The
BreakIterator class implements methods for finding
the location of boundaries in text. Instances of
BreakIterator
maintain a current position and scan over text
returning the index of characters where boundaries occur.
Internally,
BreakIterator scans text using a
CharacterIterator, and is thus able to scan text held
by any object implementing that protocol. A
StringCharacterIterator
is used to scan
String objects passed to
setText.
You use the factory methods provided by this class to create
instances of various types of break iterators. In particular,
use getWordIterator, getLineIterator,
getSentenceIterator, and getCharacterIterator
to create BreakIterators that perform
word, line, sentence, and character boundary analysis respectively.
A single BreakIterator can work only on one unit
(word, line, sentence, and so on). You must use a different iterator
for each unit boundary analysis you wish to perform.
Line boundary analysis determines where a text string can be
broken when line-wrapping. The mechanism correctly handles
punctuation and hyphenated words. Actual line breaking needs
to also consider the available line width and is handled by
higher-level software.
Sentence boundary analysis allows selection with correct interpretation
of periods within numbers and abbreviations, and trailing punctuation
marks such as quotation marks and parentheses.
Word boundary analysis is used by search and replace functions, as
well as within text editing applications that allow the user to
select words with a double click. Word selection provides correct
interpretation of punctuation marks within and following
words. Characters that are not part of a word, such as symbols
or punctuation marks, have word-breaks on both sides.
Character boundary analysis allows users to interact with characters
as they expect to, for example, when moving the cursor through a text
string. Character boundary analysis provides correct navigation
through character strings, regardless of how the character is stored.
The boundaries returned may be those of supplementary characters,
combining character sequences, or ligature clusters.
For example, an accented character might be stored as a base character
and a diacritical mark. What users consider to be a character can
differ between languages.
The BreakIterator instances returned by the factory methods
of this class are intended for use with natural languages only, not for
programming language text. It is however possible to define subclasses
that tokenize a programming language.
Examples:
Creating and using text boundaries:
public static void main(String args[]) {
if (args.length == 1) {
String stringToExamine = args[0];
//print each word in order
BreakIterator boundary = BreakIterator.getWordInstance();
boundary.setText(stringToExamine);
printEachForward(boundary, stringToExamine);
//print each sentence in reverse order
boundary = BreakIterator.getSentenceInstance(Locale.US);
boundary.setText(stringToExamine);
printEachBackward(boundary, stringToExamine);
printFirst(boundary, stringToExamine);
printLast(boundary, stringToExamine);
}
}
Print each element in order:
public static void printEachForward(BreakIterator boundary, String source) {
int start = boundary.first();
for (int end = boundary.next();
end != BreakIterator.DONE;
start = end, end = boundary.next()) {
System.out.println(source.substring(start,end));
}
}
Print each element in reverse order:
public static void printEachBackward(BreakIterator boundary, String source) {
int end = boundary.last();
for (int start = boundary.previous();
start != BreakIterator.DONE;
end = start, start = boundary.previous()) {
System.out.println(source.substring(start,end));
}
}
Print first element:
public static void printFirst(BreakIterator boundary, String source) {
int start = boundary.first();
int end = boundary.next();
System.out.println(source.substring(start,end));
}
Print last element:
public static void printLast(BreakIterator boundary, String source) {
int end = boundary.last();
int start = boundary.previous();
System.out.println(source.substring(start,end));
}
Print the element at a specified position:
public static void printAt(BreakIterator boundary, int pos, String source) {
int end = boundary.following(pos);
int start = boundary.previous();
System.out.println(source.substring(start,end));
}
Find the next word:
public static int nextWordStartAfter(int pos, String text) {
BreakIterator wb = BreakIterator.getWordInstance();
wb.setText(text);
int last = wb.following(pos);
int current = wb.next();
while (current != BreakIterator.DONE) {
for (int p = last; p < current; p++) {
if (Character.isLetter(text.codePointAt(p)))
return last;
}
last = current;
current = wb.next();
}
return BreakIterator.DONE;
}
(The iterator returned by BreakIterator.getWordInstance() is unique in that
the break positions it returns don't represent both the start and end of the
thing being iterated over. That is, a sentence-break iterator returns breaks
that each represent the end of one sentence and the beginning of the next.
With the word-break iterator, the characters between two boundaries might be a
word, or they might be the punctuation or whitespace between two words. The
above code uses a simple heuristic to determine which boundary is the beginning
of a word: If the characters between this boundary and the next boundary
include at least one letter (this can be an alphabetical letter, a CJK ideograph,
a Hangul syllable, a Kana character, etc.), then the text between this boundary
and the next is a word; otherwise, it's the material between words.)
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