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 Unicode collation support. `Collation` converts each token into its binary `CollationKey` using the provided `Collator`, allowing it to be stored as an index term. 

## Use Cases

*   Efficient sorting of terms in languages that use non-Unicode character 
    orderings.  (Lucene Sort using a Locale can be very slow.) 

*   Efficient range queries over fields that contain terms in languages that 
    use non-Unicode character orderings.  (Range queries using a Locale can be
    very slow.)

*   Effective Locale-specific normalization (case differences, diacritics, etc.).
    (<xref:Lucene.Net.Analysis.Core.LowerCaseFilter> and 
    <xref:Lucene.Net.Analysis.Miscellaneous.ASCIIFoldingFilter> provide these services
    in a generic way that doesn't take into account locale-specific needs.)

## Example Usages

### Farsi Range Queries

      // "fa" Locale is not supported by Sun JDK 1.4 or 1.5
      Collator collator = Collator.getInstance(new Locale("ar"));
      CollationKeyAnalyzer analyzer = new CollationKeyAnalyzer(version, collator);
      RAMDirectory ramDir = new RAMDirectory();
      IndexWriter writer = new IndexWriter(ramDir, new IndexWriterConfig(version, analyzer));
      Document doc = new Document();
      doc.add(new TextField("content", "\u0633\u0627\u0628", Field.Store.YES));
      writer.addDocument(doc);
      writer.close();
      IndexReader ir = DirectoryReader.open(ramDir);
      IndexSearcher is = new IndexSearcher(ir);
    
  QueryParser aqp = new QueryParser(version, "content", analyzer);
      aqp.setAnalyzeRangeTerms(true);

      // Unicode order would include U+0633 in [ U+062F - U+0698 ], but Farsi
      // orders the U+0698 character before the U+0633 character, so the single
      // indexed Term above should NOT be returned by a ConstantScoreRangeQuery
      // with a Farsi Collator (or an Arabic one for the case when Farsi is not
      // supported).
      ScoreDoc[] result
        = is.search(aqp.parse("[ \u062F TO \u0698 ]"), null, 1000).scoreDocs;
      assertEquals("The index Term should not be included.", 0, result.length);

### Danish Sorting

      Analyzer analyzer 
        = new CollationKeyAnalyzer(version, Collator.getInstance(new Locale("da", "dk")));
      RAMDirectory indexStore = new RAMDirectory();
      IndexWriter writer = new IndexWriter(indexStore, new IndexWriterConfig(version, analyzer));
      String[] tracer = new String[] { "A", "B", "C", "D", "E" };
      String[] data = new String[] { "HAT", "HUT", "H\u00C5T", "H\u00D8T", "HOT" };
      String[] sortedTracerOrder = new String[] { "A", "E", "B", "D", "C" };
      for (int i = 0 ; i < data.length="" ;="" ++i)="" {="" document="" doc="new" document();="" doc.add(new="" storedfield("tracer",="" tracer[i]));="" doc.add(new="" textfield("contents",="" data[i],="" field.store.no));="" writer.adddocument(doc);="" }="" writer.close();="" indexreader="" ir="DirectoryReader.open(indexStore);" indexsearcher="" searcher="new" indexsearcher(ir);="" sort="" sort="new" sort();="" sort.setsort(new="" sortfield("contents",="" sortfield.string));="" query="" query="new" matchalldocsquery();="" scoredoc[]="" result="searcher.search(query," null,="" 1000,="" sort).scoredocs;="" for="" (int="" i="0" ;="" i="">< result.length="" ;="" ++i)="" {="" document="" doc="searcher.doc(result[i].doc);" assertequals(sortedtracerorder[i],="" doc.getvalues("tracer")[0]);="" }="">

### Turkish Case Normalization

      Collator collator = Collator.getInstance(new Locale("tr", "TR"));
      collator.setStrength(Collator.PRIMARY);
      Analyzer analyzer = new CollationKeyAnalyzer(version, collator);
      RAMDirectory ramDir = new RAMDirectory();
      IndexWriter writer = new IndexWriter(ramDir, new IndexWriterConfig(version, analyzer));
      Document doc = new Document();
      doc.add(new TextField("contents", "DIGY", Field.Store.NO));
      writer.addDocument(doc);
      writer.close();
      IndexReader ir = DirectoryReader.open(ramDir);
      IndexSearcher is = new IndexSearcher(ir);
      QueryParser parser = new QueryParser(version, "contents", analyzer);
      Query query = parser.parse("d\u0131gy");   // U+0131: dotless i
      ScoreDoc[] result = is.search(query, null, 1000).scoreDocs;
      assertEquals("The index Term should be included.", 1, result.length);

## Caveats and Comparisons

 __WARNING:__ Make sure you use exactly the same `Collator` at index and query time -- `CollationKey`s are only comparable when produced by the same `Collator`. Since {@link java.text.RuleBasedCollator}s are not independently versioned, it is unsafe to search against stored `CollationKey`s unless the following are exactly the same (best practice is to store this information with the index and check that they remain the same at query time): 

1.  JVM vendor

2.  JVM version, including patch version

3.  The language (and country and variant, if specified) of the Locale
    used when constructing the collator via
    {@link java.text.Collator#getInstance(java.util.Locale)}.

4.  The collation strength used - see {@link java.text.Collator#setStrength(int)}

 `ICUCollationKeyAnalyzer`, available in the [icu analysis module]({@docRoot}/../analyzers-icu/overview-summary.html), uses ICU4J's `Collator`, which makes its version available, thus allowing collation to be versioned independently from the JVM. `ICUCollationKeyAnalyzer` is also significantly faster and generates significantly shorter keys than `CollationKeyAnalyzer`. See [http://site.icu-project.org/charts/collation-icu4j-sun](http://site.icu-project.org/charts/collation-icu4j-sun) for key generation timing and key length comparisons between ICU4J and `java.text.Collator` over several languages. 

 `CollationKey`s generated by `java.text.Collator`s are not compatible with those those generated by ICU Collators. Specifically, if you use `CollationKeyAnalyzer` to generate index terms, do not use `ICUCollationKeyAnalyzer` on the query side, or vice versa. 