FirebaseDatabase database = FirebaseDatabase.getInstance();
    reference = database.getReference("users");

    final String username_password = usernameText.getText().toString() + "_" + password.getText().toString();

    Query query = reference.orderByChild("username_password").equalTo(username_password);

    query.addListenerForSingleValueEvent(new ValueEventListener() {

        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            if (dataSnapshot.exists()) {

                for (DataSnapshot user : dataSnapshot.getChildren()) {

                    Toast.makeText(getApplicationContext(), "Username and Password are correct", Toast.LENGTH_SHORT).show();
                    Log.i("User Found", user.toString());

                }

            } else {

                Map<String, String> values = new HashMap<>();

                values.put("username", usernameText.getText().toString());
                values.put("password", password.getText().toString());
                values.put("username_password", username_password);

                reference.push().setValue(values, new DatabaseReference.CompletionListener() {

                    @Override
                    public void onComplete(DatabaseError databaseError, DatabaseReference databaseReference) {

                        if (databaseError == null) {

                            Toast.makeText(getApplicationContext(), "Well done", Toast.LENGTH_SHORT).show();
                            Log.i("Info", "Saved Successfully");

                        } else {

                            Toast.makeText(getApplicationContext(), "ups", Toast.LENGTH_SHORT).show();
                            Log.i("Saved Failed", databaseError.getMessage());

                        }
                    }
                });

            }
        }