In Java, I want to print just the time of day in hours and minutes and want it to correctly switch between, e.g., "13:00" and "1:00 PM" according to the locale. How do I do this?
The locale doesn't explicitly specify whether 12 or 24 hour time formats are preferred. Rather, locale specific date formats are handled by the locale defining the formats directly.
If you simply want to use the "locale preferred" time format, just call one of the three DateFormat.getTimeInstance(...) static methods, and use whatever DateFormat it returns.
If you have a SimpleDateFormat instance in your hands, you could (if you were prepared to do a significant amount of coding) call toPattern() and parse the resulting pattern to see if it used a 12 or 24 hour dates ... or neither. You could even tweak the pattern to use the "other" form and then call applyPattern(String) to alter the format.