JavaTM 2 Platform
Std. Ed. v1.3

java.text
Class RuleBasedCollator

java.lang.Object
  |
  +--java.text.Collator
        |
        +--java.text.RuleBasedCollator
All Implemented Interfaces:
Cloneable, Comparator

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.

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:

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 a-umlaut is treated as though it expanded to two characters (expressed as "a,A < b,B ... & ae; & AE;"). [ and are, of course, 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 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

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. There are two caveats, however. First, if the strings to be collated contain combining sequences that may not be in canonical order, you should set the collator to CANONICAL_DECOMPOSITION or FULL_DECOMPOSITION to enable sorting of combining sequences. Second, if the strings contain characters with compatibility decompositions (such as full-width and half-width forms), you must use FULL_DECOMPOSITION, since the rule tables only include canonical mappings. For more information, see The Unicode Standard, Version 2.0.)

Errors

The following are errors:

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 < =a?,=A? ;aa,AA< ,< ,"

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" +
                 "< =a?,=A?" +
                 ";aa,AA< ,< ,";
 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 ;  , "
                 + "< 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

Fields inherited from class java.text.Collator
CANONICAL_DECOMPOSITION, FULL_DECOMPOSITION, IDENTICAL, NO_DECOMPOSITION, PRIMARY, SECONDARY, TERTIARY
 
Constructor Summary
RuleBasedCollator(String rules)
          RuleBasedCollator constructor.
 
Method Summary
 Object clone()
          Standard override; no change in semantics.
 int compare(String source, String target)
          Compares the character data stored in two different strings based on the collation rules.
 boolean equals(Object obj)
          Compares the equality of two collation objects.
 CollationElementIterator getCollationElementIterator(CharacterIterator source)
          Return a CollationElementIterator for the given String.
 CollationElementIterator getCollationElementIterator(String source)
          Return a CollationElementIterator for the given String.
 CollationKey getCollationKey(String source)
          Transforms the string into a series of characters that can be compared with CollationKey.compareTo.
 String getRules()
          Gets the table-based rules for the collation object.
 int hashCode()
          Generates the hash code for the table-based collation object
 
Methods inherited from class java.text.Collator
compare, equals, getAvailableLocales, getDecomposition, getInstance, getInstance, getStrength, setDecomposition, setStrength
 
Methods inherited from class java.lang.Object
finalize, getClass, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

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
Method Detail

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

getCollationElementIterator

public CollationElementIterator getCollationElementIterator(CharacterIterator 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
Following copied from class: java.text.Collator
Parameters:
source - the source string.
target - the target string.
Returns:
Returns an integer value. Value is less than zero if source is less than target, value is zero if source and target are equal, value is greater than zero if source is greater than target.
See Also:
CollationKey, Collator.getCollationKey(java.lang.String)

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
Following copied from class: java.text.Collator
Parameters:
source - the string to be transformed into a collation key.
Returns:
the CollationKey for the given String based on this Collator's collation rules. If the source String is null, a null CollationKey is returned.
See Also:
CollationKey, Collator.compare(java.lang.String, java.lang.String)

clone

public Object clone()
Standard override; no change in semantics.
Overrides:
clone in class Collator
Following copied from class: java.lang.Object
Returns:
a clone of this instance.
Throws:
CloneNotSupportedException - if the object's class does not support the Cloneable interface. Subclasses that override the clone method can also throw this exception to indicate that an instance cannot be cloned.
OutOfMemoryError - if there is not enough memory.
See Also:
Cloneable

equals

public boolean equals(Object obj)
Compares the equality of two collation objects.
Overrides:
equals in class Collator
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.

hashCode

public int hashCode()
Generates the hash code for the table-based collation object
Overrides:
hashCode in class Collator
Following copied from class: java.lang.Object
Returns:
a hash code value for this object.
See Also:
Object.equals(java.lang.Object), Hashtable

JavaTM 2 Platform
Std. Ed. v1.3

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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