Java使用lambda表達式刪除集合元素中為空的值並轉為String存儲

    public String getString(List<Integer> list) {
        if (list == null || list.isEmpty()) {
            return null;
        }
        list.removeIf(e -> isNotValid(e));
        return list.stream().map(String::valueOf).collect(Collectors.joining(","));
    }

    private static Boolean isNotValid(Integer in) {
        if (in == null) {//為空的為無效數字
            return true;
        }
        return false;
    }
//輸入：[1,2,3,null,5,null,7]
//輸出："1,2,3,5,7"