Module java.base
Package java.text

Class Collator

java.lang.Object
java.text.Collator
All Implemented Interfaces:
Cloneable, Comparator<Object>
Direct Known Subclasses:
RuleBasedCollator
public abstract class Collator extends Object implements Comparator<Object>, Cloneable
The Collator class performs locale-sensitive String comparison. You use this class to build searching and sorting routines for natural language text.

Collator is an abstract base class. Subclasses implement specific collation strategies. One subclass, RuleBasedCollator, is currently provided with the Java Platform and is applicable to a wide set of languages. Other subclasses may be created to handle more specialized needs.

Like other locale-sensitive classes, you can use the static factory method, getInstance, to obtain the appropriate Collator object for a given locale. You will only need to look at the subclasses of Collator if you need to understand the details of a particular collation strategy or if you need to modify that strategy.

The following example shows how to compare two strings using the Collator for the default locale. 

// Compare two strings in the default locale
Collator myCollator = Collator.getInstance();
if (myCollator.compare("abc", "ABC") < 0) {
    System.out.println("abc is less than ABC");
} else {
    System.out.println("abc is greater than or equal to ABC");
}

You can set a Collator's strength property to determine the level of difference considered significant in comparisons. Four strengths are provided: PRIMARY, SECONDARY, TERTIARY, and IDENTICAL. The exact assignment of strengths to language features is locale dependent. For example, in Czech, "e" and "f" are considered primary differences, while "e" and "ě" are secondary differences, "e" and "E" are tertiary differences and "e" and "e" are identical. The following shows how both case and accents could be ignored for US English. 

// Get the Collator for US English and set its strength to PRIMARY
Collator usCollator = Collator.getInstance(Locale.US);
usCollator.setStrength(Collator.PRIMARY);
if (usCollator.compare("abc", "ABC") == 0) {
    System.out.println("Strings are equivalent");
}

For comparing Strings exactly once, the compare method provides the best performance. When sorting a list of Strings however, it is generally necessary to compare each String multiple times. In this case, CollationKeys provide better performance. The CollationKey class converts a String to a series of bits that can be compared bitwise against other CollationKeys. A CollationKey is created by a Collator object for a given String.

API Note:
CollationKeys from different Collators can not be compared. See the class description for CollationKey for an example using CollationKeys.
Since:
1.1
See Also:

  • Field Details

    • PRIMARY

      public static final int PRIMARY
      Collator strength value. When set, only PRIMARY differences are considered significant during comparison. The assignment of strengths to language features is locale dependent. A common example is for different base letters ("a" vs "b") to be considered a PRIMARY difference.
      See Also:

    • SECONDARY

      public static final int SECONDARY
      Collator strength value. When set, only SECONDARY and above differences are considered significant during comparison. The assignment of strengths to language features is locale dependent. A common example is for different accented forms of the same base letter ("a" vs "รค") to be considered a SECONDARY difference.
      See Also:

    • TERTIARY

      public static final int TERTIARY
      Collator strength value. When set, only TERTIARY and above differences are considered significant during comparison. The assignment of strengths to language features is locale dependent. A common example is for case differences ("a" vs "A") to be considered a TERTIARY difference.
      See Also:

    • IDENTICAL

      public static final int IDENTICAL
      Collator strength value. When set, all differences are considered significant during comparison. The assignment of strengths to language features is locale dependent. A common example is for control characters ("\u0001" vs "\u0002") to be considered equal at the PRIMARY, SECONDARY, and TERTIARY levels but different at the IDENTICAL level. Additionally, differences between pre-composed accents such as "\u00C0" (A-grave) and combining accents such as "A\u0300" (A, combining-grave) will be considered significant at the IDENTICAL level if decomposition is set to NO_DECOMPOSITION.
      See Also:

    • NO_DECOMPOSITION

      public static final int NO_DECOMPOSITION
      Decomposition mode value. With NO_DECOMPOSITION set, accented characters will not be decomposed for collation. This setting provides the fastest collation but will only produce correct results for languages that do not use accents.
      See Also:

    • CANONICAL_DECOMPOSITION

      public static final int CANONICAL_DECOMPOSITION
      Decomposition mode value. With CANONICAL_DECOMPOSITION set, characters that are canonical variants according to Unicode standard will be decomposed for collation. This should be used to get correct collation of accented characters.

      CANONICAL_DECOMPOSITION corresponds to Normalization Form D as described in Unicode Standard Annex #15: Unicode Normalization Forms.

      External Specifications
      See Also:

    • FULL_DECOMPOSITION

      public static final int FULL_DECOMPOSITION
      Decomposition mode value. With FULL_DECOMPOSITION set, both Unicode canonical variants and Unicode compatibility variants will be decomposed for collation. This causes not only accented characters to be collated, but also characters that have special formats to be collated with their norminal form. For example, the half-width and full-width ASCII and Katakana characters are then collated together. FULL_DECOMPOSITION is the most complete and therefore the slowest decomposition mode.

      FULL_DECOMPOSITION corresponds to Normalization Form KD as described in Unicode Standard Annex #15: Unicode Normalization Forms.

      External Specifications
      See Also:

  • Constructor Details

    • Collator

      protected Collator()
      Default constructor. This constructor is protected so subclasses can get access to it. Users typically create a Collator sub-class by calling the factory method getInstance.
      See Also:

  • Method Details

    • getInstance

      public static Collator getInstance()
      Gets the Collator for the current default locale. The default locale is determined by Locale.getDefault().
      Returns:
      the Collator for the default locale.(for example, en_US)
      See Also:

    • getInstance

      public static Collator getInstance(Locale desiredLocale)
      Gets the Collator for the desired locale. If the desired locale has the "ks" and/or the "kk" Unicode collation settings, this method will call setStrength(int) and/or setDecomposition(int) on the created instance, if the specified Unicode collation settings are recognized based on the following mappings:
      Strength/Decomposition mappings
      BCP 47 values for strength (ks) Collator constants for strength
      level1 PRIMARY
      level2 SECONDARY
      level3 TERTIARY*
      identic IDENTICAL
      BCP 47 values for normalization (kk) Collator constants for decomposition
      true CANONICAL_DECOMPOSITION
      false NO_DECOMPOSITION*
      Asterisk (*) denotes the default value. If the specified setting value is not recognized, the strength and/or decomposition is not overridden, as if there were no BCP 47 collation options in the desired locale.
      API Note:
      Implementations of Collator class may produce different instances based on the "co" Unicode collation identifier in the desiredLocale. For example: 
      Collator.getInstance(Locale.forLanguageTag("sv-u-co-trad"));
      may return a Collator instance with the Swedish traditional sorting, which gives 'v' and 'w' the same sorting order, while the Collator instance for the Swedish locale without "co" identifier distinguishes 'v' and 'w'.
      Parameters:
      desiredLocale - the desired locale.
      Returns:
      the Collator for the desired locale.
      External Specifications
      See Also:

    • compare

      public abstract int compare(String source, String target)
      Compares the source string to the target string according to the collation rules for this Collator. Returns an integer less than, equal to or greater than zero depending on whether the source String is less than, equal to or greater than the target string. See the Collator class description for an example of use.

      For a one time comparison, this method has the best performance. If a given String will be involved in multiple comparisons, CollationKey.compareTo has the best performance. See the Collator class description for an example using CollationKeys.

      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:

    • compare

      public int compare(Object o1, Object o2)
      Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

      This implementation merely returns compare((String)o1, (String)o2) .

      Specified by:
      compare in interface Comparator<Object>
      Parameters:
      o1 - the first object to be compared.
      o2 - the second object to be compared.
      Returns:
      a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
      Throws:
      ClassCastException - the arguments cannot be cast to Strings.
      Since:
      1.2
      See Also:

    • getCollationKey

      public abstract CollationKey getCollationKey(String source)
      Transforms the String into a series of bits that can be compared bitwise to other CollationKeys. CollationKeys provide better performance than Collator.compare when Strings are involved in multiple comparisons. See the Collator class description for an example using CollationKeys.
      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:

    • equals

      public boolean equals(String source, String target)
      Convenience method for comparing the equality of two strings based on this Collator's collation rules.
      Parameters:
      source - the source string to be compared with.
      target - the target string to be compared with.
      Returns:
      true if the strings are equal according to the collation rules. false, otherwise.
      See Also:

    • getStrength

      public int getStrength()
      Returns this Collator's strength property. The strength property determines the minimum level of difference considered significant during comparison. See the Collator class description for an example of use.
      Returns:
      this Collator's current strength property.
      See Also:

    • setStrength

      public void setStrength(int newStrength)
      Sets this Collator's strength property. The strength property determines the minimum level of difference considered significant during comparison. See the Collator class description for an example of use.
      Parameters:
      newStrength - the new strength value.
      Throws:
      IllegalArgumentException - If the new strength value is not one of PRIMARY, SECONDARY, TERTIARY or IDENTICAL.
      See Also:

    • getDecomposition

      public int getDecomposition()
      Get the decomposition mode of this Collator. Decomposition mode determines how Unicode composed characters are handled. Adjusting decomposition mode allows the user to select between faster and more complete collation behavior.

      The three values for decomposition mode are:

      • NO_DECOMPOSITION,
      • CANONICAL_DECOMPOSITION
      • FULL_DECOMPOSITION.
      See the documentation for these three constants for a description of their meaning.
      Returns:
      the decomposition mode
      See Also:

    • setDecomposition

      public void setDecomposition(int decompositionMode)
      Set the decomposition mode of this Collator. See getDecomposition for a description of decomposition mode.
      Parameters:
      decompositionMode - the new decomposition mode.
      Throws:
      IllegalArgumentException - If the given value is not a valid decomposition mode.
      See Also:

    • getAvailableLocales

      public static Locale[] getAvailableLocales()
      Returns an array of all locales for which the getInstance methods of this class can return localized instances. The returned array represents the union of locales supported by the Java runtime and by installed CollatorProvider implementations. At a minimum, the returned array must contain a Locale instance equal to Locale.ROOT and a Locale instance equal to Locale.US.
      Returns:
      An array of locales for which localized Collator instances are available.

    • clone

      public Object clone()
      Overrides Cloneable
      Overrides:
      clone in class Object
      Returns:
      a clone of this instance.
      See Also:

    • equals

      public boolean equals(Object that)
      Compares the equality of two Collators.
      Specified by:
      equals in interface Comparator<Object>
      Overrides:
      equals in class Object
      Parameters:
      that - the Collator to be compared with this.
      Returns:
      true if this Collator is the same as that Collator; false otherwise.
      See Also:

    • hashCode

      public abstract int hashCode()
      Generates the hash code for this Collator.
      Overrides:
      hashCode in class Object
      Returns:
      a hash code value for this object.
      See Also: