How to store the number of children in a Firebase Database to a variable in Android Studio

Question

I want to be store the number of children in a Firebase Database to a variable in Android Studio. I am trying to get the total all the nodes under 2 different children in Firebase but it returns java.lang.ArithmeticException: divide by zero. Any help will be greatly appreciated.

private long absent, present, total, attendanceRecord;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_student_stats);

    statsResult = (TextView) findViewById(R.id.statsResult);

    ValueEventListener eventListener = new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot ds : dataSnapshot.getChildren()) {
                String name = ds.getKey();


                String selectedClass;
                Bundle extras = getIntent().getExtras();
                if (extras == null) {
                    selectedClass = null;
                } else {
                    selectedClass = extras.getString("classSelected");
                }
                absentNumber = FirebaseDatabase.getInstance().getReference().child("Users").child(user.getUid()).child("Classes").child(selectedClass).child("AbsentStudents").child(name);
                presentNumber = FirebaseDatabase.getInstance().getReference().child("Users").child(user.getUid()).child("Classes").child(selectedClass).child("PresentStudents").child(name);


                presentNumber.addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        for (DataSnapshot snap: dataSnapshot.getChildren()) {
                            present = snap.getChildrenCount() ;
                        }
                    }
                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });

                absentNumber.addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        for (DataSnapshot snap: dataSnapshot.getChildren()) {
                            absent = snap.getChildrenCount() ;
                        }
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });

                total = present + absent;
                attendanceRecord = present / total;

                allStudentsList.add(name + "\t" + attendanceRecord + "%");

            }

Answer 1

The problem is that data is loaded from Firebase asynchronously, and when you run the division the data hasn't loaded yet (so total is 0). To understand how asynchronous loading works, let's add some simple logging to your code:

System.out.println("Before attaching listener");
presentNumber.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        System.out.println("Got data");
    }
    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException(); // don't ignore errors
    }
});
System.out.println("After attaching listener");

When you run this code, you'll get:

Before attaching listener

After attaching listener

Got data

That is probably not the order you expected. Because it takes some time to get the data from the Firebase servers, your main code continues (and runs the "After" log). Only once the data is available from Firebase, does your onDataChange get called.

Knowing this, I hope the error makes more sense: by the time you run attendanceRecord = present / total, the total will still be 0 because the data hasn't been loaded yet.

The solution to deal with asynchronous loading is to reframe your problem. Right now your code says: "first load A, then load B, then do something with A and B". Instead I find it helpful to frame it as: "first start loading A and B. Once A and B are loaded, do something with them." In your code that would translate to adding an extra function:

public void checkAttendanceRecord(long present, long absent) {
  if (present >= 0 && absent >= 0) {
    total = present + absent;
    attendanceRecord = present / total;

    allStudentsList.add(name + "\t" + attendanceRecord + "%");
  }
}

And then

long present = -1;
long absent = -1;

presentNumber.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot snap: dataSnapshot.getChildren()) {
            present = snap.getChildrenCount() ;
        }
        checkAttendanceRecord(present, absent);
    }
    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException(); // don't ignore errors
    }
});

absentNumber.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot snap: dataSnapshot.getChildren()) {
            absent = snap.getChildrenCount() ;
        }
        checkAttendanceRecord(present, absent);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException(); // don't ignore errors
    }
});

Note that I'm not really sure what the loops in onDataChange are for, so I left them as is and focused only on the asynchronous nature of the code.

To learn more about asynchronous data loading, have a look at:

• My answer to Setting Singleton property value in Firebase Listener
• Doug's blog post on asynchronous loading in Firebase
• Wait Firebase async retrive data in android