public class DatabaseCheckTask extends AsyncTask<String,Integer,Integer>{
    int isRegistered = 7;
    @Override
    protected Integer doInBackground(String... strings) {
        DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference("students");
        databaseReference.child(strings[0]).addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if(dataSnapshot.exists()){
                    Log.d("db_checker","ROLL_MATCHED");
                    isRegistered = 2;
                }
                else {
                    Log.d("db_checker","ROLL_NOT_MATCHED");
                    isRegistered = 1;
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Log.d("checker","database error");
                isRegistered = 3;
            }
        });
        return isRegistered;
    }

    @Override
    protected void onPostExecute(Integer integer) {
        super.onPostExecute(integer);
        switch (integer){
            case 1:
                Log.d("signup","SIGNUP");
                break;
            case 2:
                Log.d("signup","NOSIGNUP");
                break;
            case 3:
                Log.d("signup","DATABASE_ERROR");
                break;
            case 7:
                Log.d("signup","CASE_7");
                break;
            default:
                Log.d("signup","DEFAULT_CASE");
        }
    }
}