The builtin Java classes don't offer this, but ICU's TimeZone class does, and TimeZone.getAvailableIDs("US") provides the correct answer.

A Locale is not a TimeZone, and vice versa. Check the Javadoc for the method you're using - the very first line says

Gets a calendar using the default time zone and specified locale.

That's why you're getting the default timezone - since you didn't specify one when obtaining a Calendar.

Think about what Jon said - if you know what timezone you would want to use in the situation where you've worked out a user is from the US, then you can call the Calendar.getInstance method that takes a timezone and a locale. On the other hand, if you can't say definitely what you would do here, then go back to the drawing board and think about your requirements a little more, rather than looking at your implementation.

If you can't answer the previous question, I think the standard recourse of most websites is to allow users to specify their preferred timezone (if they have a persistent account on the server) and default them to the server's timezone if they haven't said otherwise. If they don't have persistent accounts, and they're supplying information to you with times in (e.g. an XML upload), then they will have to either specify what time zone they're using in the request, or (probably better) you mandate the use of UTC for all times.

I think the best alternative is to get these IANA's files:

• https://github.com/eggert/tz/blob/main/zone.tab
• https://github.com/eggert/tz/blob/main/zone1970.tab

Those files have entries in the format below:

#codes  coordinates TZ  comments
AD      +4230+00131 Europe/Andorra
AE,OM   +2518+05518 Asia/Dubai
AF      +3431+06912 Asia/Kabul

If I'm not wrong, zone1970.tab is newer and covers cases where a timezone is used by more than one country, but you can check both and see which one solves your problem.

And there's also the iso3166.tab file that maps countries codes to their respective names.

I'm not sure if Java has an API that directly reads those files, but looking at their formats, it seems to be straighforward to read them and find the info you want.

If you want your JVM's default timezone as a ZoneId object, just call ZoneId.systemDefault(). If you want the zone's ID as a String (such as Europe/Berlin or America/New_York), just call ZoneId.systemDefault().getId().

If I understand your question, you could try something like this -

String[] tzIds = TimeZone.getAvailableIDs();
List<String> al = new ArrayList<String>();
for (String timeZoneId : tzIds) {
  if (timeZoneId.startsWith("Canada")) {
    al.add(timeZoneId);
  }
}
System.out.println(al);

The output (slightly re-formatted) here is

[Canada/Pacific, Canada/Yukon, Canada/Mountain, Canada/Central, 
 Canada/East-Saskatchewan, Canada/Saskatchewan, Canada/Eastern, 
 Canada/Atlantic, Canada/Newfoundland]

For a solution covering all thinkable city names this will require a database containing cities and their corresponding time zones. It will be further complicated by the fact that cities with the same name exist, so you may have ambiguous input. For example, by Paris I suppose you intended the capital and largest city of France, Europe, but towns called Paris exist in other places too. I don’t know if a suitable database exists, you may search.

I can get you close, though, with what is built into Java. Time zones have IDs in the form region/city, for example Australia/Sydney and Asia/Dubai. The city used in naming the time zone is the largest populated area of the time zone, so even in the case where a country or state is only one time zone, the city needs not be the capital. But if the city coincides, we can find the zone.

    Set<String> zids = ZoneId.getAvailableZoneIds();

    String[] cityNames = { "Abu Dhabi", "Dubai", "Sydney", "Dhaka", "Paris", "Indianapolis", "São Tomé" };
    for (String cityName : cityNames) {
        String tzCityName = Normalizer.normalize(cityName, Normalizer.Form.NFKD)
                .replaceAll("[^\\p{ASCII}-_ ]", "")
                .replace(' ', '_');
        List<String> possibleTimeZones = zids.stream()
                .filter(zid -> zid.endsWith("/" + tzCityName))
                .collect(Collectors.toList());
        System.out.format("%-12s %s%n", cityName, possibleTimeZones);
    }

The output from this snippet is:

Abu Dhabi    []
Dubai        [Asia/Dubai]
Sydney       [Australia/Sydney]
Dhaka        [Asia/Dhaka]
Paris        [Europe/Paris]
Indianapolis [America/Indianapolis, America/Indiana/Indianapolis]
São Tomé     [Africa/Sao_Tome]

You will notice, though, that it didn’t find any time zone for Abu Dhabi because although the capital of the United Arab Emirates, it is not the largest city; Dubai is. You will notice too that two time zones were found for Indianapolis. The former is just an alias for the latter, though.

The city names used in the time zone database are the English names (when they exist) stripped of any accents. When a name is in two or three words, they are separated by underscores rather than spaces. So São Tomé becomes Sao_Tome. Therefore in the code I am performing this conversion. The way to strip off the accents was taken from another Stack Overflow answer, link below.

Links

• List of tz database time zones on Wikipedia
• Answer by Erick Robertson to Is there a way to get rid of accents and convert a whole string to regular letters?

I think ICU4J package will help you.