RuleBasedCollator

Class Overview

A concrete implementation class for Collation.

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. If a character is not located in the RuleBasedCollator, the default Unicode Collation Algorithm (UCA) rule-based table is automatically searched as a backup.

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 rule elements are defined as follows:

• 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
• Text-Argument: A text-argument is any sequence of characters, excluding special characters (that is, common whitespace characters [0009-000D, 0020] and rule syntax characters [0021-002F, 003A-0040, 005B-0060, 007B-007E]). If those characters are desired, you can put them in single quotes (for example, use '&' for ampersand). Note that unquoted white space characters are ignored; for example, b c is treated as bc.
• 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. For example "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 if it contracts to a single character (expressed as "c < ch < d"), while in traditional German a-umlaut is treated as if it expands to two characters (expressed as "a,A < b,B ... & ae;ã & AE;Ã", where ã and à are the escape sequences for a-umlaut).

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 text-arguments up to the first "<" are ignorable. For example, ", - < a < b" makes "-" an ignorable character.

Normalization and Accents

RuleBasedCollator automatically processes its rule table to include both pre-composed and combining-character versions of accented characters. Even if the provided rule string contains only base characters and separate combining accent characters, the pre-composed accented characters matching all canonical combinations of characters from the rule string will be entered in the table.

This allows you to use a RuleBasedCollator to compare accented strings even when the collator is set to NO_DECOMPOSITION. However, if the strings to be collated contain combining sequences that may not be in canonical order, you should set the collator to CANONICAL_DECOMPOSITION to enable sorting of combining sequences. For more information, see The Unicode Standard, Version 3.0.

Errors

The following rules are not valid:

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

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

Examples

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=å,Å=Å"
         + ";aa,AA< æ,Æ< ø,Ø";
 RuleBasedCollator myNorwegian = new RuleBasedCollator(Norwegian);
 

Combining Collators is as simple as concatenating strings. Here is 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 collators
 // 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
 

The next example shows 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 = "= ¨ ; ¯ ; ¿" + "< 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 the 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 ア
 String jaString = "& ア , ー < ト";

 RuleBasedCollator myJapaneseCollator =
     new RuleBasedCollator(en_USCollator.getRules() + jaString);
 

Summary

[Expand] Inherited Constants

From class java.text.Collator int CANONICAL_DECOMPOSITION Constant used to specify the decomposition rule. int FULL_DECOMPOSITION Constant used to specify the decomposition rule. int IDENTICAL Constant used to specify the collation strength. int NO_DECOMPOSITION Constant used to specify the decomposition rule. int PRIMARY Constant used to specify the collation strength. int SECONDARY Constant used to specify the collation strength. int TERTIARY Constant used to specify the collation strength.

[Expand] Inherited Methods
From class java.text.Collator Object clone() Returns a new collator with the same decomposition mode and strength value as this collator. abstract int compare( String string1, String string2) Compares two strings to determine their relative order. int compare( Object object1, Object object2) Compares two objects to determine their relative order. boolean equals( Object object) Compares this collator with the specified object and indicates if they are equal. boolean equals( String string1, String string2) Compares two strings using the collation rules to determine if they are equal. static Locale[] getAvailableLocales() Returns an array of locales for which custom Collator instances are available. abstract CollationKey getCollationKey( String string) Returns a CollationKey for the specified string for this collator with the current decomposition rule and strength value. int getDecomposition() Returns the decomposition rule for this collator. static Collator getInstance() Returns a Collator instance which is appropriate for the user's default Locale. static Collator getInstance( Locale locale) Returns a Collator instance which is appropriate for locale. int getStrength() Returns the strength value for this collator. abstract int hashCode() Returns an integer hash code for this object. void setDecomposition(int value) Sets the decomposition rule for this collator. void setStrength(int value) Sets the strength value for this collator.
From class java.lang.Object Object clone() Creates and returns a copy of this Object. boolean equals( Object o) Compares this instance with the specified object and indicates if they are equal. void finalize() Invoked when the garbage collector has detected that this instance is no longer reachable. final Class<?> getClass() Returns the unique instance of Class that represents this object's class. int hashCode() Returns an integer hash code for this object. final void notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. final void notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. String toString() Returns a string containing a concise, human-readable description of this object. final void wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object. final void wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires. final void wait(long millis) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
From interface java.util.Comparator abstract int compare(T lhs, T rhs) Compares the two specified objects to determine their relative ordering. abstract boolean equals( Object object) Compares this Comparator with the specified Object and indicates whether they are equal.