How to format time objects in Java into the correct timezone

I want to use the Java Time API to get the central Europen summer time (CEST) and format it correctly. I have the following code:

LocalDateTime localDateTime= LocalDateTime.now();
DateTimeFormatter myFormatObj = DateTimeFormatter.ofPattern("dd-MM-yyyy HH:mm:ss");
localDateTime.format(myFormatObj);
ZoneId europeBerlin = ZoneId.of("Europe/Berlin");
ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, europeBerlin);

The command zonedDateTime.toString()leaves to the following output:

2020-09-27T08:42:33.660+02:00[Europe/Berlin]

But I would like to have an output as specified before in the DateTimeFormatter ("dd-MM-yyyy HH:mm:ss"). I have already formatted the localDateTime into this format and now I just want to get the CEST time. How can I do that? I'd appreciate every comment.

Please note that date-time objects from the Java Time API are immutable. Therefore, whenevery you want to modify an existing date-time instance, a new fresh copy is returned, leaving the old one untouched.

Also, a minor optimization: DateTimeFormatter is thread-safe. Thus, there is no need to construct a new instance every time, since the format is constant. You could declare it at the top like this:

private static final DateTimeFormatter FORMATTER;

static {
    FORMATTER = DateTimeFormatter.ofPattern("dd-MM-yyyy HH:mm:ss");
}

To print a formatted String, use the following:

LocalDateTime localDateTime = LocalDateTime.now();
ZoneId europeBerlin = ZoneId.of("Europe/Berlin");
ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, europeBerlin);

// please note that #format has a return value
// originally, you called 'localDateTime.format(myFormatObj);' while ignoring
// the return value
String formatted = FORMATTER.format(zonedDateTime);

System.out.println(formatted); // e.g. 27-09-2020 11:44:27

Edit 1: Regarding thread-safety
Thread-safety refers to when an object can safely be used by mutliple threads concurrently, without breaking internals of the class. If a class is thread-safe, you can call it from mutliple threads at the same time (therefore you do not need to create a new instance every time, but only one). If a class is not thread-safe, a new instance is needed for each thread.

	DateTimeFormatter myFormatObj = DateTimeFormatter.ofPattern("dd-MM-yyyy HH:mm:ss");
	ZoneId europeBerlin = ZoneId.of("Europe/Berlin");
	ZonedDateTime zonedDateTime = ZonedDateTime.now(europeBerlin);
	
	String formattedDateTime = zonedDateTime.format(myFormatObj);
	System.out.println(formattedDateTime);

Output when running in UTC time zone just now:

> 27-09-2020 20:33:53

We got the Berlin time (not the UTC time).

What went wrong in your code?

Two things:

1. LocalDateTime.now() gives you the current time in the default time zone of the JVM. It seems that this was not Eurpoe/Berlin (perhaps it was UTC, it could be something else). Then ZonedDateTime.of(localDateTime, europeBerlin) takes that date and time of day and claims that it is Europe/Berlin time zone, which is wrong and the reason why you got an incorrect result. You don’t often need the LocalDateTime class and virtually never the no-arg LocalDateTime.now() method.
2. To get the time in a specific format you need to format the date and time into a string. The LocalDateTime and the ZonedDateTime objects haven’t got any format.

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