How can I mock a method that has a parameter with a private constructor?

I have an abstract cache client with an implementation that I'm trying to add unit tests to, and it has a protected class implementation of the key. Like this:

public abstract class SimpleCacheClient<V extends Serializable> {    // Autowired RedissonClient and RedisKeyGenerator    public V get(SimpleCacheKey key) {        // return the cache entry from the autowired RedissonClient    }    public void set(SimpleCacheKey key, V value) {        // set the cache entry    }    public SimpleCacheKey getCacheKey(Object...keys) {        return new SimpleCacheKey(keyGenerator.generateKey(keys));    }    /**    * Simple wrapper for cache key to guarantee that implementations    * are using the key generator provided in this class    */    protected class SimpleCacheKey {        private String key;        SimpleCacheKey(String key) {            this.key = key;        }        public String getKey() {            return key;        }        @Override        public String toString() {            return getKey();        }    }}

And here's the implementation I'm trying to test:

public class CacheClientImplementation extends SimpleCacheClient<ArrayList<DateTime>> {    public void addEventDateTimes(String key, ArrayList<DateTime> eventDateTimes) {        // Do stuff with eventDateTimes and then        set(getCacheKey(key), eventDateTimes);    }    public ArrayList<DateTime> getEventDateTimes(String key) {        ArrayList<DateTime> eventDateTimes = get(getCacheKey(key));        // Do stuff with eventDateTimes.        return eventDateTimes;    }}

I'm trying to test to make sure that CacheClientImplementation performs certain operations on the values provided to it before setting and getting.I'm trying to mock the redis cache itself by hijacking the get() and set() methods to read and write from/to a HashMap so that I can check the contents of the "cache" in my tests.

@RunWith(MockitoJUnitRunner.class)public class CacheClientImplementationTest{    @Mock    private RedissonClient redissonClient;    @Mock    private RedisKeyGenerator redisKeyGenerator;    @Spy    @InjectMocks    private CacheClientImplementation cacheClient = new CacheClientImplementation();    private final HashMap<String, ArrayList<DateTime>> cacheMap = new HashMap<>();    @Before    public void setup() {        Mockito.doAnswer((ver) -> {            cacheMap.put(ver.getArgumentAt(0, Object.class).toString(), ver.getArgumentAt(1, ArrayList.class));            return null;        }).when(cacheClient).set(Mockito.any(), Mockito.any(ArrayList.class));        Mockito.doAnswer((ver) -> cacheMap.getOrDefault(ver.getArgumentAt(0, Object.class).toString(), null))            .when(cacheClient).get(Mockito.any());    }    @After    public void teardown() {        cacheMap.clear();    }}

However, I end up with this problem when I run a test in the file.

C:\...\CacheClientImplementationTest.java:20: error: SimpleCacheClient.SimpleCacheKey has protected access in SimpleCacheClient        }).when(cacheClient).set(Mockito.any(), Mockito.any(ArrayList.class));

Is there any way I can doAnswer for these methods without changing SimpleCacheKey?

Thanks!