Future Timestamp Error

[Ch-Tima]

Error:

      Fatal Exception: java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
   at java.util.ArrayList.get(ArrayList.java:437)
   at com.chtima.currencyconverter.MainActivity$3.onResponse(MainActivity.java:129)
   at retrofit2.DefaultCallAdapterFactory$ExecutorCallbackCall$1.lambda$onResponse$0$retrofit2-DefaultCallAdapterFactory$ExecutorCallbackCall$1(DefaultCallAdapterFactory.java:89)
   at retrofit2.DefaultCallAdapterFactory$ExecutorCallbackCall$1$$ExternalSyntheticLambda0.run(:6)
   at android.os.Handler.handleCallback(Handler.java:938)
   at android.os.Handler.dispatchMessage(Handler.java:99)
   at android.os.Looper.loop(Looper.java:223)
   at android.app.ActivityThread.main(ActivityThread.java:7680)
   at java.lang.reflect.Method.invokeNative(Method.java)
   at java.lang.reflect.Method.invoke(Method.java:423)
   at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

Current real Date & Time: 25.12.2024. However, the user can change the Date & Time on their device, for example, to 26.12.2025.
Request to API: https://api.privatbank.ua/p24api/exchange_rates?date=26.12.2025

{
  "date": "26.12.2024",
  "bank": "PB",
  "baseCurrency": 980,
  "baseCurrencyLit": "UAH",
  "exchangeRate": [] <<< here problem!
}

private void loadDataFromAPI(){
        private24Service = retrofit.create(Private24Service.class);

        var dateFormat = new SimpleDateFormat("dd.MM.yyyy");
        var dateNow = dateFormat.format(Calendar.getInstance().getTime());

        private24Service.GetCurrencyRoot(dateNow).enqueue(new Callback<CurrencyRoot>() {
            @Override
            public void onResponse(Call<CurrencyRoot> call, Response<CurrencyRoot> response) {
                if(response.code() == 200 && response.body() != null && response.body().getExchangeRate() != null){
                    CurrencyRoot root = response.body();
                    // BEGIN_LINE_129
                    UserTempData tempData = new UserTempData(
                            root.getExchangeRate().get(0), 
                            root.getExchangeRate().get(1)
                    );
                    // END_LINE_129
                } else {...}
            }

            @Override
            public void onFailure(Call<CurrencyRoot> call, Throwable t) {...}
        });
    }

[Ch-Tima]

Better Solution to the Bug: Using the Network Time Protocol (NTP)

The best approach to solve this bug would be to request the "Network Time Protocol" (NTP). However, making an NTP request is a bit challenging. Instead, I found the Play Services Time API by Google, which provides an easier way to get accurate time.

Thanks to TrustedTime, we can obtain the real time from Google Services even "without internet" (though not always, I suppose). This solution seems much more convenient than creating an NTP request or querying an external API.

Here's the updated version of my code to fix the issue:

class MainActivity 
//...
    protected void onCreate(Bundle savedInstanceState) {
        //...code...

        Task<TrustedTimeClient> initializeTrustedTimeClientTask = 
                TrustedTime.createClient(this); <<<< This fixed all
        SharedPreferences sp = SharedPreferencesKeys.getSharedPreferences(this);

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

        initializeTrustedTimeClientTask.addOnCompleteListener(task -> {
            var dateFormat = new SimpleDateFormat("dd.MM.yyyy", Locale.getDefault());
            dateFormat.setTimeZone(TimeZone.getTimeZone("UTC+2"));

            if(task.isSuccessful()){
                isTrueTime = true;
                Long tms = task.getResult().computeCurrentUnixEpochMillis();
                trueTimeNow = dateFormat.format(new Date(tms));//1730001052195L
                //...code...
            }else {
                //true - the mistake was already there
                if(!sp.getBoolean(SharedPreferencesKeys.ConstKeys.TRUE_TIME_FAIL, false)){
                    //show warning
                }
                isTrueTime = false;
                trueTimeNow = dateFormat.format(new Date(System.currentTimeMillis()));
            }

            CurrencyRoot data = loadCurrenciesFromLocalStorage();
            UserTempData user = loadUserDataFromLocalStorage();

            if(data != null && (user == null ||
                    user.getLastFirstCurrency() == null ||
                    user.getLastSecondCurrency() == null)){
                user = new UserTempData(data.getExchangeRate().get(0), data.getExchangeRate().get(1));
            }

            if(data == null){//first start
                loadDataFromAPI();
            }else if(!data.getDate().equals(trueTimeNow)){//just old data
                this.userTempData = user;
                this.currencyRoot = data;
                isOldData = true;
                loadDataFromAPI();
            }else {//data is exists and new ones
                openMainFragment(data, user);
            }

        });

    }

//...

@Ch-Tima

[Ch-Tima]

Currently, this bug has only been fixed in the Play Store version of the app.
The new version 1.4 will be available soon.