title: java.lang.UnsupportedOperationException when testing
body: <p>Try to write test, operating with mocks. When assertion happens there is an exception.</p>
  <p><strong>CardRecordServiceTest.java</strong></p>
  <pre><code>@ExtendWith(MockitoExtension.class)
  public class CartRecordServiceTest {
      private MockMvc mockMvc;
  
      private ObjectMapper objectMapper = new ObjectMapper();
  
      @Mock
      private CartRecord cartRecords;
      @Mock
      private GoodsService goodsService;
      @Mock
      private GoodsRepository goodsRepository;
  
      CartRecordService cartRecordService;
  
  
      @BeforeEach
      public void setUp(){
          cartRecordService = new CartRecordService(cartRecords, goodsRepository, goodsService);
          mockMvc = MockMvcBuilders
                  .standaloneSetup(cartRecordService)
                  .build();
      }
      @Test
      public void enougthGoodsToAddRecordNew(){
          CartAdditionDTO cartAdditionDTO = new CartAdditionDTO(1L, 15L);
  
          Mockito.when(goodsService.enoughQuantity(cartAdditionDTO)).thenReturn(true);
          Mockito.when(cartRecords.getRecords()).thenReturn(Map.of(2L,2L));
  
          Assertions.assertTrue(cartRecordService.addRecord(cartAdditionDTO));
  
          verify(cartRecords,times(2)).getRecords();
          verify(goodsService).enoughQuantity(any(CartAdditionDTO.class));
      }
  </code></pre>
  <p><strong>CartRecordService.java</strong></p>
  <pre><code>    public boolean addRecord(CartAdditionDTO cartAdditionDTO) {
          if(!goodsService.enoughQuantity(cartAdditionDTO)){
              return false;
          }
          if (cartRecords.getRecords().containsKey(cartAdditionDTO.getId())) {
              Long newQuantity = cartRecords.getRecords().get(cartAdditionDTO.getId()) + cartAdditionDTO.getQuantity();
              if(goodsService.enoughQuantity(new CartAdditionDTO(cartAdditionDTO.getId(), newQuantity))){
                  cartRecords.getRecords().put(cartAdditionDTO.getId(), newQuantity);
                  return true;
              }
              return false;
          }
  // here is an error at put
          cartRecords.getRecords().put(cartAdditionDTO.getId(), cartAdditionDTO.getQuantity());
          return true;
      }
  </code></pre>
  <p><strong>Log</strong></p>
  <p>java.lang.UnsupportedOperationException
  at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:142)
  at java.base/java.util.ImmutableCollections$AbstractImmutableMap.put(ImmutableCollections.java:1072)
  at com.example.store.service.CartRecordService.addRecord(CartRecordService.java:77)
  at com.example.store.ServiceUnitTests.CartRecordServiceTest.enougthGoodsToAddRecordNew(CartRecordServiceTest.java:69)</p>