code: @NotNull
      public String getStoreSku(@NotNull String appstoreName, @NotNull String sku) {
          if (TextUtils.isEmpty(appstoreName)) {
              throw SkuMappingException.newInstance(SkuMappingException.REASON_STORE_NAME);
          }
          if (TextUtils.isEmpty(sku)) {
              throw SkuMappingException.newInstance(SkuMappingException.REASON_SKU);
          }
  
          Map<String, String> storeSku = sku2storeSkuMappings.get(appstoreName);
          if (storeSku != null && storeSku.containsKey(sku)) {
              final String s = storeSku.get(sku);
              Logger.d("getStoreSku() using mapping for sku: ", sku, " -> ", s);
              return s;
          }
          return sku;
      }
docs: Returns a store-specific SKU by the base internal SKU.
  
  @param appstoreName The name of an app store.
  @param sku          The internal SKU.
  @return store-specific SKU by a base internal one.
  @throws java.lang.IllegalArgumentException When appstoreName or sku param is empty or null value.
  @see #mapSku(String, String, String)