private void onRegisterUser() {
    String name = username.getText().toString();
    Log.d("Name",""+name);
    boolean exist = retreiveUserNames(name);
    if (!validate()) {
    }
    else if (exist) {
        username.setError("El nombre de usuario ya existe");

    } else {
        signUp(getUserEmail(), getUserPassword());
    }
}

public boolean retreiveUserNames(final String sUserName) {
    mDatabase = FirebaseDatabase.getInstance().getReference("userNames");

    mDatabase.addValueEventListener(new ValueEventListener() {
        @Override

        public void onDataChange(DataSnapshot dataSnapshot) {
            for(DataSnapshot userSnapshot : dataSnapshot.getChildren()) {
                String existingUsername = userSnapshot.getKey();

                Log.d("Ex",""+sUserName);
                Log.d("Shot ", "" + existingUsername);

                if(sUserName.equals(existingUsername)){
                    isTaken = true;
                    Log.d("BooleanShot ", "" + isTaken);
                    break;
                }
                else if(!(sUserName.equals(existingUsername))){
                    isTaken = false;
                }
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Toast.makeText(getApplicationContext(), "Error de conexión, intente mas tarde...", Toast.LENGTH_SHORT).show();
        }

    });
    return isTaken;
}