java.lang.UnsupportedOperationException when testing

Try to write test, operating with mocks. When assertion happens there is an exception.

CardRecordServiceTest.java

@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));
    }

CartRecordService.java

    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;
    }

Log

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)

>Solution :

The problem is that your code attempts to modify a map, but your test code is setup with Map.of(...), which returns an immutable map. Use new HashMap<>(Map.of(....)) instead.