On Wed, 8 Oct 2025 00:33:20 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 incrementally with one additional
> commit since the last revision:
>
> minor api doc updates
Given this patch obviously has so many performance optimization opportunities,
I recommend handling those in subsequent RFEs so that we can review this purely
from a specification point of view.
make/modules/java.base/gensrc/GensrcCharacterData.gmk line 76:
> 74:
> 75:
> 76: GENSRC_STRINGCASEFOLDING :=
> $(SUPPORT_OUTPUTDIR)/gensrc/java.base/jdk/internal/java/lang/CaseFolding.java
Can we target the package `jdk.internal.lang` instead of
`jdk.internal.java.lang`? I think the previous one is the convention set forth
by stable values.
-------------
PR Review: https://git.openjdk.org/jdk/pull/27628#pullrequestreview-3314954963
PR Review Comment: https://git.openjdk.org/jdk/pull/27628#discussion_r2413957017
