Trail: Internationalization
Lesson: Working with Text
Section: Detecting Text Boundaries

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Line Boundaries

Applications that format text or that 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.

The two examples that follow use the markBoundaries method of BreakIteratorDemo.java to show the line boundaries detected by a BreakIterator. The markBoundaries method indicates line boundaries by printing carets (^) beneath the target string.

According to a BreakIterator, a line boundary occurs after the termination of a sequence of whitespace characters (space, tab, new line). In the following example, note that you 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.
^     ^   ^      ^    ^     ^  ^ ^   ^

The next example breaks a long string of text into fixed-length lines with a method called formatLines. This method uses a BreakIterator to locate the potential line breaks. 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();
    }
}

The BreakIteratorDemo program invokes the formatLines method 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."

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