Question

I want to show a formatted string with minutes, seconds, and only one character of milliseconds. This is what I put together (mostly from other posts around here):

public static final String getTimeDurationAsString(long milliseconds) {
    int millis  = (int) (milliseconds % 1000);
    int seconds = (int) (milliseconds / 1000) % 60;
    int minutes = (int) ((milliseconds / (1000*60)) % 60);
    int hours   = (int) ((milliseconds / (1000*60*60)) % 24);

    StringBuilder sb = new StringBuilder();
    if (hours > 0) {
        sb.append(String.format("%02d", hours));
        sb.append(":");
    }
    sb.append(String.format("%02d", minutes));
    sb.append(":");
    sb.append(String.format("%02d", seconds));
    sb.append(".");
    sb.append(String.format("%03d", millis).substring(0, 1));

    return sb.toString();
}

So I will hide the hours position if the duration didn’t exceed 59.9 minutes (which will usually be the case). I did a pretty bad substring() for the milliseconds position just to grab the first digit. Is there a better way to do the above? I’d like strings like:

00:14.9
00:05.1
00:05.2
33:20:4
etc

I have to generate this string repeatedly for a game I’m making (the above rendered every frame) so afraid it’s doing a lot of unnecessary work?

Thanks!

Answer 1

Without a doubt there are more efficient ways to code this up (there are practically always are).

However, on my box your function can format ~130,000 timestamps per second. Clearly, this is plenty fast enough for your stated use case.

That said, if I were coding this up and thought a priori that it’s likely to be on the performance-critical path, I’d avoid String.format():

public static final String getTimeDurationAsString(long milliseconds) {
    int millis  = (int) (milliseconds % 1000);
    int seconds = (int) (milliseconds / 1000) % 60;
    int minutes = (int) ((milliseconds / (1000*60)) % 60);
    int hours   = (int) ((milliseconds / (1000*60*60)) % 24);
    StringBuilder sb = new StringBuilder();
    if (hours > 0) {
        sb.append((char)('0' + hours / 10))
          .append((char)('0' + hours % 10)).append(":");
    }
    sb.append((char)('0' + minutes / 10))
      .append((char)('0' + minutes % 10)).append(":")
      .append((char)('0' + seconds / 10))
      .append((char)('0' + seconds % 10)).append(".")
      .append((char)('0' + millis / 100));
    return sb.toString();
}

On my box this can do about 8 million conversions per second. It also creates significantly fewer temporary objects that end up having to be garbage collected.