How to test cache with Mockito

So i have this class which count unique symbols .This method contain cache too.I need to test exactly cahce . I know for this target can use Mockito, however i have some problems with it.

public class UniqueCharacterCounterService {
    private Map<String, Map<Character, Integer>> cache = new HashMap<>();

public Map<Character,Integer> countCharacters(final String inputData) throws IncorrectInputDataException {
    if (inputData == null) {
        throw new IncorrectInputDataException("The method parameter can't be null");
    }
    Map<Character, Integer> value = cache.get(inputData);
    Map<Character, Integer> result = new LinkedHashMap<>();
    if (value != null) {
        return value;
    }
    char symbol;
    Integer counter;
    char inputAsCharArray[] = inputData.toCharArray();
    for (int j = 0; j < inputAsCharArray.length; j++) {
        symbol = inputAsCharArray[j];
        counter = result.get(symbol);
        if (counter != null) {
            result.put(symbol, counter + 1);
        } else {
            result.put(symbol, 1);
        }
    }
    cache.put(inputData, result);
    return result;
}

public void printResult(Map<Character, Integer> input) {
    input.forEach((key, value) -> System.out.println("\"" + key + "\" " + "- " + value));
}

}

Main

> public class Main {
    final static Logger logger = Logger.getLogger(Main.class);

    public static void main(String[] args) throws IncorrectInputDataException {
        try {
            UniqueCharacterCounterService characterCounter = new UniqueCharacterCounterService();
            Map<Character, Integer> result = characterCounter.countCharacters("Hello");
            characterCounter.printResult(result);
            result = characterCounter.countCharacters("world");
            characterCounter.printResult(result);
            result = characterCounter.countCharacters("Hello");
            characterCounter.printResult(result);
        } catch (IncorrectInputDataException e) {
            logger.error(e.getMessage());
        }
    }
}`