Trail: Internationalization
Lesson: Working with Text
Section: Comparing Strings
Performing Locale-Independent Comparisons
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Performing Locale-Independent Comparisons

Collation rules define the sort sequence of strings. These rules vary with locale, because various natural languages sort words differently. You can use the predefined collation rules provided by the Collator class to sort strings in a locale-independent manner.

To instantiate the Collator class invoke the getInstance method. Usually, you create a Collator for the default Locale, as in the following example:

Collator myDefaultCollator = Collator.getInstance();

You can also specify a particular Locale when you create a Collator, as follows:

Collator myFrenchCollator = Collator.getInstance(Locale.FRENCH);

The getInstance method returns a RuleBasedCollator, which is a concrete subclass of Collator. The RuleBasedCollator contains a set of rules that determine the sort order of strings for the locale you specify. These rules are predefined for each locale. Because the rules are encapsulated within the RuleBasedCollator, your program won't need special routines to deal with the way collation rules vary with language.

You invoke the Collator.compare method to perform a locale-independent string comparison. The compare method returns an integer less than, equal to, or greater than zero when the first string argument is less than, equal to, or greater than the second string argument. The following table contains some sample calls to Collator.compare:

Example Return Value Explanation
myCollator.compare("abc", "def") -1 "abc" is less than "def"
myCollator.compare("rtf", "rtf") 0 the two strings are equal
myCollator.compare("xyz", "abc") 1 "xyz" is greater than "abc"

You use the compare method when performing sort operations. The sample program called CollatorDemo uses the compare method to sort an array of English and French words. This program shows what can happen when you sort the same list of words with two different collators:

Collator fr_FRCollator = Collator.getInstance(new Locale("fr","FR"));
Collator en_USCollator = Collator.getInstance(new Locale("en","US"));

The method for sorting, called sortStrings, can be used with any Collator. Notice that the sortStrings method invokes the compare method:

public static void sortStrings(Collator collator, String[] words) {
    String tmp;
    for (int i = 0; i < words.length; i++) {
        for (int j = i + 1; j < words.length; j++) { 
            if (collator.compare(words[i], words[j]) > 0) {
                tmp = words[i];
                words[i] = words[j];
                words[j] = tmp;
            }
        }
    }
}

The English Collator sorts the words as follows:

peach
péché
pêche
sin

According to the collation rules of the French language, the preceding list is in the wrong order. In French péché should follow pêche in a sorted list. The French Collator sorts the array of words correctly, as follows:

peach
pêche
péché
sin

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