Basically, I am using the following code to parse string as LocalDateTime, which works fine most of the time.

DateTimeFormatter dtformatter = DateTimeFormatter.ofPattern("yyyyMMddHHmmssSSS");

However, I encounter cases where the seconds and millseconds are 00000 and this is when the parser fails and print a LocalDateTime 2018-03-01T09:16 instead of 2018-03-01T09:16:00.000.

System.out.println(LocalDateTime.parse("20180301091600000",dtformatter));

(Note that in my code, I have to parse string as LocalDateTime, do some comparison and then at the end, print LocalDateTime to csv)

How can I fix it to make it print 2018-03-01T09:16:00.000 instead of 2018-03-01T09:16 ?

FYI, I am using jdk10.

I'm not sure why it's not work, it seems it is a bug because when I use :

20180301091600001        result is      2018-03-01T09:16:00.001
----------------^                       -----------------^^^^^^

Also another test :

2018030100000000         result is      2018-03-01T00:00
--------^^^-----                        -----------^^^^^^^^^^^

It seems that the parser ignore the seconds and millisecond when it is zero, why?

The full explanation why?, is in the answer of Basil Bourque.

Solution

Here is a quick fix where you can use another formatter like this :

var result = LocalDateTime.parse("20180301091600000", dtformatter)
                .format(DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss:SSS"));

Output

2018-03-01T09:16:00:000