Java8提取两个String的公共字符集

没找到轮子，简单写一下

// 方法1：
private List getDuplicateCharacters(String checkWord, String key) {
        List characters = new ArrayList<>();
        Arrays.stream(key.split("")).forEach(character -> {
            if (checkWord.contains(character)) {
                characters.add(character);
            }
        });
        return characters;
    }

// 方法2：
private Pattern BLANK_SPLIT_PATTERN = Pattern.compile("");

private List getDuplicateCharacters(String checkWord, String key) {
        if (Strings.isNullOrEmpty(checkWord) || Strings.isNullOrEmpty(key)) {
           return new ArrayList<>();
        }
        return BLANK_SPLIT_PATTERN.splitAsStream(key).filter(character -> checkWord.contains(character)).collect(
              Collectors.toList());
    }