Java DateTimeFormatterBuilder with optional pattern results in DateTimeParseException

Question

Goal

Provide a flexible parser for LocalDate instances that can handle input in one of the following formats:

• yyyy
• yyyyMM
• yyyyMMdd<

Answer 1

Here is the solution. You can define possible patterns inside appendPattern(). And to optional put defaults.

   DateTimeFormatter parser = new DateTimeFormatterBuilder()
            .appendPattern("[yyyy][yyyyMM][yyyyMMdd]")
            .optionalStart()
              .parseDefaulting(ChronoField.MONTH_OF_YEAR, 1)
              .parseDefaulting(ChronoField.DAY_OF_MONTH, 1)
            .optionalEnd()
            .toFormatter();
    System.out.println(parser.parse("2014",LocalDate::from)); // Works
    System.out.println(parser.parse("201411",LocalDate::from)); // Works
    System.out.println(parser.parse("20141102",LocalDate::from)); // Works

The output is

2014-01-01
2014-11-01
2014-11-02