How to Convert Milliseconds to \'hh:mm:ss\' Format in Java?

Formatting Milliseconds to "hh:mm:ss"

Problem:

A developer working with a countdown timer encounters difficulty converting milliseconds to the desired "hh:mm:ss" format. Their initial attempt resulted in incorrect time display, particularly for values exceeding an hour.

Solution:

The main issue stemmed from a logical flaw in converting hours to milliseconds using minutes instead of hours. Specifically, the code utilized:

TimeUnit.MINUTES.toMinutes(TimeUnit.MILLISECONDS.toHours(millis))

while it should have been:

TimeUnit.HOURS.toMinutes(TimeUnit.MILLISECONDS.toHours(millis))

Code Demonstration:

Here's a revised code snippet that correctly formats milliseconds to "hh:mm:ss":

String hms = String.format("%02d:%02d:%02d", TimeUnit.MILLISECONDS.toHours(millis),
    TimeUnit.MILLISECONDS.toMinutes(millis) - TimeUnit.HOURS.toMinutes(TimeUnit.MILLISECONDS.toHours(millis)),
    TimeUnit.MILLISECONDS.toSeconds(millis) - TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes(millis)));

Test Case:

long millis = 3600000;
String hms = ... // Replace with revised code from above

System.out.println(hms); // Expected output: 01:00:00

Additionally, the code can be optimized using modulus division instead of subtraction:

String hms = String.format("%02d:%02d:%02d", TimeUnit.MILLISECONDS.toHours(millis),
    TimeUnit.MILLISECONDS.toMinutes(millis) % TimeUnit.HOURS.toMinutes(1),
    TimeUnit.MILLISECONDS.toSeconds(millis) % TimeUnit.MINUTES.toSeconds(1));

Both variations achieve the correct "hh:mm:ss" formatting and leverage the TimeUnit API to handle conversions seamlessly.

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