Class RuleBasedCollator

Class java.text.RuleBasedCollator

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java.lang.Object
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   +----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) :

The French secondary ordering is applied to the whole collator object.
All non-mentioned Unicode characters are at the end of the collation order.
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 => '&').
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 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 not preceded by either a reset or relation character (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 Collators is as simple as concatenating strings. Here's an example that combines two Collators 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