Comparing Strings

Customizing Collation Rules

In the previous section, we discussed how to perform string comparisons using the predefined rules for a locale. These collation rules determine the sort order of strings. If the predefined collation rules do not meet your needs, you can design your own rules and assign them to a RuleBasedCollator object.

Customized collation rules are contained in a String object which is passed to the RuleBasedCollator constructor. Here's a simple example:

String simpleRule = "< a < b < c < d";
RuleBasedCollator simpleCollator = new RuleBasedCollator(simpleRule);

For the simpleCollator object in the preceeding example, "a" is less than "b," which is less that "c," and so forth. The simpleCollator.compare method references these rules when comparing strings. The full syntax used to construct a collation rule is more flexible and complex than this simple example. For a full description of the syntax, please refer to the API documentation for the RuleBasedCollatorclass.

In the example that follows we'll sort a list of Spanish words with two collators. The full source code for this example is in the file called RulesDemo.java.

We start by defining our own collation rules for English and Spanish. We've decided to sort the Spanish words in the traditional manner. When sorting by the traditional rules, the letters "ch," and "ll," and their upper case equivalents, each have their own position in the sort order. These character pairs compare as if they were one character. For example, "ch" sorts as a single letter, following "cz" in the sort order. Note how the rules for the two collators differ:

String englishRules = 
   ("< 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");
  
String smallnTilde  = new String("\u00F1");
String capitalNTilde = new String("\u00D1");
  
String traditionalSpanishRules = 
   ("< a,A < b,B < c,C " +
    "< ch, cH, Ch, CH "  +
    "< d,D < e,E < f,F " +
    "< g,G < h,H < i,I < j,J < k,K < l,L " +
    "< ll, lL, Ll, LL "  +
    "< m,M < n,N " +
    "< " + smallnTilde + "," + capitalNTilde + " " +
    "< o,O < p,P < q,Q < r,R " +
    "< s,S < t,T < u,U < v,V < w,W < x,X " +
    "< y,Y < z,Z");

In the following lines of code, we create the collators and invoke our sort routine:

try {
   RuleBasedCollator enCollator = 
      new RuleBasedCollator(englishRules);
   RuleBasedCollator spCollator = 
      new RuleBasedCollator(traditionalSpanishRules);

   sortStrings(enCollator, words);
   printStrings(words);
     
   System.out.println();
     
   sortStrings(spCollator, words);
   printStrings(words);
}
catch (ParseException pe) {
   System.out.println("Parse exception for rules");
}

The sort routine, called sortStrings, is generic. It will sort any array of words according to the rules of any Collator object:

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++) {
            // Compare elements of the words array
            if( collator.compare(words[i], words[j] ) > 0 ) {
                // Swap words[i] and words[j] 
                tmp = words[i];
                words[i] = words[j];
                words[j] = tmp;
            }
        }
    }
}

When sorted with the English collation rules, the array of words is as follows:

chalina
curioso
llama
luz

Compare the preceeding list with the following, which is sorted according to traditional Spanish rules of collation:

curioso
chalina
luz
llama

Comparing Strings