RFR: 8336741: Optimize LocalTime.toString with StringBuilder.repeat

[Brett Okken]

On Thu, 18 Jul 2024 11:32:47 GMT, Shaojin Wen <duke at openjdk.org> wrote:

> class LocalTime {
>     public String toString() {
>         // ...
>                 if (nanoValue % 1000_000 == 0) {
>                     buf.append(Integer.toString((nanoValue / 1000_000) + 1000).substring(1));
>                 } else if (nanoValue % 1000 == 0) {
>                     buf.append(Integer.toString((nanoValue / 1000) + 1000_000).substring(1));
>                 } else {
>                     buf.append(Integer.toString((nanoValue) + 1000_000_000).substring(1));
>                 }
>        // ...
>     }
> }
> 
> Currently, LocalTime.toString handles nanos by adding a value and then subString(1) to fill it with zeros. Using StringBuilder.repeat is more concise and has better performance.

src/java.base/share/classes/java/time/LocalTime.java line 1653:

> 1651:                 int digits;
> 1652:                 if (nanoValue % 1000_000 == 0) {
> 1653:                     digits = nanoValue / 1000_000;

should it be `1_000_000` - breaking on every 3 digits?

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PR Review Comment: https://git.openjdk.org/jdk/pull/20232#discussion_r1683016529