How do you parse and format date/time values using DateTimeFormatter, and how does this replace the older, notoriously thread-unsafe SimpleDateFormat class?

DateTimeFormatter (immutable and thread-safe, unlike the older SimpleDateFormat which was famously NOT thread-safe and required careful per-thread instance management or external synchronization to use safely in concurrent code) provides both predefined standard formatters (ISO_LOCAL_DATE, ISO_DATE_TIME) and a pattern-based custom formatter (via ofPattern(), using largely similar pattern letters to SimpleDateFormat's own pattern syntax) for both parsing a String into a date/time object and formatting a date/time object into a String, with a single DateTimeFormatter instance safely shareable and reusable across multiple threads simultaneously, unlike SimpleDateFormat which required a new instance per use (or ThreadLocal-based pooling) to avoid subtle, hard-to-diagnose concurrency bugs.

// Modern, thread-safe DateTimeFormatter -- safe to share as a static final constant across threads
private static final DateTimeFormatter FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

LocalDateTime dateTime = LocalDateTime.parse("2024-06-01 14:30:00", FORMATTER);
String formatted = dateTime.format(FORMATTER);

// vs. the OLD, unsafe-to-share SimpleDateFormat, which required per-use or per-thread instances
// SimpleDateFormat oldFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  // NOT safe as a shared static field!