RuleBasedCollator

Class Overview

A concrete subclass of Collator. It is based on the ICU RuleBasedCollator which implements the CLDR and Unicode collation algorithms.

Most of the time, you create a Collator instance for a Locale by calling the getInstance() factory method. You can construct a RuleBasedCollator if you need a custom sort order.

The root collator's sort order is the CLDR root collation order which in turn is the Unicode default sort order with a few modifications. A RuleBasedCollator is built from a rule String which changes the sort order of some characters and strings relative to the default order.

A rule string usually contains one or more rule chains. A rule chain consists of a reset followed by one or more rules. The reset anchors the following rules in the default sort order. The rules change the order of the their characters and strings relative to the reset point.

A reset is an ampersand & followed by one or more characters for the reset position. A rule is a relation operator, which specifies the level of difference, also followed by one or more characters. A multi-character rule creates a "contraction". A multi-character reset position usually creates "expansions".

For example, the following rules make "ä" sort with a diacritic-like (secondary) difference from "ae" (like in German phonebook sorting), and make "å" and "aa" sort as a base letter (primary) after "z" (like in Danish). Uppercase forms sort with a case-like (tertiary) difference after their lowercase forms.

 &AE<<ä <<<Ä
 &z<å<<<Å<<<aa<<<Aa<<<AA
 

For details see

• CLDR Collation Rule Syntax
• ICU User Guide Collation Customization

Note: earlier versions of RuleBasedCollator up to and including Android 4.4 (KitKat) allowed the omission of the reset from the first rule chain. This was interpreted as an implied reset after the last non-Han script in the default order. However, this is not a useful reset position, except for large tailorings of Han characters themselves. Starting with the CLDR 24 collation specification and the ICU 53 implementation, the initial reset is required.

If the rule string does not follow the syntax, then RuleBasedCollator throws a ParseException.

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.