Difference between year-of-era and week-based-year

Question

Java 8's DateTimeFormatter class has a method, ofPattern(String pattern), that lets you define a format from a string of A-z, a-z letters. The examples don't clarify the difference between y, year-of-era and Y, week-based-year. What is it?

Symbol  Meaning                     Presentation      Examples
------  -------                     ------------      -------
 y       year-of-era                 year              2004; 04
 Y       week-based-year             year              1996; 96

Answer 1

That's year value for "year-week" style dates, as in 2006W52. It may be off the year-of-era value by +1 or -1 if the week in question straddles year boundary.

See http://en.wikipedia.org/wiki/ISO_8601#Week_dates

Answer 2

Each field is documented in a "field" class, such as ChronoField, WeekFields or IsoFields.

The "year-of-era" field is documented in ChronoField.

The "week-based-year" field is documented in WeekFields.