DateTimeFormatterBuilder fail to chose format

Question

I've configured formatter:

public static final DateTimeFormatter DATE_FORMATTER = new DateTimeFormatterBuilder()
        .append(forPattern("yyyy-MM-dd"))
        .append(forPattern("MM/dd/yy"))
        .append(forPattern("MMM dd, yyyy"))
        .toFormatter();

and trying to parse a string 2017-08-29

LocalDate.parse(dt, DATE_FORMATTER).toDateTimeAtStartOfDay().toLocalDateTime()

I'm getting error:

IllegalArgumentException: Invalid format: "2017-08-29" is too short

Error gone if I leave "yyyy-MM-dd" the only format in the builder.

Am I misusing the API? I would like the parser to try another format if it fails with first one.

Answer 1

When you use the append method, you're creating a formatter that accepts all the three patterns, one after the another (all the three are required).

If you want to accept any of the three formats (just one of them), you must use appendOptional instead:

DateTimeFormatter DATE_FORMATTER = new DateTimeFormatterBuilder()
    .appendOptional(DateTimeFormat.forPattern("yyyy-MM-dd").getParser())
    .appendOptional(DateTimeFormat.forPattern("MM/dd/yy").getParser())
    .appendOptional(DateTimeFormat.forPattern("MMM dd, yyyy").getParser())
    .toFormatter();

Now you can parse any of the three formats:

System.out.println(LocalDate.parse("2017-08-29", DATE_FORMATTER).toDateTimeAtStartOfDay().toLocalDateTime());
System.out.println(LocalDate.parse("08/29/17", DATE_FORMATTER).toDateTimeAtStartOfDay().toLocalDateTime());
System.out.println(LocalDate.parse("Aug 29, 2017", DATE_FORMATTER).toDateTimeAtStartOfDay().toLocalDateTime());

All of the above outputs:

2017-08-29T00:00:00.000

---

Just one note: the third formatter uses the month short name (MMM), and the code above assumes that the system's default locale is English (when you create a formatter, by default it uses the language that corresponds to the system's default locale).

But this can be changed without notice, even at runtime, so it's better to specify a java.util.Locale in your formatter.

Example: if month name is always in English, just use the equivalent locale:

DateTimeFormatter DATE_FORMATTER = new DateTimeFormatterBuilder()
    .appendOptional(DateTimeFormat.forPattern("yyyy-MM-dd").getParser())
    .appendOptional(DateTimeFormat.forPattern("MM/dd/yy").getParser())
    .appendOptional(DateTimeFormat.forPattern("MMM dd, yyyy").getParser())
    // use English locale
    .toFormatter().withLocale(Locale.ENGLISH);

Just change the locale to the one that best fits your needs. Check the javadoc for more details.

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As reminded in the comments, you can also create an array of parsers and use in the DateTimeFormatterBuilder:

// array with all possible patterns
DateTimeParser[] parsers = new DateTimeParser[] {
    DateTimeFormat.forPattern("yyyy-MM-dd").getParser(),
    DateTimeFormat.forPattern("MM/dd/yy").getParser(),
    DateTimeFormat.forPattern("MMM dd, yyyy").getParser() };

DateTimeFormatter DATE_FORMATTER = new DateTimeFormatterBuilder()
    // use array of all possible parsers
    .append(null, parsers)
    // use English locale
    .toFormatter().withLocale(Locale.ENGLISH);

This works the same way as the previous one.

---

Java new Date/Time API

Joda-Time is in maintainance mode and is being replaced by the new APIs, so I don't recommend start a new project with it. Even in joda's website it says: "Note that Joda-Time is considered to be a largely “finished” project. No major enhancements are planned. If using Java SE 8, please migrate to java.time (JSR-310).".

If you can't (or don't want to) migrate from Joda-Time to the new API, you can ignore this section.

If you're using Java 8, consider using the new java.time API. It's easier, less bugged and less error-prone than the old APIs.

If you're using Java <= 7, you can use the ThreeTen Backport, a great backport for Java 8's new date/time classes. And for Android, you'll also need the ThreeTenABP (more on how to use it here).

The code below works for both. The only difference is the package names (in Java 8 is java.time and in ThreeTen Backport (or Android's ThreeTenABP) is org.threeten.bp), but the classes and methods names are the same.

The API is very similar when it comes to create the formatter and parsing it:

DateTimeFormatter DATE_FORMATTER = new DateTimeFormatterBuilder()
    .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd"))
    .appendOptional(DateTimeFormatter.ofPattern("MM/dd/yy"))
    .appendOptional(DateTimeFormatter.ofPattern("MMM dd, yyyy"))
    // use English locale
    .toFormatter(Locale.ENGLISH);

System.out.println(LocalDate.parse("2017-08-29", DATE_FORMATTER).atStartOfDay());
System.out.println(LocalDate.parse("08/29/17", DATE_FORMATTER).atStartOfDay());
System.out.println(LocalDate.parse("Aug 29, 2017", DATE_FORMATTER).atStartOfDay());

All the above create a LocalDateTime with the value that corresponds to 2017-08-29T00:00.

You can also use optional patterns (delimited by []):

DateTimeFormatter DATE_FORMATTER = DateTimeFormatter.ofPattern("[yyyy-MM-dd][MM/dd/yy][MMM dd, yyyy]", Locale.ENGLISH);

This works the same way as the above.