| Concrete implementation class for Collation.
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 >
RuleBasedCollator has the following restrictions for efficiency
(other subclasses may be used for more complex languages) :
- If a French secondary ordering is specified it applies to the whole
collator object.
- All non-mentioned Unicode characters are at the end of the collation
order.
- If a character is not located in the RuleBasedCollator, the default
Unicode Collation Algorithm (UCA) rulebased table is automatically
searched as a backup.
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, 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 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 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. 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 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 or allocated in the
default UCA table.
(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
< ?=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 Diaeresis 00A8, Macron 00AF
// Acute 00BF
+ "< a , A ; ae, AE ; ? , ?"
+ "< b , B < c, C < e, E & C < d, D";
// change the order of accent characters
String addOn = "& ?;?;?;"; // Acute 00BF, Macron 00AF, Diaeresis 00A8
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 = "& \\u30A2 , \\u30FC < \\u30C8";
RuleBasedCollator myJapaneseCollator = new
RuleBasedCollator(en_USCollator.getRules() + jaString);
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