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ICU-11493 document Charset.name() not unique for mapping table variants

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X-SVN-Rev: 37955
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Markus Scherer 2015-09-15 16:08:15 +00:00
parent 536f59d246
commit 6d31a2bfa2
1 changed files with 37 additions and 4 deletions
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41
icu4j/main/classes/charset/src/com/ibm/icu/charset/CharsetICU.java
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@ -19,13 +19,46 @@ import com.ibm.icu.text.UnicodeSet;
/**
* <p>A subclass of java.nio.Charset for providing implementation of ICU's charset converters.
* This API is used to convert codepage or character encoded data to and
* from UTF-16. You can open a converter with {@link Charset#forName } and {@link #forNameICU }. With that
* from UTF-16. You can open a converter with {@link Charset#forName} and {@link #forNameICU}. With that
* converter, you can get its properties, set options, convert your data.</p>
*
* <p>Since many software programs recogize different converter names for
* <p>Since many software programs recognize different converter names for
* different types of converters, there are other functions in this API to
* iterate over the converter aliases.
*
* iterate over the converter aliases.
*
* <p>Note that {@link #name()} cannot always return a unique charset name.
* {@link Charset} documents that,
* for charsets listed in the IANA Charset Registry,
* the {@link #name()} must be listed there,
* and it “must be the MIME-preferred name” if there are multiple names.
*
* <p>However, there are different implementations of many if not most charsets,
* ICU provides multiple variants for some of them,
* ICU provides variants of some java.nio-system-supported charsets,
* and ICU users are free to add more variants.
* This is so that applications can be compatible with multiple implementations at the same time.
*
* <p>This is in conflict with the {@link Charset#name()} requirements.
* It is not possible to offer variants of an IANA charset and
* always use the MIME-preferred name and also have those names be unique.
*
* <p>{@link #name()} returns the MIME-preferred name, or IANA name,
* so that it can always be used for the charset field in internet protocols.
*
* <p>Same-name charsets are accessible via {@link Charset#forName} or {@link #forNameICU}
* by using unique aliases (e.g., the ICU-canonical names).
*
* <p>{@link Charset} also documents that
* “Two charsets are equal if, and only if, they have the same canonical names.”
* This is not possible.
*
* <p>Unfortunately, {@link Charset#equals} is final, and
* {@link Charset#availableCharsets} returns
* “a sorted map from canonical charset names to charset objects”.
* Since {@link #name()} cannot be unique,
* {@link #equals} cannot work properly in such cases, and
* {@link Charset#availableCharsets} can only include one variant for a name.
*
* @stable ICU 3.6
*/
public abstract class CharsetICU extends Charset{