How to Use DateTimeFormatter in Java: Simple Guide

Use DateTimeFormatter in Java to format and parse date-time objects by defining a pattern string. Create a formatter with DateTimeFormatter.ofPattern() and apply it to LocalDate, LocalTime, or LocalDateTime objects using format() or parse() methods.

Syntax

The basic syntax to create a DateTimeFormatter is using DateTimeFormatter.ofPattern(String pattern). The pattern defines how the date and time will be formatted or parsed.

• Pattern: A string like "yyyy-MM-dd" that specifies the date/time format.
• format(): Method to convert a date/time object to a string.
• parse(): Method to convert a string back to a date/time object.

java

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String formattedDate = formatter.format(LocalDateTime.now());
LocalDateTime dateTime = LocalDateTime.parse("2024-06-01 15:30:00", formatter);

Example

This example shows how to format the current date and time into a string and then parse a string back into a LocalDateTime object using DateTimeFormatter.

java

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class DateTimeFormatterExample {
    public static void main(String[] args) {
        // Create a formatter with a pattern
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

        // Format current date-time to string
        LocalDateTime now = LocalDateTime.now();
        String formattedDate = now.format(formatter);
        System.out.println("Formatted date-time: " + formattedDate);

        // Parse string back to LocalDateTime
        String dateString = "2024-06-01 15:30:00";
        LocalDateTime parsedDate = LocalDateTime.parse(dateString, formatter);
        System.out.println("Parsed LocalDateTime: " + parsedDate);
    }
}

Output

Formatted date-time: 2024-06-01 15:30:00
Parsed LocalDateTime: 2024-06-01T15:30:00

Common Pitfalls

Common mistakes when using DateTimeFormatter include:

• Using a pattern that does not match the input string when parsing, causing exceptions.
• Confusing uppercase and lowercase letters in patterns (e.g., MM for month vs mm for minutes).
• Not handling exceptions like DateTimeParseException when parsing invalid strings.

java

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;

public class FormatterPitfall {
    public static void main(String[] args) {
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

        String wrongDateString = "2024-06-01 15:30"; // Missing seconds

        try {
            // This will throw an exception because pattern expects seconds
            LocalDateTime.parse(wrongDateString, formatter);
        } catch (DateTimeParseException e) {
            System.out.println("Parsing failed: " + e.getMessage());
        }

        // Correct way: match pattern exactly
        String correctDateString = "2024-06-01 15:30:00";
        LocalDateTime dateTime = LocalDateTime.parse(correctDateString, formatter);
        System.out.println("Parsed correctly: " + dateTime);
    }
}

Output

Parsing failed: Text '2024-06-01 15:30' could not be parsed at index 16
Parsed correctly: 2024-06-01T15:30:00

Quick Reference

Pattern Symbol	Meaning	Example
yyyy	Year (4 digits)	2024
MM	Month (2 digits)	06
dd	Day of month (2 digits)	01
HH	Hour (24-hour, 2 digits)	15
mm	Minute (2 digits)	30
ss	Second (2 digits)	00
a	AM/PM marker	PM

Key Takeaways

Use DateTimeFormatter.ofPattern() with a pattern string to format or parse dates and times.

Ensure the pattern exactly matches the date/time string format when parsing to avoid errors.

Remember uppercase and lowercase letters in patterns have different meanings (e.g., MM vs mm).

Handle DateTimeParseException when parsing strings to catch format mismatches.

DateTimeFormatter works with LocalDate, LocalTime, and LocalDateTime classes for flexible date-time handling.