Convert timestamp in milliseconds to string formatted time in Java

Utilizing SimpleDateFormat patterns

When using SimpleDateFormat, be aware of the pattern characters it expects. Here's a cheat sheet:

• yyyy: Full year.
• MM: Month.
• dd: Day of the month.
• HH: Hours (24hr format).
• mm: Minutes.
• ss: Seconds.
• SSS: Milliseconds (barely visible to humans, but we include it anyway for completeness 🕒).

sdf.setTimeZone(TimeZone.getTimeZone("UTC")); // Consistent across the globe!
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By setting the time zone to UTC, you evade daylight saving time changes and any location-based oddities.

Shifting to java.time (post Java 8)

From Java 8 onwards, we swap out SimpleDateFormat with java.time. Here's the updated alternative:

Instant instant = Instant.ofEpochMilli(System.currentTimeMillis());
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss").withZone(ZoneId.of("UTC"));
String formattedTime = formatter.format(instant);
System.out.println(formattedTime); // "2023-01-28 13:45:30" - Time travel, anyone?
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With DateTimeFormatter, we ensure immutability and thread-safety.

Addressing exceptions and edge cases

Formatting timestamps may sometimes feel like defusing a bomb. Erroneous inputs, out-of-range values, even time zones can become ticking time bombs. To maintain calm and ensure peace:

• Always validate the input timestamp. Don't let "dirty" data interfere.
• Watch out for limit breakers. Keep an eye on the API's maximum capacity.
• Don't let time zones give you the blues. Present times responsibly to global audiences.

Always prepare a try-catch contingency, because exceptions are ninjas - they can appear out of nowhere.

Optimizing performance and exploring alternatives

For the performance-obsessed, be mindful of creating new Date or SimpleDateFormat objects. Reuse these instances wherever possible to give the garbage collector less clean up duty.

sdf.setTimeZone(TimeZone.getTimeZone("UTC")); // It's the world standard after all!
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If you can stray from the Java Standard Library, Joda-Time or Apache Commons Lang offer simplifications in duration formatting:

String formattedDuration = DurationFormatUtils.formatDurationHMS(timeMillis);
System.out.println(formattedDuration); // "1:23:45.678", proof that every millisecond counts!
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