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# Class java.text.RuleBasedCollator

    java.lang.Object
       |
       +----java.text.Collator
               |
               +----java.text.RuleBasedCollator

public class RuleBasedCollator
extends Collator

The `RuleBasedCollator` class is a concrete subclass of `Collator` that provides a simple, data-driven, table collator. With this class you can create a customized table-based `Collator`. `RuleBasedCollator` maps characters to sort keys.

`RuleBasedCollator` has the following restrictions for efficiency (other subclasses may be used for more complex languages) :

1. If a French secondary ordering is specified it applies to the whole collator object.
2. All non-mentioned Unicode characters are at the end of the collation order.
3. Private use characters are treated as identical. The private use area in Unicode is `0xE800`-`0xF8FF`.

The collation table is composed of a list of collation rules, where each rule is of three forms:

        < modifier >
        < relation > < text-argument >
        < reset > < text-argument >

The following demonstrates how to create your own collation rules:

- Text-Argument: A text-argument is any sequence of characters, excluding special characters (that is, whitespace characters and the characters used in modifier, relation and reset). If those characters are desired, you can put them in single quotes (e.g. ampersand => '&'). Note that unquoted white space characters are ignored; e.g. `b c` is treated as `bc`.
- Modifier: There is a single modifier which is used to specify that all accents (secondary differences) are backwards.

  '@' : Indicates that accents are sorted backwards, as in French.

- Relation: The relations are the following:
  - '<' : Greater, as a letter difference (primary)
  - ';' : Greater, as an accent difference (secondary)
  - ',' : Greater, as a case difference (tertiary)
  - '=' : Equal
- Reset: There is a single reset which is used primarily for contractions and expansions, but which can also be used to add a modification at the end of a set of rules.

  '&' : Indicates that the next rule follows the position to where the reset text-argument would be sorted.

This sounds more complicated than it is in practice. For example, the following are equivalent ways of expressing the same thing:

>      a < b < c
>      a < b & b < c
>      a < c & a < b

Notice that the order is important, as the subsequent item goes immediately after the text-argument. The following are not equivalent:

>      a < b & a < c
>      a < c & a < b

Either the text-argument must already be present in the sequence, or some initial substring of the text-argument must be present. (e.g. "a < b & ae < e" is valid since "a" is present in the sequence before "ae" is reset). In this latter case, "ae" is not entered and treated as a single character; instead, "e" is sorted as if it were expanded to two characters: "a" followed by an "e". This difference appears in natural languages: in traditional Spanish "ch" is treated as though it contracts to a single character (expressed as "c < ch < d"), while in traditional German "ä" (a-umlaut) is treated as though it expands to two characters (expressed as "a & ae ; ä < b").

Ignorable Characters

For ignorable characters, the first rule must start with a relation (the examples we have used above are really fragments; "a < b" really should be "< a < b"). If, however, the first relation is not "<", then all the all text-arguments up to the first "<" are ignorable. For example, ", - < a < b" makes "-" an ignorable character, as we saw earlier in the word "black-birds". In the samples for different languages, you see that most accents are ignorable.

Normalization and Accents

The `Collator` object automatically normalizes text internally to separate accents from base characters where possible. This is done both when processing the rules, and when comparing two strings. `Collator` also uses the Unicode canonical mapping to ensure that combining sequences are sorted properly (for more information, see The Unicode Standard, Version 2.0.)

Errors

The following are errors:

- A text-argument contains unquoted punctuation symbols (e.g. "a < b-c < d").
- A relation or reset character not followed by a text-argument (e.g. "a < , b").
- A reset where the text-argument (or an initial substring of the text-argument) is not already in the sequence. (e.g. "a < b & e < f")

If you produce one of these errors, a `RuleBasedCollator` throws a `ParseException`.

Examples

Simple: "< a < b < c < d"

Norwegian: "< a,A< b,B< c,C< d,D< e,E< f,F< g,G< h,H< i,I< j,J < k,K< l,L< m,M< n,N< o,O< p,P< q,Q< r,R< s,S< t,T < u,U< v,V< w,W< x,X< y,Y< z,Z < å=a?,Å=A? ;aa,AA< æ,Æ< ø,Ø"

Normally, to create a rule-based Collator object, you will use `Collator`'s factory method `getInstance`. However, to create a rule-based Collator object with specialized rules tailored to your needs, you construct the `RuleBasedCollator` with the rules contained in a `String` object. For example:

>      String Simple = "< a < b < c < d";
>      RuleBasedCollator mySimple = new RuleBasedCollator(Simple);

Or:

>      String Norwegian = "< a,A< b,B< c,C< d,D< e,E< f,F< g,G< h,H< i,I< j,J" +
>                      "< k,K< l,L< m,M< n,N< o,O< p,P< q,Q< r,R< s,S< t,T" +
>                      "< u,U< v,V< w,W< x,X< y,Y< z,Z" +
>                      "< å=a?,Å=A?" +
>                      ";aa,AA< æ,Æ< ø,Ø";
>      RuleBasedCollator myNorwegian = new RuleBasedCollator(Norwegian);

Combining `Collator`s is as simple as concatenating strings. Here's an example that combines two `Collator`s from two different locales:

>      // Create an en_US Collator object
>      RuleBasedCollator en_USCollator = (RuleBasedCollator)
>          Collator.getInstance(new Locale("en", "US", ""));
>      // Create a da_DK Collator object
>      RuleBasedCollator da_DKCollator = (RuleBasedCollator)
>          Collator.getInstance(new Locale("da", "DK", ""));
>      // Combine the two
>      // First, get the collation rules from en_USCollator
>      String en_USRules = en_USCollator.getRules();
>      // Second, get the collation rules from da_DKCollator
>      String da_DKRules = da_DKCollator.getRules();
>      RuleBasedCollator newCollator =
>          new RuleBasedCollator(en_USRules + da_DKRules);
>      // newCollator has the combined rules

Another more interesting example would be to make changes on an existing table to create a new `Collator` object. For example, add "& C < ch, cH, Ch, CH" to the `en_USCollator` object to create your own:

>      // Create a new Collator object with additional rules
>      String addRules = "& C < ch, cH, Ch, CH";
>      RuleBasedCollator myCollator =
>          new RuleBasedCollator(en_USCollator + addRules);
>      // myCollator contains the new rules

The following example demonstrates how to change the order of non-spacing accents,

>      // old rule
>      String oldRules = "=?;?;?;?"    // main accents
>                      + ";?;?;?;?"    // main accents
>                      + ";?;?;?;?"    // main accents
>                      + ";?;?;?;?"    // main accents
>                      + ";?;?;?;?"    // main accents
>                      + "< a , A ; ae, AE ; æ , Æ"
>                      + "< b , B < c, C < e, E & C < d, D";
>      // change the order of accent characters
>      String addOn = "& ? ; ? ; ?";
>      RuleBasedCollator myCollator = new RuleBasedCollator(oldRules + addOn);

The last example shows how to put new primary ordering in before the default setting. For example, in Japanese `Collator`, you can either sort English characters before or after Japanese characters,

>      // get en_US Collator rules
>      RuleBasedCollator en_USCollator = (RuleBasedCollator)Collator.getInstance(Locale.US);
>      // add a few Japanese character to sort before English characters
>      // suppose the last character before the first base letter 'a' in
>      // the English collation rule is ?
>      String jaString = "& ? < ?, ? < ?, ?";
>      RuleBasedCollator myJapaneseCollator = new
>          RuleBasedCollator(en_USCollator.getRules() + jaString);

See Also:
  Collator, CollationElementIterator

##

RuleBasedCollator(String)
  RuleBasedCollator constructor.

##

clone()
  Standard override; no change in semantics.
compare(String, String)
  Compares the character data stored in two different strings based on the collation rules.
equals(Object)
  Compares the equality of two collation objects.
getCollationElementIterator(String)
  Return a CollationElementIterator for the given String.
getCollationKey(String)
  Transforms the string into a series of characters that can be compared with CollationKey.compareTo.
getRules()
  Gets the table-based rules for the collation object.
hashCode()
  Generates the hash code for the table-based collation object

##

RuleBasedCollator

     public RuleBasedCollator(String rules) throws ParseException

  RuleBasedCollator constructor. This takes the table rules and builds a collation table out of them. Please see RuleBasedCollator class description for more details on the collation rule syntax.

  Parameters:
    rules - the collation rules to build the collation table from.
  Throws: ParseException
    A format exception will be thrown if the build process of the rules fails. For example, build rule "a < ? < d" will cause the constructor to throw the ParseException because the '?' is not quoted.
  See Also:
    Locale

##

getRules

     public String getRules()

  Gets the table-based rules for the collation object.

  Returns:
    returns the collation rules that the table collation object was created from.

getCollationElementIterator

     public CollationElementIterator getCollationElementIterator(String source)

  Return a CollationElementIterator for the given String.

  See Also:
    CollationElementIterator

compare

     public int compare(String source,
                        String target)

  Compares the character data stored in two different strings based on the collation rules. Returns information about whether a string is less than, greater than or equal to another string in a language. This can be overriden in a subclass.

  Overrides:
    compare in class Collator

getCollationKey

     public CollationKey getCollationKey(String source)

  Transforms the string into a series of characters that can be compared with CollationKey.compareTo. This overrides java.text.Collator.getCollationKey. It can be overriden in a subclass.

  Overrides:
    getCollationKey in class Collator

clone

     public Object clone()

  Standard override; no change in semantics.

  Overrides:
    clone in class Collator

equals

     public boolean equals(Object obj)

  Compares the equality of two collation objects.

  Parameters:
    obj - the table-based collation object to be compared with this.
  Returns:
    true if the current table-based collation object is the same as the table-based collation object obj; false otherwise.
  Overrides:
    equals in class Collator

hashCode

     public int hashCode()

  Generates the hash code for the table-based collation object

  Overrides:
    hashCode in class Collator

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