On Fri, 3 Oct 2025 19:41:20 GMT, Xueming Shen <[email protected]> wrote:
>> ### Summary
>>
>> Case folding is a key operation for case-insensitive matching (e.g., string
>> equality, regex matching), where the goal is to eliminate case distinctions
>> without applying locale or language specific conversions.
>>
>> Currently, the JDK does not expose a direct API for Unicode-compliant case
>> folding. Developers now rely on methods such as:
>>
>> **String.equalsIgnoreCase(String)**
>>
>> - Unicode-aware, locale-independent.
>> - Implementation uses Character.toLowerCase(Character.toUpperCase(int)) per
>> code point.
>> - Limited: does not support 1:M mapping defined in Unicode case folding.
>>
>> **Character.toLowerCase(int) / Character.toUpperCase(int)**
>>
>> - Locale-independent, single code point only.
>> - No support for 1:M mappings.
>>
>> **String.toLowerCase(Locale.ROOT) / String.toUpperCase(Locale.ROOT)**
>>
>> - Based on Unicode SpecialCasing.txt, supports 1:M mappings.
>> - Intended primarily for presentation/display, not structural
>> case-insensitive matching.
>> - Requires full string conversion before comparison, which is less efficient
>> and not intended for structural matching.
>>
>> **1:M mapping example, U+00DF (ß)**
>>
>> - String.toUpperCase(Locale.ROOT, "ß") → "SS"
>> - Case folding produces "ss", matching Unicode caseless comparison rules.
>>
>>
>> jshell> "\u00df".equalsIgnoreCase("ss")
>> $22 ==> false
>>
>> jshell>
>> "\u00df".toUpperCase(Locale.ROOT).toLowerCase(Locale.ROOT).equals("ss")
>> $24 ==> true
>>
>>
>> ### Motivation & Direction
>>
>> Add Unicode standard-compliant case-less comparison methods to the String
>> class, enabling & improving reliable and efficient Unicode-aware/compliant
>> case-insensitive matching.
>>
>> - Unicode-compliant **full** case folding.
>> - Simpler, stable and more efficient case-less matching without workarounds.
>> - Brings Java's string comparison handling in line with other programming
>> languages/libraries.
>>
>> This PR proposes to introduce the following comparison methods in `String`
>> class
>>
>> - boolean equalsFoldCase(String anotherString)
>> - int compareToFoldCase(String anotherString)
>> - Comparator<String> UNICODE_CASEFOLD_ORDER
>>
>> These methods are intended to be the preferred choice when Unicode-compliant
>> case-less matching is required.
>>
>> *Note: An early draft also proposed a String.toCaseFold() method returning a
>> new case-folded string.
>> However, during review this was considered error-prone, as the resulting
>> string could easily be mistaken for a general transformation like
>> toLowerCase() and then pass...
>
> Xueming Shen has updated the pull request with a new target base due to a
> merge or a rebase. The pull request now contains two commits:
>
> - Merge branch 'master' of https://git.openjdk.org/jdk into JDK-8365675
> - 8365675: Add String Unicode Case-Folding Support
>
> to update api
src/java.base/share/classes/jdk/internal/lang/CaseFolding.java.template line 1:
> 1: /*
We're going to need to find a more compact format for the data, individual 1 or
2 entry char arrays have a large overhead. Plus the map entries take a lot of
space for the data and indexing.
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PR Review Comment: https://git.openjdk.org/jdk/pull/26892#discussion_r2403194785
