How to automatically save last listview values ​​and display offline?

Question

Java

When creating an application that takes data (json) from the bank's api using retrofit2, I receive the latest exchange rate values ​​in the listview in response. I was able to implement what I wanted, but when I re-open the application offline, the application automatically requests data from the server, an obvious error arrives in response and therefore: I cannot work with the data without an Internet connection.
Question: How to implement automatic saving of data, in my case: the exchange rate. So that the user re-enters the application already in offline mode and can use the values ​​​​that were automatically saved in the listview.

the code

public class MainActivity extends AppCompatActivity {

    Float trip;
    Float trip2 = 34.2f;

    SharedPreferences sPref;

    String flo3 = "EUR: ";

    EditText etText;

    Button saveBtn;

    final  String SAVED_TEXT = "saved_text";

    final String LOG_TAG = "myLogs";
    private float selectedFromList;



    public void onSaveInstanceState(Bundle savedState) {
        super.onSaveInstanceState(savedState);

    }


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final ListView listView = (ListView) findViewById(R.id.listView);


        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(Api.BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        Api api = retrofit.create(Api.class);

        Call<List<Hero>> call = api.getHeroes();

        call.enqueue(new Callback<List<Hero>>() {
            @Override
            public void onResponse(Call<List<Hero>> call, Response<List<Hero>> response) {
                List<Hero> heroes = response.body();


                Float[] heroNames = new Float[heroes.size()];

                for(int i = 0; i < heroes.size(); i++) {

                    heroNames[i] = heroes.get(i).getBuy();

                }

                listView.setAdapter(
                        new ArrayAdapter<Float>(
                                getApplicationContext(),
                                android.R.layout.simple_list_item_single_choice,
                                heroNames
                        )


                );


                Float selectedFromList = (Float) listView.getItemAtPosition(1);
                System.out.println("EUR = : " + selectedFromList);

                listView.setItemChecked(1, true);

                trip = selectedFromList;

                TextView ob5 = (TextView)findViewById(R.id.textView23);
                ob5.setText(Float.toString(trip));


            }


            @Override
            public void onFailure(Call<List<Hero>> call, Throwable t) {
                Toast.makeText(getApplicationContext(), t.getMessage(),Toast.LENGTH_SHORT).show();
            }

        });

    }

Answer 1

In ListView, as in other views, nothing is saved. You want persistent saving. You can save to preferences, file, database. Choose whichever is more convenient for you.