BreakIterator

Class Overview

Locates boundaries in text. This class defines a protocol for objects that break up a piece of natural-language text according to a set of criteria. Instances or subclasses of BreakIterator can be provided, for example, to break a piece of text into words, sentences, or logical characters according to the conventions of some language or group of languages. We provide four built-in types of BreakIterator:

• getSentenceInstance() returns a BreakIterator that locates boundaries between sentences. This is useful for triple-click selection, for example.
• getWordInstance() returns a BreakIterator that locates boundaries between words. This is useful for double-click selection or "find whole words" searches. This type of BreakIterator makes sure there is a boundary position at the beginning and end of each legal word (numbers count as words, too). Whitespace and punctuation are kept separate from real words.
• getLineInstance() returns a BreakIterator that locates positions where it is legal for a text editor to wrap lines. This is similar to word breaking, but not the same: punctuation and whitespace are generally kept with words (you don't want a line to start with whitespace, for example), and some special characters can force a position to be considered a line break position or prevent a position from being a line break position.
• getCharacterInstance() returns a BreakIterator that locates boundaries between logical characters. Because of the structure of the Unicode encoding, a logical character may be stored internally as more than one Unicode code point. (A with an umlaut may be stored as an a followed by a separate combining umlaut character, for example, but the user still thinks of it as one character.) This iterator allows various processes (especially text editors) to treat as characters the units of text that a user would think of as characters, rather than the units of text that the computer sees as "characters".

BreakIterator's interface follows an "iterator" model (hence the name), meaning it has a concept of a "current position" and methods like first(), last(), next(), and previous() that update the current position. All BreakIterators uphold the following invariants:

• The beginning and end of the text are always treated as boundary positions.
• The current position of the iterator is always a boundary position (random- access methods move the iterator to the nearest boundary position before or after the specified position, not to the specified position).
• DONE is used as a flag to indicate when iteration has stopped. DONE is only returned when the current position is the end of the text and the user calls next(), or when the current position is the beginning of the text and the user calls previous().
• Break positions are numbered by the positions of the characters that follow them. Thus, under normal circumstances, the position before the first character is 0, the position after the first character is 1, and the position after the last character is 1 plus the length of the string.
• The client can change the position of an iterator, or the text it analyzes, at will, but cannot change the behavior. If the user wants different behavior, he must instantiate a new iterator.

BreakIterator accesses the text it analyzes through a CharacterIterator, which makes it possible to use BreakIterator to analyze text in any text-storage vehicle that provides a CharacterIterator interface.

Note: Some types of BreakIterator can take a long time to create, and instances of BreakIterator are not currently cached by the system. For optimal performance, keep instances of BreakIterator around as long as it makes sense. For example, when word-wrapping a document, don't create and destroy a new BreakIterator for each line. Create one break iterator for the whole document (or whatever stretch of text you're wrapping) and use it to do the whole job of wrapping the text.

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 the 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 the 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.charAt(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.)

Summary

Constants
int	DONE	This constant is returned by iterate methods like previous() or next() if they have returned all valid boundaries.

Protected Constructors
	BreakIterator() Default constructor, for use by subclasses.

Public Methods
Object	clone() Creates a copy of this iterator, all status information including the current position are kept the same.
abstract int	current() Returns this iterator's current position.
abstract int	first() Sets this iterator's current position to the first boundary and returns that position.
abstract int	following(int offset) Sets the position of the first boundary to the one following the given offset and returns this position.
static Locale[]	getAvailableLocales() Returns an array of locales for which custom BreakIterator instances are available.
static BreakIterator	getCharacterInstance(Locale where) Returns a new instance of BreakIterator to iterate over characters using the given locale.
static BreakIterator	getCharacterInstance() Returns a new instance of BreakIterator to iterate over characters using the user's default locale.
static BreakIterator	getLineInstance(Locale where) Returns a new instance of BreakIterator to iterate over line breaks using the given locale.
static BreakIterator	getLineInstance() Returns a new instance of {BreakIterator to iterate over line breaks using the user's default locale.
static BreakIterator	getSentenceInstance(Locale where) Returns a new instance of BreakIterator to iterate over sentence-breaks using the given locale.
static BreakIterator	getSentenceInstance() Returns a new instance of BreakIterator to iterate over sentence-breaks using the default locale.
abstract CharacterIterator	getText() Returns a CharacterIterator which represents the text being analyzed.
static BreakIterator	getWordInstance() Returns a new instance of BreakIterator to iterate over word-breaks using the default locale.
static BreakIterator	getWordInstance(Locale where) Returns a new instance of BreakIterator to iterate over word-breaks using the given locale.
boolean	isBoundary(int offset) Indicates whether the given offset is a boundary position.
abstract int	last() Sets this iterator's current position to the last boundary and returns that position.
abstract int	next(int n) Sets this iterator's current position to the next boundary after the given position, and returns that position.
abstract int	next() Sets this iterator's current position to the next boundary after the current position, and returns this position.
int	preceding(int offset) Returns the position of last boundary preceding the given offset, and sets the current position to the returned value, or DONE if the given offset specifies the starting position.
abstract int	previous() Sets this iterator's current position to the previous boundary before the current position and returns that position.
void	setText(String newText) Sets the new text string to be analyzed, the current position will be reset to the beginning of this new string, and the old string will be lost.
abstract void	setText(CharacterIterator newText) Sets the new text to be analyzed by the given CharacterIterator.

[Expand] Inherited Methods

From class java.lang.Object Object clone() Creates and returns a copy of this Object. boolean equals(Object o) Compares this instance with the specified object and indicates if they are equal. void finalize() Called before the object's memory is reclaimed by the VM. final Class<?> getClass() Returns the unique instance of Class that represents this object's class. int hashCode() Returns an integer hash code for this object. final void notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. final void notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. String toString() Returns a string containing a concise, human-readable description of this object. final void wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object. final void wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires. final void wait(long millis) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.