How to get ArrayList from second Activity and show it in a Recycler view in the Main Activity

I have a Main Activity that start a Second Activity where countries are selected. In the Second Activity, when the user selects the countries and presses “Add”, I want to close the Second Activity and show the selected countries in a RecyclerView in the Main Activity. I’m not able to retrieve arraylist with selected countries in Main Activity. Any idea? Thank you.

Main Activity
1: https://i.stack.imgur.com/xMmL8.png

Second Activity
2: https://i.stack.imgur.com/lVMYS.jpg

Main Activity Code:

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public class MainActivity extends AppCompatActivity {
private Button addCountry;
private RecyclerView countrySelectedRecyclerView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbarId);
toolbar.setTitle("Main Activity");
setSupportActionBar(toolbar);
countrySelectedRecyclerView = findViewById(R.id.idCountrySelectedRecyclerView);
addCountry = findViewById(R.id.addCountryId);
selectedCountryAdapter adapter = new selectedCountryAdapter();
RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getApplicationContext());
countrySelectedRecyclerView.setLayoutManager(layoutManager);
countrySelectedRecyclerView.setHasFixedSize(true);
countrySelectedRecyclerView.setAdapter(adapter);
addCountry.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, ChooseCountry.class);
startActivity(intent);
}
});
}
}

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Second Activity Code:

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public class ChooseCountry extends AppCompatActivity implements CountryListener{
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_choose_country);
Toolbar toolbar = findViewById(R.id.toolbarId);
toolbar.setTitle("Second Activity");
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
RecyclerView countryRecyclerView = findViewById(R.id.recyclerViewCoutryId);
List<Countries> countriesList = new ArrayList<>();
Countries unitedStates = new Countries();
unitedStates.image = R.drawable.united_states;
unitedStates.unselected = R.drawable.icon_unselected;
unitedStates.country = "United States";
countriesList.add(unitedStates);
Countries italy = new Countries();
italy.image = R.drawable.italy;
italy.unselected = R.drawable.icon_unselected;
italy.country = "Italy";
countriesList.add(italy);
Countries canada = new Countries();
canada.image = R.drawable.canada;
canada.unselected = R.drawable.icon_unselected;
canada.country = "Canada";
countriesList.add(canada);
Countries spain = new Countries();
spain.image = R.drawable.spain;
spain.unselected = R.drawable.icon_unselected;
spain.country = "Spain";
countriesList.add(spain);
final selectionCountryAdapter selectCountryAdapter = new selectionCountryAdapter(countriesList, this);
RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getApplicationContext());
countryRecyclerView.setLayoutManager(layoutManager);
countryRecyclerView.setHasFixedSize(true);
countryRecyclerView.setAdapter(selectCountryAdapter);
//Arraylist with the selected countries
List<Countries> newSelectedCountries = selectCountryAdapter.getSelectedCountries();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.menu_add, menu);
return super.onCreateOptionsMenu(menu);
}
@Override
public boolean onOptionsItemSelected(@NonNull MenuItem item) {
switch (item.getItemId()){
case R.id.idMenuAdd:
//Required code
finish();
break;
case android.R.id.home:
onBackPressed();
return true;
}
return super.onOptionsItemSelected(item);
}

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There are a few ways to achieve the result you are looking for.

If you want to continue using two activities, then save the selection in a Room database, and observe the Data using LiveData and a Query.

If you would like an easier solution, create two fragments:

1. Put all your first Activity's View related code in the first fragment.
2. Put your second Activity's View related code in the second fragment.

Then create a ViewModel which both of your fragments will have access to. In your optionsItemSelected() just set a variable in the viewModel to a list of selected countries.

You can also move your country list initialization code into the ViewModel, which you can call from the second fragment's onViewCreated() callback.

Remove the second Activity because it can be replaced entirely by a Fragment. You can access supportFragmentManager from inside the first Activity, and then call .beginTransaction() followed by .replace() and .commit() to use Fragments.

Another benefit is that you don't have to initialize your Toolbar twice.