Formatting

18.2.1 Why Format DateTime?

LocalDateTime always stores date-time in a standard way —
but users want readable formats:

• YYYY-MM-DD

• MM/DD/YYYY

• Friday, July 3rd, 2025 3:30 PM

This is where DateTimeFormatter comes in.

Key concept:

• DateTimeFormatter formats a LocalDateTime → String.

• It also parses Strings → LocalDateTime.

• You can use built-in patterns or custom patterns.

18.2.2 Common Format Patterns

18.2.3 Mini Lab: Formatting

Below shows how to format LocalDateTime in multiple ways.

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

public class FormattingMiniLab {
    public static void main(String[] args) {
        LocalDateTime now = LocalDateTime.now();

        // Built-in ISO
        System.out.println("ISO: " + now);

        // Custom format 1
        DateTimeFormatter fmt1 = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
        System.out.println("Custom 1: " + now.format(fmt1));

        // Custom format 2
        DateTimeFormatter fmt2 = DateTimeFormatter.ofPattern("MM-dd-yyyy HH:mm");
        System.out.println("Custom 2: " + now.format(fmt2));

        // Day name
        DateTimeFormatter fmt3 = DateTimeFormatter.ofPattern("EEEE, MMM dd yyyy HH:mm");
        System.out.println("Custom 3: " + now.format(fmt3));
    }
}

Key points:

• Use ofPattern() with custom patterns.

• format() turns a date-time into a String.

• You can adjust output for region, language, style.