How to Convert Milliseconds to \"hh:mm:ss\" Format?

Converting Milliseconds to "hh:mm:ss" Format

You may encounter a situation where you need to convert milliseconds to the familiar "hh:mm:ss" format, typically used for representing time. This can be a common requirement in applications, such as countdown timers.

Understanding the Problem

You may have tried using the following code to perform the conversion:

<code class="java">//hh:mm:ss
String.format("%02d:%02d:%02d", TimeUnit.MILLISECONDS.toHours(millis),
    TimeUnit.MILLISECONDS.toMinutes(millis) - TimeUnit.MINUTES.toMinutes(TimeUnit.MILLISECONDS.toHours(millis)),
    TimeUnit.MILLISECONDS.toSeconds(millis) - TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes(millis)));</code>

However, this code might not be working as expected. For instance, for an input of 3600000ms (representing 1 hour), you get "01:59:00" instead of the correct "01:00:00".

Identifying the Issue

Upon closer examination, the issue lies in this line:

<code class="java">TimeUnit.MILLISECONDS.toMinutes(millis) - TimeUnit.MINUTES.toMinutes(TimeUnit.MILLISECONDS.toHours(millis))</code>

Here, you are converting hours to milliseconds using minutes instead of hours.

Correcting the Logic

The correct code should use TimeUnit.HOURS.toMinutes(TimeUnit.MILLISECONDS.toHours(millis)) instead:

<code class="java">//hh:mm:ss
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)));</code>

This change ensures that the time conversion is done correctly, and you will get the desired result of "01:00:00" for 3600000ms.

Simplified Version

You can further simplify the code by using a modulus division instead of subtraction:

<code class="java">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));</code>

This version remains efficient while providing the same accurate results.

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