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Character Boundaries

You need to locate character boundaries if your application allows the end user to highlight individual characters or to move a cursor through text one character at a time. To create a BreakIterator that locates character boundaries, you invoke the getCharacterInstance method, as follows:

BreakIterator characterIterator =
         BreakIterator.getCharacterInstance(currentLocale);

This type of BreakIterator detects boundaries between user characters, not just Unicode characters.

A user character may be composed of more than one Unicode character. For example, the user character ü can be composed by combining the Unicode characters \u0075 (u) and \u00a8 (¨). This isn't the best example, however, because the character ü may also be represented by the single Unicode character \u00fc. We'll draw on the Arabic language for a more realistic example.

In Arabic the word for house is:

This word contains three user characters, but it is composed of the following six Unicode characters:

String house = "\u0628" + "\u064e" + "\u064a" + 
	       "\u0652" + "\u067a" + "\u064f";

The Unicode characters at positions 1, 3, and 5 in the house string are diacritics. Arabic requires diacritics because they can alter the meanings of words. The diacritics in the example are nonspacing characters, since they appear above the base characters. In an Arabic word processor you cannot move the cursor on the screen once for every Unicode character in the string. Instead you must move it once for every user character, which may be composed by more than one Unicode character. Therefore you must use a BreakIterator to scan the user characters in the string.

The sample program BreakIteratorDemo, creates a BreakIterator to scan Arabic characters. The program passes this BreakIterator, along with the String object created previously, to a method named listPositions:

BreakIterator arCharIterator =
		BreakIterator.getCharacterInstance(new Locale ("ar","SA"));

listPositions (house, arCharIterator);

The listPositions method uses a BreakIterator to locate the character boundaries in the string. Note that the BreakIteratorDemo assigns a particular string to the BreakIterator with the setText method. The program retrieves the first character boundary with the first method and then invokes the next method until the constant BreakIterator.DONE is returned. The code for this routine is as follows:

static void listPositions(String target, BreakIterator iterator) {
		
    iterator.setText(target);
    int boundary = iterator.first();

    while (boundary != BreakIterator.DONE) {
        System.out.println (boundary);
        boundary = iterator.next();
    }
}

The listPositions method prints out the following boundary positions for the user characters in the string house. Note that the positions of the diacritics (1, 3, 5) are not listed:

0
2
4
6

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