/

Answer 1

What are you trying to erase the teams from?

If they were in a list, for example...

       Data t1 = new Data(110,42,"Juventus");
       Data t2= new Data(105,45,"Manchester City");
       Data t3= new Data(240,50,"Barcelona");
       Data t4= new Data(122,36,"Arsenal");

       List<Data> teams = Arrays.asList(t1, t2, t3, t4);

...you could create a list with a team erased like this...

public List<Data> erase(List<Data> team, int id) {
    return team.stream()
               .filter(t -> t.getId() != id)
               .collect(Collectors.toList());
}

So...

List<Data> remainingTeam = erase(team, 122); // Removes Arsenal

...would remove the first element from the list

Answer 2

I will not answer this to elaborately since it is homework. I will try to give you a hint though.

If you have a team and want to do something with it. Otherwise you just have a team which just stays there in a particular scope (if you do not know what scope is, look it up!). If you have a team you most likely want do do something with it. In this case you seem to want to store information about the teams to use in a championship. Important to note here is that the teams are not the focus here. The real focus is the Championship. The teams are just a part of the championship. There can still be a championship even if all teams does not choose to participate. But you want all teams choosing to participate to be registered to this particular championship (eg UEFA Champions League).

This leads to something called aggregate or association depending on how hard you want to tie the object to the championship. However you do probably not need to pursue these terms any further at this point. What is important to remember is that there is an "has a" relation between the championship and the teams. The championship "has a" collection of participating teams. This is normally reflected in this way in code,

public class Championship {
   private Team[] teams; // Or List<Team>, Collection<Team>, HashMap<Team>, ...
}

The Championship can then have methods for registering a team, removing a team, updating status, etc...

public void register(Team t) {
    if (numberOfTeams < teams.length) {
        teams[numberOfTeams] = t; // Index starts at zero
        numberOfTeams++;
    } else {
        throw new IndexOutOfBoundsException("The list is full. " + 
            "No more teams may be registered!")
    }
}

Even though the function erasing a team was requested, I believe I will not write it down. This design is so different from your original intent, so that writing the erase function will likely solve your complete homework. However, you do actually not have to erase the team it is perfectly possible to just overwrite the position with the next team as,

teams[i] = teams[i+1];

Hope this helps!

Answer 3

Short answer:

public void erase(int id) {
  // who needs an if statement, if we can use predicates?
  teams.removeIf(team -> team.getId() == id); 
}

But this will not work with your current code. Your current code misses the container for your teams.

Longer answer:

For the fun of it. Solving your homework:

class Team {
  int id;
  String name;
  int points;
  Team(int id, String name, int points) {
    this.id = id;
    this.name = name;
    this.points = points;
  }

  @Override
  public String toString() {
     // ugly formatted... another homework? ;-)
     return "Team '" + name + "' (" + id + "): " + points;
  }
}

Note, that I will not add any getter or setter, nor will I care about visibility here. I will leave that as another homework for you.

class Championship {
  List<Team> teams = new ArrayList<>();

  void register(Team team) {
    teams.add(team);
  }

  void erase(int id) {
     teams.removeIf(team -> team.id == id);
  }

  @Override
  public String toString() {
     // for additional fun... sorted by descending points
     return "=== Championship table ===\n"
             + teams.stream()
             .sorted((o1, o2) -> Integer.compare(o2.points, o1.points))
             .map(Objects::toString)
             .collect(Collectors.joining("\n"));
  }
}

Somewhere else:

public static void main(String[] args) {
  Championship championship = new Championship();
  championship.register(new Team(1, "not the best ones", 3));
  championship.register(new Team(2, "The better ones", 7));
  championship.register(new Team(3, "The winners", 11));
  System.out.println(championship);

  championship.erase(3);
  System.out.println(championship);
}

Output:

=== Championship table ===
Team 'The winners' (3): 11
Team 'The better ones' (2): 7
Team 'not the best ones' (1): 3
=== Championship table ===
Team 'The better ones' (2): 7
Team 'not the best ones' (1): 3

Too much of information? Just start with something like a championship-class or at least use a collection of Teams (e.g. List<Team>).

By the way... Do not deliver this solution as your homework, except you understand what is going on and you can explain it with your own words. Otherwise you are only betraying yourself.