Online Presence with Firebase and Android based off article https://firebase.googleblog.com/2013/06/how-to-build-presence-system.html . Read the article as it explains the whole .onDisconnect().removeValue() nicely.

MainActivity.java

 private void initialiseOnlinePresence() {
        final DatabaseReference onlineRef = databaseReference.child(".info/connected");
        final DatabaseReference currentUserRef = databaseReference.child("/presence/" + userId);
        onlineRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(final DataSnapshot dataSnapshot) {
                Log.d(TAG, "DataSnapshot:" + dataSnapshot);
                if (dataSnapshot.getValue(Boolean.class)){
                    currentUserRef.onDisconnect().removeValue();
                    currentUserRef.setValue(true);
                }
            }

            @Override
            public void onCancelled(final DatabaseError databaseError) {
                Log.d(TAG, "DatabaseError:" + databaseError);
            }
        });
        final DatabaseReference onlineViewersCountRef = databaseReference.child("/presence");
        onlineViewersCountRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(final DataSnapshot dataSnapshot) {
                Log.d(TAG, "DataSnapshot:" + dataSnapshot);
                onlineViewerCountTextView.setText(String.valueOf(dataSnapshot.getChildrenCount()));
            }

            @Override
            public void onCancelled(final DatabaseError databaseError) {
                Log.d(TAG, "DatabaseError:" + databaseError);
            }
        });
    }