In Java, the commonly used class to handle date and time formatting is DateTimeFormatter. DateTimeFormatter provides rich formatting options and can flexibly meet different needs. This article will introduce how to use DateTimeFormatter to format date and time.

1. Basic usage

The use of DateTimeFormatter is very simple. You can use the ofPattern method to create a DateTimeFormatter instance. The ofPattern method accepts a string parameter indicating the date and time format.

For example, to format the date and time as "yyyy-MM-dd HH:mm:ss", you can use the following code:

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

Next, you can use the format method of DateTimeFormatter Format the date and time into the specified format:

LocalDateTime dateTime = LocalDateTime.now();
String formattedDateTime = formatter.format(dateTime);
System.out.println(formattedDateTime); // 2022-01-01 10:00:00

In the above code, we first obtain the current date and time, and then use the format method of DateTimeFormatter to format it as "yyyy-MM-dd HH: mm:ss" format.

2. Date and time formatting options

DateTimeFormatter supports many date and time formatting options to meet different needs. Here are some common date and time formatting options:

• y: year, such as "2022"
• M: month, such as "01"
• d: Number of days, such as "01"
• H: hour (24-hour clock), such as "10"
• h: hour (12-hour clock), such as "10" or "10 PM"
• m: Number of minutes, such as "00"
• s: Number of seconds, such as "00"
• S: Number of milliseconds, such as "000"
• a: AM/PM mark, such as "AM" or "PM"

For example, to format the date time as "yyyy/MM/dd HH:mm:ss", you can use the following code :

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
LocalDateTime dateTime = LocalDateTime.now();
String formattedDateTime = formatter.format(dateTime);
System.out.println(formattedDateTime); // 2022/01/01 10:00:00

3. Localized date and time formatting

In addition to basic date and time formatting options, DateTimeFormatter also supports localized date and time formatting. Localized formatting formats date and time according to different languages, regions, and cultural conventions. Use the ofLocalizedDateTime method of DateTimeFormatter to create a localized date and time formatter.

For example, to format the date and time into English format, you can use the following code:

DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.FULL).withLocale(Locale.ENGLISH);
LocalDateTime dateTime = LocalDateTime.now();
String formattedDateTime = formatter.format(dateTime);
System.out.println(formattedDateTime); // Saturday, January 1, 2022 10:00:00 AM UTC

In the above code, we created a localized date and time formatter and specified Use English formatting. Use the withLocale method to specify the region and language used.

4. Parsing date and time

In addition to formatting date and time, DateTimeFormatter also supports parsing date and time. Use the parse method of DateTimeFormatter to parse a string format date and time into a LocalDateTime object.

For example, to parse the string "2022-01-01 10:00:00" into a LocalDateTime object, you can use the following code:

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String dateTimeString = "2022-01-01 10:00:00";
LocalDateTime dateTime = LocalDateTime.parse(dateTimeString, formatter);
System.out.println(dateTime); // 2022-01-01T10:00

In the above code, we created a Date time formatter and use the parse method to parse the string "2022-01-01 10:00:00" into a LocalDateTime object.

It should be noted that when using the parse method to parse date and time, the format of the formatter must exactly match the format of the string, otherwise a DateTimeParseException will be thrown.

5. Summary

This article introduces how to use the DateTimeFormatter function in Java for date and time formatting. DateTimeFormatter provides rich formatting options and localization support, making date and time formatting more flexible and convenient. At the same time, DateTimeFormatter also supports parsing date and time, and can convert date and time in string format into LocalDateTime objects. I hope this article can help readers better understand and use DateTimeFormatter.

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