Detecting Text Boundaries

Character Boundaries

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

BreakIterator characterIterator = 
   BreakIterator.getCharacterInstance(currentLocale);

This type of BreakIterator detects boundaries between user characters, not just Unicode characters. User characters differ with language, but the BreakIterator class can recognize these differences because it is locale-sensitive. 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 upon the Arabic language for a more realistic example.

In Arabic the word for house is:

Although this word contains three user characters, it is composed by 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. In Arabic diacritics are required, because they can alter the meanings of words. The diacritics in our example are non-spacing characters since they appear above the base characters. In an Arabic word processor, we cannot move the cursor on the screen once for every Unicode character in the string. Instead, we must move it once for every user character, which may be composed by more than one Unicode character. Therefore, we must use a BreakIterator to scan the user characters in the string.

In our sample program, BreakIteratorDemo.java, we create a BreakIterator to scan Arabic characters. Then we pass this BreakIterator, along with the String object we 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 we assign a particular string to the BreakIterator with the setText method. We retrieve the first character boundary with the first method, then invoke 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. The positions of the diacritics (1, 3, 5) are not listed:

0
2
4
6

Detecting Text Boundaries