| 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) :
- If a special collation rule controlled by a <modifier> is
specified it applies to the whole collator object.
- All non-mentioned 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 one of three forms:
<modifier>
<relation> <text-argument>
<reset> <text-argument>
The definitions of the rule elements is as follows:
- 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
(e.g. ampersand => '&'). Note that unquoted white space characters
are ignored; e.g.
b c is treated as bc.
- Modifier: There are currently two modifiers that
turn on special collation rules.
- '@' : Turns on backwards sorting of accents (secondary
differences), as in French.
- '!' : Turns on Thai/Lao vowel-consonant swapping. If this
rule is in force when a Thai vowel of the range
\U0E40-\U0E44 precedes a Thai consonant of the range
\U0E01-\U0E2E OR a Lao vowel of the range \U0EC0-\U0EC4
precedes a Lao consonant of the range \U0E81-\U0EAE then
the vowel is placed after the consonant for collation
purposes.
'@' : 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
a-umlaut is treated as though it expanded to two characters
(expressed as "a,A < b,B ... &ae;\u00e3&AE;\u00c3").
[\u00e3 and \u00c3 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.
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
< \u00E5=a\u030A,\u00C5=A\u030A
;aa,AA< \u00E6,\u00C6< \u00F8,\u00D8"
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" +
"< \u00E5=a\u030A,\u00C5=A\u030A" +
";aa,AA< \u00E6,\u00C6< \u00F8,\u00D8";
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 = "=\u0301;\u0300;\u0302;\u0308" // main accents
+ ";\u0327;\u0303;\u0304;\u0305" // main accents
+ ";\u0306;\u0307;\u0309;\u030A" // main accents
+ ";\u030B;\u030C;\u030D;\u030E" // main accents
+ ";\u030F;\u0310;\u0311;\u0312" // main accents
+ "< a , A ; ae, AE ; \u00e6 , \u00c6"
+ "< b , B < c, C < e, E & C < d, D";
// change the order of accent characters
String addOn = "& \u0300 ; \u0308 ; \u0302";
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 \u2212
String jaString = "& \u2212 < \u3041, \u3042 < \u3043, \u3044";
RuleBasedCollator myJapaneseCollator = new
RuleBasedCollator(en_USCollator.getRules() + jaString);
See Also: Collator
See Also: CollationElementIterator
version:
1.25 07/24/98
author:
Helena Shih, Laura Werner, Richard Gillam |
