### Question:
StripeApiHandler { @VisibleForTesting static String getAddCustomerSourceUrl(@NonNull String customerId) { return String.format(Locale.ENGLISH, "%s/%s", getRetrieveCustomerUrl(customerId), SOURCES); }    }### Answer:
@Test public void testGetAddCustomerSourceUrl() { String customerId = "cus_123abc"; String addSourceUrl = StripeApiHandler.getAddCustomerSourceUrl(customerId); assertEquals("https: addSourceUrl); }