Date formatting

Format dates with thread-safe, immutable DateTimeFormatter.

Code Comparison

✕ Pre-Java 8

// Not thread-safe!
SimpleDateFormat sdf =
    new SimpleDateFormat("yyyy-MM-dd");
String formatted = sdf.format(date);
// Must synchronize for concurrent use

✓ Java 8+

DateTimeFormatter fmt =
    DateTimeFormatter.ofPattern(
        "uuuu-MM-dd");
String formatted =
    LocalDate.now().format(fmt);
// Thread-safe, immutable

Share 𝕏

See a problem with this code? Let us know.

Why the modern way wins

🛡️

Thread-safe

Share formatters across threads without synchronization.

📋

Built-in formats

ISO_LOCAL_DATE, ISO_INSTANT, etc. for standard formats.

🔒

Immutable

Store as static final constants safely.

Old Approach

SimpleDateFormat

Modern Approach

DateTimeFormatter

Since JDK

Difficulty

Beginner

JDK Support

Date formatting

Available

Widely available since JDK 8 (March 2014)

How it works

DateTimeFormatter is immutable and thread-safe, unlike SimpleDateFormat. It can be stored as a constant and shared. Predefined formatters like ISO_LOCAL_DATE are available for common formats.

Instant with nanosecond precision

Java 8

// Millisecond precision only
long millis =
    System.currentTimeMillis();
// 1708012345678

Java 9+

// Microsecond/nanosecond precision
Instant now = Instant.now();
// 2025-02-15T20:12:25.678901234Z
long nanos = now.getNano();

Hover to see modern ➜

JDK 9+ →

Date/Time Beginner

java.time API basics

Pre-Java 8

// Mutable, confusing, zero-indexed months
Calendar cal = Calendar.getInstance();
cal.set(2025, 0, 15); // January = 0!
Date date = cal.getTime();
// not thread-safe

Java 8+

LocalDate date = LocalDate.of(
    2025, Month.JANUARY, 15);
LocalTime time = LocalTime.of(14, 30);
Instant now = Instant.now();
// immutable, thread-safe

Hover to see modern ➜

JDK 8+ →

Date/Time Intermediate

HexFormat

Java 8

// Pad to 2 digits, uppercase
String hex = String.format(
    "%02X", byteValue);
// Parse hex string
int val = Integer.parseInt(
    "FF", 16);

Java 17+

var hex = HexFormat.of()
    .withUpperCase();
String s = hex.toHexDigits(
    byteValue);
byte[] bytes =
    hex.parseHex("48656C6C6F");

Hover to see modern ➜

JDK 17+ →