java - Importance of order when using multiple optional patterns -

how order of optional patterns in datetimeformatter affect parsing operation?

i running program , wondered why last line throws exception not first three.

public static void main(string[] args) {   string p1 = "[eeee][e] dd-mm-yyyy";   string p2 = "[e][eeee] dd-mm-yyyy";   string date1 = "thu 07-01-2016";   string date2 = "thursday 07-01-2016";   parse(date1, p1); //ok   parse(date1, p2); //ok   parse(date2, p1); //ok   parse(date2, p2); //exception }  private static void parse(string date, string pattern) {   datetimeformatter fmt = datetimeformatter.ofpattern(pattern, locale.english);   system.out.println(fmt.parse(date)); } 

the exception on last line is:

java.time.format.datetimeparseexception: text 'thursday 07-01-2016' not parsed @ index 3

the documentation not mention precedence , i'll argue result getting normal. result of reading string format left right.

• let's consider first format "[eeee][e] dd-mm-yyyy".

  • "thu 07-01-2016": api tries find if first optional section "[eeee]" can matched. quoting datetimeformatter javadoc text token

    exactly 4 pattern letters use full form.

    which in case full form of day of week. doesn't match "thu" optional section skipped. second optional section, however, "[e]", , still quoting

    less 4 pattern letters use short form.

    so match "thu". string parse can understood correctly

  • "thursday 07-01-2016": same above, except match on first optional section "thursday". api still continue search valid section next optional, "[e]" , won't find optional section skipped.
• let's consider second format "[e][eeee] dd-mm-yyyy".
  • "thu 07-01-2016": api tries find if first optional section "[e]" can matched , work "thu". above, api try find match "[eeee]" won't find optional section skipped.
  • "thursday 07-01-2016": api tries match "[e]" again , that's thing happens: match. "thursday" starts "thu" formatter able find match. then, tries parse rest "rsday 07-01-2016". [eeee] optional section won't matched skipped. fails space because there no space on what's left (there's "r" instead).

so if run code

parse("thuthursday 07-01-2016", "[e][eeee] dd-mm-yyyy"); 

you'll see works: "[e]" matched "thu" , "[eeee]" matched "thursday".

notice how exception message hints @ (emphasis mine):

java.time.format.datetimeparseexception: text 'thursday 07-01-2016' not parsed at index 3

index 3 corresponds "r" of "rsday" means able parse, right point.