JapaneseDate.get(DAY_OF_YEAR) returns the number of days from the beginning of the Gregorian year, not the number of days from the beginning of the Era (for YearOfEra1).
However, when fetching the DAY_OF_YEAR from a JapaneseDate for the start of an Era, an exception can be thrown because the dayOfYear is larger than the actualRange computed for that first year.

BTW, there are no specific tests for JapaneseDate.get(DAY_OF_YEAR).