How to get Offset from a Time Zone of a country which uses different time zone like EEST and EET

I want to get the offset of a time zone from a country where a device is in. Let's take an example of a device which is in Finland, Finland is currently using EEST, but for some reason my device has stored the
time zone as EET. Now we know EEST and EET have different offset's (+3 and +2 respectively). So I want a java code which will give me the correct offset by taking the time zone in EET, determining if the current time zone used in this country is actually EET or EEST (or determine if DST is active or no), and returning me the correct offset.

Note: EEST and EET is just an example I have used, I would like the code to work with any country which uses more than one Time zone at different time of the year.

I cannot use Continent/city format since the Only information I have from the device is EET timezone.

tl;dr

> determine if DST is active or no

ZoneId
.of( "Europe/Helsinki" ) 
.getRules()
.isDaylightSavings(
    Instant.now()
)

Details

EEST and EET are not time zones. They merely give an ambiguous hint as to what time zone might be intended, with an indication of whether Daylight Saving Time (DST) is in effect. These pseudo-zones are not standardized, and are not even unique/distinct. These should be used only when localizing for presentation to a user. These should never be used for data storage, data exchange, or business logic.

You said:

> Now we know EEST and EET have different offset's (+3 and +2 respectively)

No, we do not know that. There is no official/standard definition for the offsets implied by EEST and EET. The time zones implied by EEST and EET may have different offsets on some dates as their politicians may start/stop DST on different dates. Or the politicians may introduce other anomalies beside DST; politicians are unpredictable.

You said:

>I want to get the offset of a time zone from a country where a device is in.

As for determining time zone by location, that is presumptuous. The only time zone that matters for presentation is the user’s expected/desired time zone. You can guess what that might be. But where critical, you must confirm with the user explicitly.

You said:

>Calendar calendar = …

You are using terribly flawed date-time classes that were years ago supplanted by the modern java.time classes defined in JSR 310.

You said:

> determining if the current time zone used in this country is actually EET or EEST (or determine if DST is active or no)

First, determine a real time zone. Real time zones have a name in format of Continent/Region. For example, Africa/Casablanca or Pacific/Auckland. For Finland, the only time zone is Europe/Helsinki.

ZoneId z = ZoneId.of( "Europe/Helsinki" ) ;

Get the rules defined for that zone.

ZoneRules rules = z.getRules() ;

Capture the current moment as seen with an offset from UTC of zero hours-minutes-seconds.

Instant now = Instant.now() ;

Interrogate the rules for that moment, asking if DST is in effect.

boolean isDST = rules.isDaylightSavings( now ) ;

You said:

> I cannot use Continent/city format since the Only information I have from the device is EET timezone.

Then you have incomplete information, and your task is impossible.

This is like asking for the current money exchange rate for “dollar” but you don’t know if that is the Australia dollar, the Canada dollar, the Hong Kong dollar, or the United States dollar.

Here is tutorial in reference to TimeZone offsets.
Time Zone and Offset Classes (The Java™ Tutorials > Date Time > Standard Calendar).

To get the UTC offset you can use the TimeZone class.
To create a new TimeZone object, you can use the getTimeZone method.

For this example, I'll use EET, or Eastern European Time.

TimeZone timeZone = TimeZone.getTimeZone("EET");

The TimeZone class offers the getOffset method, which takes either a set of values, or a Unix-epoch timestamp as a parameter.
We can use a Calendar object to obtain a time value for the UTC, and a timestamp using the getTimeInMillis method.

Calendar calendar = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
long offset = timeZone.getOffset(calendar.getTimeInMillis());

If we printed the offset we'd get the following.

10800000

Which is equivalent to 3 hours, or 3 × 60 × 60 × 1000.

In terms of DST, or Daylight Saving Time, you can utilize the following methods: useDaylightTime, and getDSTSavings,

The first method will return true if the time-zone observes daylight savings, while the latter will return a value in milliseconds to be negated from the UTC offset.

Daylight Saving Time occurs at different times for different areas.

You can use a Date object, obtainable via a Calendar object, to query whether the timestamp is currently observing DST or not.

TimeZone timeZone = TimeZone.getTimeZone("EET");
Calendar calendar = Calendar.getInstance(timeZone);
boolean dst = timeZone.inDaylightTime(calendar.getTime());