FireStore v6

private void addNewContact(){
        save    = findViewById(R.id.save);
        name    = findViewById(R.id.name);
        email   = findViewById(R.id.email);
        phone   = findViewById(R.id.phone);
        
        String mName = name.getText().toString();
        String mEmail = email.getText().toString();
        String mPhone = phone.getText().toString();
        
        Map<String, Object>newContact = new HashMap<>();
        newContact.put(NAME_KEY, mName);
        newContact.put(EMAIL_KEY, mEmail);
        newContact.put(PHONE_KEY, mPhone);
        
        db.collection("PhoneBook").document("Contacts").set(newContact)
                .addOnSuccessListener(new OnSuccessListener<Void>() {
                    @Override
                    public void onSuccess(Void aVoid) {
                        Toast.makeText(MainActivity.this, "User Registered",
                                Toast.LENGTH_SHORT).show();
                    }
                });