Firebase Database - Unable to retrieve nested data

Question

I'm unable to retrieve the data which is nested inside "Leagues" and "Season".

In the database below, I cannot retrieve Season 2016/2017 points and results.

I am able to access the data that is not nested, such as Email and Username without problems

"Users" : 
    "User1" : {
      "Email" : "dfs@sdf.com",
      "Last_login" : "5:15pm",
      "Username" : "Test",
      "Leagues" : {
        "FootballLeague" : true,
        "CricketLeague" : true
      },
      "Season" : {
         "2017" : {
               "Points" : 5,
               "Results" : 2
               "newdata" : {
                     "randomdata1: data1",
                     "randomdata2: data2"
                           },
                 }
         "2018" : {
                "Points" : 7,
                "Results" : 2
      }
}

The following class is what I'm using to store data as objects:

public class Users {

private String Username;
private String Email;
private String Last_login;
private Map<String, Boolean> Leagues;
private Map<String, thisSeason> Season;

public Users() {

}

//All getters and setters for the strings. 

public Map<String, Boolean> get_joined_Leagues() {
   return Leagues;
}

public void set_joined_Leagues(Map<String, Boolean> leagues) {
  Leagues = leagues;
}


public Map<String, thisSeason> getSeason() {
   return Season;
}

public void set_thisSeason(Map<String, thisSeason> season) {
  Season = season;
}

public static class thisSeason {
private int Points;
private int Results;
private Map <String, thisNewData> newdata;

    public thisSeason() {
    }

    public int getthisSeason_results() {
        return Results;
    }

    public void setthisSeason_results(int resultsin) {
        Results = resultsin;
    }

    public int getthisSeason_points() {
        return Points;
    }

    public void setSeason_points(int Pointsin) {
        Points = Pointsin;
    }

    public Map<String, thisNewData> getNewData() {
   return newdata;
    }

    public void set_thisNewData(Map<String, thisNewData> newdata_in) {
    newdata= newdata_in;
    }

    public static class thisNewData {
      private String randomdata1;
      private String randomdata2;

      //Getters and Setters
      }

Here is part of my Java class where I access the Database:

    List<Users> usersList= new ArrayList<Users>();
DatabaseReference fbDB = FirebaseDatabase.getInstance().getReference();

        fbDB.child("Users").addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            if (dataSnapshot.exists()) {

                for (DataSnapshot ds : dataSnapshot.getChildren()) {
                    Users C = ds.getValue(Users.class);
                    usersList.add(C);

                    System.out.println(C.getSeason().getthisSeason_points()); //ERROR occurs here - null object reference

                }
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });


}

I get a null object reference error as shown in the code above.

Also, on a slight side note - I'm aware my implementation for getting Leagues is incorrect. How can I retrieve the keys instead?

Answer 1

The main problems that I found in your code:

First, your Leagues object: Well, it should not be a custom object at all, if it stays in the current structure, but a Map. This is the map object - a collection which has a key, and a value, just like a child in the Realtime Database.

If a league name (=key) didn't have a true (=value) next to it - the names could have been stored in String variables, but this is not the case, because every child in the Realtime Database must have a value, and storing a "dummy" value next to a league name makes each league a part of a Map. This current "dummy" is a boolean, and that's why this will be the leagues' data type:Map<String, Boolean> leagues.

---

Second, your Season object: You are confusing between one season and many seasons. Your child - "Season" actually stores a lot of seasons. Thats why this case will be similar to the one above - each season's time is the key and the season itself is the value.

---

Therefore, the User class's variables should be:

private String Username;
private String Email;
private String Last_login;
private Map<String,Boolean> Leagues;
private Map<String, Season> Seasons;