Detecting Text Boundaries

Line Boundaries

Applications that format text or perform line wrapping must locate potential line breaks. You can find these line breaks, or boundaries, with a BreakIterator that has been created with the getLineInstance method:

BreakIterator lineIterator = 
   BreakIterator.getLineInstance(currentLocale);

This BreakIterator determines the positions in a string where text can break to continue on the next line. The positions detected by the BreakIterator are potential line breaks. The actual line breaks displayed on the screen may not be the same.

In the two examples that follow, we'll use the markBoundaries method to view the line boundaries detected by a BreakIterator. This method prints marks line boundaries by printing carets ('^') beneath the target string.

According to a BreakIterator, a line boundary occurs after the end of a sequence of whitespace characters (space, tab, newline). In the following example, note that we can break the line at any of the boundaries detected:

She stopped.  She said, "Hello there," and then went on.
^   ^         ^   ^     ^      ^       ^   ^    ^    ^  ^

Potential line breaks also occur immediately after a hyphen:

There are twenty-four hours in a day.
^     ^   ^      ^    ^     ^  ^ ^   ^

In the next example, we break a long string of text into fixed length lines with a method called formatLines. We use a BreakIterator to locate the potential line breaks. To break a line, we execute a System.out.println() whenever the length of the current line reaches the maxLength parameter. The formatLines method is short, simple, and thanks to the BreakIterator, locale-independent. Here is the source code:

static void formatLines(String target, int maxLength, 
                        Locale currentLocale) {

   BreakIterator boundary = BreakIterator.getLineInstance(currentLocale);
   boundary.setText(target);
   int start = boundary.first();
   int end = boundary.next();
   int lineLength = 0;

   while (end != BreakIterator.DONE) {
      String word = target.substring(start,end);
      lineLength = lineLength + word.length();
      if (lineLength >= maxLength) {
         System.out.println();
         lineLength = word.length();
      }
      System.out.print(word);
      start = end;
      end = boundary.next();
   }
} 

In the BreakIteratorDemo.java program, we invoke formatLines as follows:

String moreText = "She said, \"Hello there,\" and then " +
                  "went on down the street.  When she stopped " +
                  "to look at the fur coats in a shop window, " +
                  "her dog growled.  \"Sorry Jake,\" she said. " +
                  " \"I didn't know you would take it personally.\"";

formatLines(moreText, 30, currentLocale);

The output from this call to formatLines is:

She said, "Hello there," and 
then went on down the 
street.  When she stopped to 
look at the fur coats in a 
shop window, her dog 
growled.  "Sorry Jake," she 
said.  "I didn't know you 
would take it personally."

Detecting Text Boundaries