DateTimeFormatter ​

When using the old Date object, we relied on SimpleDateFormat for formatting and displaying dates. With the new LocalDateTime or ZonedDateTime, we use DateTimeFormatter for formatting.

Unlike SimpleDateFormat, DateTimeFormatter is an immutable object and is thread-safe. The concept of threads will be discussed later, but for now, remember that since SimpleDateFormat is not thread-safe, you must create a new local variable each time you use it. In contrast, you can create a single instance of DateTimeFormatter and use it throughout your code.

You create a DateTimeFormatter by passing a formatting string:

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm");

The usage of the formatting string is identical to that of SimpleDateFormat.

Another way to create a DateTimeFormatter is by passing a formatting string while specifying a Locale:

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("E, yyyy-MMMM-dd HH:mm", Locale.US);

This approach formats according to the default habits of the specified locale. Here’s an example:

import java.time.*;
import java.time.format.*;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        ZonedDateTime zdt = ZonedDateTime.now();
        var formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm ZZZZ");
        System.out.println(formatter.format(zdt));

        var zhFormatter = DateTimeFormatter.ofPattern("yyyy MMM dd EE HH:mm", Locale.CHINA);
        System.out.println(zhFormatter.format(zdt));

        var usFormatter = DateTimeFormatter.ofPattern("E, MMMM/dd/yyyy HH:mm", Locale.US);
        System.out.println(usFormatter.format(zdt));
    }
}

In the formatting string, if you need to output fixed characters, you can use 'xxx' to denote them.

When you run the above code, you will see the current time displayed in different formats based on the locale:

2019-09-15T23:16 GMT+08:00
2019 9月 15 周日 23:16
Sun, September/15/2019 23:16

When you directly call System.out.println() on a ZonedDateTime or LocalDateTime instance, it actually invokes their toString() method, which formats the output according to ISO 8601 by default. You can also use the pre-defined static variables of DateTimeFormatter for this purpose:

var ldt = LocalDateTime.now();
System.out.println(DateTimeFormatter.ISO_DATE.format(ldt));
System.out.println(DateTimeFormatter.ISO_DATE_TIME.format(ldt));

The output will be similar to that of toString():

2019-09-15
2019-09-15T23:16:51.56217

Summary ​

To format ZonedDateTime or LocalDateTime, you need to use the DateTimeFormatter class.

DateTimeFormatter allows for custom output of dates and times using formatting strings and Locale.