How to mock a parameter which is not passed to constructor but derived?

Question

I am trying to unit test my redis map cache, the class has the following implementation:

public class ProductCacheTemplate extends CacheTemplate<Integer, Product> {

private RMapReactive<Integer, Product> map;

public ProductCacheTemplate(RedissonReactiveClient client) {
    this.map = client.getMap("product", new TypedJsonJacksonCodec(Integer.class, Product.class));
}

@Override
protected Mono<Product> getFromCache(Integer id) {
    return this.map.get(id);
}



@Override
protected Mono<Product> updateCache(Integer id, Product product) {
    return this.map.fastPut(id, product).thenReturn(product);
}

@Override
protected Mono<Void> deleteFromCache(Integer id) {
    return this.map.fastRemove(id).then();
}
}

I am wondering how do I mock the RMapReactive<Integer, Product> map in this case? I have tried the following approach but it doesn't work. I still get NullPointerException when trying to get from cache.

@ExtendWith(MockitoExtension.class) class ProductCacheTemplateTest {

private ProductCacheTemplate productCache;

@Mock
private RedissonReactiveClient client;

@BeforeEach
public void setUp() {
    this.client = Mockito.mock(RedissonReactiveClient.class);
    this.redisCacheProperties = getRedisCacheProperties();
    when(this.client.getMapCache(redisCacheProperties.getBadgeCache().getName(),
                                 new TypedJsonJacksonCodec(Integer.class, Product.class)))
            .thenAnswer(
                    invocation -> {
                        Integer key = invocation.getArgument(0);
                        if (key.equals(TEST_KEY)) {
                            return Mono.empty();
                        }
                        return null;
                    });
    this.productCache = Mockito.spy(new ProductCache(client));
}

@Test
void testGetFromCache() {
    Mono<Product> result = productCache.get(TEST_KEY);
    StepVerifier.create(result)
            .verifyComplete();
}

@Test
void putIntoCache() {
    Mono<Product> result = productCache.put(TEST_KEY, TEST_VALUE);
    StepVerifier.create(result)
            .verifyComplete();
}

@Test
void deleteFromCache() {
    Mono<Void> result = productCache.delete(TEST_KEY);
    StepVerifier.create(result)
            .verifyComplete();
}

private static final Integer TEST_KEY = 1;
private static final Product TEST_VALUE = new Product();
}