Collator performs locale-sensitive string comparison. A concrete subclass, RuleBasedCollator, allows customization of the collation ordering by the use of rule sets.

A Collator is thread-safe only when frozen. See isFrozen() and Freezable.

Following the Unicode Consortium's specifications for the Unicode Collation Algorithm (UCA), there are 5 different levels of strength used in comparisons:

• PRIMARY strength: Typically, this is used to denote differences between base characters (for example, "a" < "b"). It is the strongest difference. For example, dictionaries are divided into different sections by base character.
• SECONDARY strength: Accents in the characters are considered secondary differences (for example, "as" < "às" < "at"). Other differences between letters can also be considered secondary differences, depending on the language. A secondary difference is ignored when there is a primary difference anywhere in the strings.
• TERTIARY strength: Upper and lower case differences in characters are distinguished at tertiary strength (for example, "ao" < "Ao" < "aò"). In addition, a variant of a letter differs from the base form on the tertiary strength (such as "A" and "Ⓐ"). Another example is the difference between large and small Kana. A tertiary difference is ignored when there is a primary or secondary difference anywhere in the strings.
• QUATERNARY strength: When punctuation is ignored (see Ignoring Punctuations in the User Guide) at PRIMARY to TERTIARY strength, an additional strength level can be used to distinguish words with and without punctuation (for example, "ab" < "a-b" < "aB"). This difference is ignored when there is a PRIMARY, SECONDARY or TERTIARY difference. The QUATERNARY strength should only be used if ignoring punctuation is required.
• IDENTICAL strength: When all other strengths are equal, the IDENTICAL strength is used as a tiebreaker. The Unicode code point values of the NFD form of each string are compared, just in case there is no difference. For example, Hebrew cantillation marks are only distinguished at this strength. This strength should be used sparingly, as only code point value differences between two strings is an extremely rare occurrence. Using this strength substantially decreases the performance for both comparison and collation key generation APIs. This strength also increases the size of the collation key.

Unlike the JDK, ICU4J's Collator deals only with 2 decomposition modes, the canonical decomposition mode and one that does not use any decomposition. The compatibility decomposition mode, java.text.Collator.FULL_DECOMPOSITION is not supported here. If the canonical decomposition mode is set, the Collator handles un-normalized text properly, producing the same results as if the text were normalized in NFD. If canonical decomposition is turned off, it is the user's responsibility to ensure that all text is already in the appropriate form before performing a comparison or before getting a CollationKey.

For more information about the collation service see the User Guide.

Examples of use

// 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");
}

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();
myCollator.setDecomposition(NO_DECOMPOSITION);
if (myCollator.compare("à\u0325", "a\u0325̀") != 0) {
    System.out.println("à\u0325 is not equals to a\u0325̀ without decomposition");
    myCollator.setDecomposition(CANONICAL_DECOMPOSITION);
    if (myCollator.compare("à\u0325", "a\u0325̀") != 0) {
        System.out.println("Error: à\u0325 should be equals to a\u0325̀ with decomposition");
    }
    else {
        System.out.println("à\u0325 is equals to a\u0325̀ with decomposition");
    }
}
else {
    System.out.println("Error: à\u0325 should be not equals to a\u0325̀ without decomposition");
}