public class UserDatabase {
  
    public final static int CAPACITY = 5;
    User[] users = new User[CAPACITY];

    public User[] getAllUsers(){
        return users; 
    }

    public void addUser (User u) {
        int id = u.getId();
        users[id] = u
    }

    public void addUser(int id) {      
        if (id<0 || id >= users.length) {
            System.out.println("Over capacity. The maximum capacity is " + UserDatabase.CAPACITY);
            return;
        }
         users[id] = user;
    }

    public User getUserById(int id) {
        if (id<0 || id>= users.length ) {
            return null;
        }
        return users[id];
    }

    public User getUserByUserName(String userName) {
        for (User u: users) {
             if (u.getUserName().equals(userName)) {
               return user;
        }
        return null;
    }
}

    public boolean userExists(User user) {
        for (User u: users) {            
            String un = user.getUserName();
            String pw = user.getPassword();
            if (u.getUserName().equals(un) && u.getPassword().equals(pw)){
                return true;
            }
        }
        return false;
    }
}