### Question:
BillingApiService { public List<String> getAllBillingCenters() { return billingManager.getAllBillingCenters(); }  List<BillingChartInfo> getBillingChartInfo(final BillingChartRequest request); List<BillingChartInfo> getBillingChartInfoPaginated(final BillingChartRequest request); List<String> getAllBillingCenters();  }### Answer:
@Test public void shouldReturnAllBillingCenters() { when(mockBillingManager.getAllBillingCenters()).thenReturn(billingCenters); assertThat(billingApiService.getAllBillingCenters()).isEqualTo(billingCenters); }