在Java 8中格式化LocalDateTime时，默认的DateTimeFormatter返回空字符串

Question

The following code returns an empty string:

LocalDateTime.parse("2018-01-01T00:00:00.00")
    .format(new DateTimeFormatterBuilder().toFormatter())

I'd expect either an exception or the date formatted in some default way.

Have I found a bug in Java or this is as per specification? I cannot find any information in Javadoc about this behaviour.

Answer 1

DateTimeFormatterBuilder is for building a format, it starts off empty. You have to call its various methods such as appendPattern to add the format you want.

DateTimeFormatter has some standard formats you can use straight off. These use DateTimeFormatterBuilder to build the format. For example:

public static final DateTimeFormatter ISO_LOCAL_DATE;
static {
    ISO_LOCAL_DATE = new DateTimeFormatterBuilder()
            .appendValue(YEAR, 4, 10, SignStyle.EXCEEDS_PAD)
            .appendLiteral('-')
            .appendValue(MONTH_OF_YEAR, 2)
            .appendLiteral('-')
            .appendValue(DAY_OF_MONTH, 2)
            .toFormatter(ResolverStyle.STRICT, IsoChronology.INSTANCE);
}

Answer 2

Your code is equivalent to:

LocalDateTime.parse("2018-01-01T00:00:00.00").format(DateTimeFormatter.ofPattern(""))

that produces an empty string in all cases.

Answer 3

I'd expect either exception or the date formatted in some default way

That's your opinion. But IMO, creating an empty builder and add nothing to it should result in a - guess what - "empty" formatter - or a formatter with an empty pattern, or whatever.

The documentation doesn't explicity say this will happen, but it also doesn't say it'll throw an exception or format in some default way . And if the docs don't say that, so why on earth would you expect that? Based on what?

But the worst thing is thinking that, just because your based-on-nothing expectations weren't satisfied, then it's obviously a bug! Why developers - specially the crappy ones - tend to think "wow, I found a bug" when things doesn't go as expected, when most of the time it's their fault?

And why are you creating an empty pattern in the first place?

"OMG, things didn't happen as I expected, so certainly it's a bug!"

-- crappy developer