public DateTimeFormatterBuilder appendOptional (DateTimeFormatter formatter)

Appends a formatter to the builder which will optionally format/parse.

This method has the same effect as appending each of the constituent parts directly to this builder surrounded by an optionalStart() and optionalEnd().

The formatter will format if data is available for all the fields contained within it. The formatter will parse if the string matches, otherwise no error is returned.

formatter    the formatter to add, not null

Returns:  this, for chaining, not null