How to get locale specific Date/Time patterns in Java in a future-proof way-CodePudding

TL;DR: How do I get from having locale and SHORT/MEDIUM/LONG etc to the pattern String to parse a date.

Full version:

Accessing the pattern of a locale-specific date format seems to be problem not well covered in Java.

This is in the context of

the JDK8 DateTime API not providing access, and
the classic SimpleDateFormat not looking future-proof enough.

I'm bringing this question back due to the JDK-specificity of the first, and the implementation-specific-ness of the second question, this time to be answered in a non-version-specific way, long after 2017 (the date of the first question):

Use case:

On the user interface, show the date format that a date will be parsed with, when entered: E.g. For Locale.US display start date (M/d/yy), for Locale.GERMANY show Startdatum (dd.MM.yy) next to an input (or, in HTML, as a placeholder).

This would be trivial to achieve - as long as it still works - with

DateFormat usFormat = DateFormat.getDateInstance(DateFormat.SHORT, Locale.US);
DateFormat deFormat = DateFormat.getDateInstance(DateFormat.SHORT, Locale.GERMANY);
System.out.println(((SimpleDateFormat) usFormat).toPattern()); // M/d/yy
System.out.println(((SimpleDateFormat) deFormat).toPattern()); // dd.MM.yy

but this code involves the old API and an implementation specific typecast - both are assumptions that I'm not too confident using.

Maintaining my own library of locale-specific patterns seems even less advisable, but with the DateTime API not granting any access to its internal patterns (they must be there):

Is there a way to solve this problem in a future-proof way?

Due to the linked questions above, this likely involves a specific minimal Java version, and that's fine. I'm currently still bound to be 8 and 11 compatible, but this could either push the version further, or provide an alternative future proof implementation for instances running under newer Java versions.

CodePudding user response：

You can use the DateTimeFormatterBuilder to get the format string:

String usFormat = DateTimeFormatterBuilder.getLocalizedDateTimePattern(FormatStyle.SHORT, null, IsoChronology.INSTANCE, Locale.US);
String deFormat = DateTimeFormatterBuilder.getLocalizedDateTimePattern(FormatStyle.SHORT, null, IsoChronology.INSTANCE, Locale.GERMANY);
System.out.println(usFormat); // M/d/yy
System.out.println(deFormat); // dd.MM.yy