A one-shot function to get a default country for a locale. I was needed this method in order to display a country flag from a language code only (ISO 639-2 & 639-3)

LocaleUtils.java

import androidx.annotation.NonNull;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;
import java.util.Map;

public final class LocaleUtils
{
    private static Map<String, String> sCountryForLocale;

    /**
     * @return a Map listing for each locale (ISO 639-2 & 639-3)
     * the corresponding default country (ISO 3166-1)
     */
    @NonNull
    public static final Map<String, String> countryForLocale()
    {
        if (sCountryForLocale == null)
        {
            sCountryForLocale = new HashMap<String, String>() {{
              // Add country for locale which could not be determined by logic
              // i.e. more than 1 possible country and country code != locale code.
              // Based on ISO 639-2 & 639-3 wikipedia articles:
              // https://en.wikipedia.org/wiki/List_of_ISO_639-2_codes
              // https://en.wikipedia.org/wiki/Wikipedia:WikiProject_Languages/List_of_ISO_639-3_language_codes_(2019)
              // Selection has been done in this order:
              // - Official language in only 1 country
              // - Official language in more than 1 country but spoken widely only in 1 (e.g. Bengali)
              // - Official language in more than 1 country ordered by speakers (e.g. Hausa)
              // - Recognised regional/minority language in only 1 country
              // - Recognised regional/minority language in more than 1 country ordered by speakers (e.g. Low German)
              // - Native country ordered by population (e.g. Alemannic)
              // - Flag likeness (e.g. Arabic spoken by countries of the Arab League)
                put("af", "za"); put("ar", "sa"); put("bn", "bd"); put("bo", "cn"); put("ca", "ad");
                put("ccp", "bd"); put("ckb", "iq"); put("da", "dk"); put("ee", "gh"); put("el", "gr");
                put("en", "gb"); put("fa", "ir"); put("ff", "cm"); put("gsw", "ch"); put("ha", "ng");
                put("ko", "kr"); put("ln", "cd"); put("lrc", "ir"); put("mas", "ke"); put("ms", "my");
                put("nb", "no"); put("nds", "de"); put("ne", "np"); put("nmg", "cm"); put("om", "et");
                put("os", "ru"); put("pa", "in"); put("qu", "bo"); put("se", "no"); put("sq", "al");
                put("sr", "rs"); put("sv", "se"); put("sw", "tz"); put("ta", "lk"); put("teo", "ug");
                put("ti", "er"); put("ur", "pk"); put("ji", "ru"); put("yo", "ng"); put("yue", "hk");
                put("zh", "cn");
            }};

            // For others, do it by logic ;)
            Locale[] availableLocales = Locale.getAvailableLocales();
            String previousLanguage = availableLocales[0].getLanguage();
            List<String> previousCountries = new ArrayList<>();
            // Firstly, we create a clean map of countries by locale
            HashMap<String, List<String>> countriesForLocales = new HashMap<>(availableLocales.length);
            for (Locale locale : availableLocales)
            {
                String language = locale.getLanguage();
                // We ignore already added locales
                if (sCountryForLocale.containsKey(language))
                {
                    continue;
                }

                String country = locale.getCountry();

                if (! language.equals(previousLanguage))
                {
                    // This happens for Esperanto
                    if (! previousCountries.isEmpty())
                    {
                        countriesForLocales.put(previousLanguage, previousCountries);
                    }
                    previousLanguage = language;
                    previousCountries = country.isEmpty()   ? new ArrayList<>()
                                                            : new ArrayList<>(Collections.singleton(country));
                }
                // This happens for base locale (without specified country)
                // or locale with several alphabets (e.g. Serbian)
                else if (! country.isEmpty() && ! previousCountries.contains(country))
                {
                    previousCountries.add(country);
                }
            }
            if (! sCountryForLocale.containsKey(previousLanguage)
                && ! countriesForLocales.containsKey(previousLanguage)
                && ! previousCountries.isEmpty())
            {
                countriesForLocales.put(previousLanguage, previousCountries);
            }

            // Now we populate our prefilled map with our locale-country tuples
            for (Map.Entry<String, List<String>> countriesForLocale : countriesForLocales.entrySet())
            {
                String locale = countriesForLocale.getKey();
                List<String> countries = countriesForLocale.getValue();

                // If only one, we take it
                if (countries.size() == 1)
                {
                    sCountryForLocale.put(locale, countries.get(0).toLowerCase());
                }
                // Else we search for country code == locale code
                else
                {
                    int index = countries.indexOf(locale.toUpperCase());
                    if (index > -1)
                    {
                        sCountryForLocale.put(locale, countries.get(index).toLowerCase());
                    }
                }
            }
        }

        return sCountryForLocale;
    }
}