java8 lanbda表達式對list對象去重，對象屬性轉成string

需求：對ulist中的mobilTel去重合並成逗號分隔的String，以下三行代碼含義

參數類型說明： List<User> ulist，String userinfo

1 因為部分用戶的mobileTel為空，則先去除空的數據
2 對非空的list進行去重
3 對特定屬性進行逗號分隔

ulist = ulist.stream().filter(c -> c.getMobilTel() != null).collect(Collectors.toList());
ulist = ulist.stream().collect(Collectors.collectingAndThen(
        Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(User::getMobilTel))), ArrayList::new));
userInfo = ulist.stream().map(c -> c.getMobilTel()).collect(Collectors.joining(","));

　頭兩行代碼可以合並

  ulist = ulist.stream().filter(c -> c.getMobilTel() != null).collect(Collectors.collectingAndThen(
                    Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(User::getMobilTel))), ArrayList::new));
  userInfo = ulist.stream().map(c -> c.getMobilTel()).collect(Collectors.joining(","));