### Question:
OAuthClientManager implements ClientDetailsService, IOAuthClientManager { @Override public void deleteClient(String clientId) { if(clientId != null) { clientRepo.deleteById(clientId); } } OAuthClientManager(OAuthClientRepository repo, BCryptPasswordEncoder bCryptPasswordEncoder, int accessTokenValidity); @Override ClientDetails loadClientByClientId(String clientId); @Override OAuthCredentials create(String name, String description, List<OAuthScope> scopes, Set<String> grantTypes, String redirectUrl); @Override OAuthClientResultPage getAllClientDetails(Pageable pageable); @Override void deleteClient(String clientId); @Override OAuthCredentials updateClientSecret(String clientId); @Override List<OAuthClient> getClientsDetails(List<String> clientList);  }### Answer:
@Test public void test_deleteClient() { String clientId = "clientId"; managerToTest.deleteClient(clientId); Mockito.verify(clientRepo).deleteById(clientId); }
@Test public void test_deleteClient_nullId() { String clientId = null; managerToTest.deleteClient(clientId); Mockito.verify(clientRepo, Mockito.never()).deleteById(clientId); }