Java™ Platform
Standard Ed. 6

java.text
Class BreakIterator

java.lang.Object
  extended by java.text.BreakIterator
All Implemented Interfaces:
Cloneable

public abstract class BreakIterator
extends Object
implements Cloneable

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.)

See Also:
CharacterIterator

Field Summary
static int DONE
          DONE is returned by previous(), next(), next(int), preceding(int) and following(int) when either the first or last text boundary has been reached.
 
Constructor Summary
protected BreakIterator()
          Constructor.
 
Method Summary
 Object clone()
          Create a copy of this iterator
abstract  int current()
          Returns character index of the text boundary that was most recently returned by next(), next(int), previous(), first(), last(), following(int) or preceding(int).
abstract  int first()
          Returns the first boundary.
abstract  int following(int offset)
          Returns the first boundary following the specified character offset.
static Locale[] getAvailableLocales()
          Returns an array of all locales for which the get*Instance methods of this class can return localized instances.
static BreakIterator getCharacterInstance()
          Returns a new BreakIterator instance for character breaks for the default locale.
static BreakIterator getCharacterInstance(Locale locale)
          Returns a new BreakIterator instance for character breaks for the given locale.
static BreakIterator getLineInstance()
          Returns a new BreakIterator instance for line breaks for the default locale.
static BreakIterator getLineInstance(Locale locale)
          Returns a new BreakIterator instance for line breaks for the given locale.
static BreakIterator getSentenceInstance()
          Returns a new BreakIterator instance for sentence breaks for the default locale.
static BreakIterator getSentenceInstance(Locale locale)
          Returns a new BreakIterator instance for sentence breaks for the given locale.
abstract  CharacterIterator getText()
          Get the text being scanned
static BreakIterator getWordInstance()
          Returns a new BreakIterator instance for word breaks for the default locale.
static BreakIterator getWordInstance(Locale locale)
          Returns a new BreakIterator instance for word breaks for the given locale.
 boolean isBoundary(int offset)
          Returns true if the specified character offset is a text boundary.
abstract  int last()
          Returns the last boundary.
abstract  int next()
          Returns the boundary following the current boundary.
abstract  int next(int n)
          Returns the nth boundary from the current boundary.
 int preceding(int offset)
          Returns the last boundary preceding the specified character offset.
abstract  int previous()
          Returns the boundary preceding the current boundary.
abstract  void setText(CharacterIterator newText)
          Set a new text for scanning.
 void setText(String newText)
          Set a new text string to be scanned.
 
Methods inherited from class java.lang.Object
equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

DONE

public static final int DONE
DONE is returned by previous(), next(), next(int), preceding(int) and following(int) when either the first or last text boundary has been reached.

See Also:
Constant Field Values
Constructor Detail

BreakIterator

protected BreakIterator()
Constructor. BreakIterator is stateless and has no default behavior.

Method Detail

clone

public Object clone()
Create a copy of this iterator

Overrides:
clone in class Object
Returns:
A copy of this
See Also:
Cloneable

first

public abstract int first()
Returns the first boundary. The iterator's current position is set to the first text boundary.

Returns:
The character index of the first text boundary.

last

public abstract int last()
Returns the last boundary. The iterator's current position is set to the last text boundary.

Returns:
The character index of the last text boundary.

next

public abstract int next(int n)
Returns the nth boundary from the current boundary. If either the first or last text boundary has been reached, it returns BreakIterator.DONE and the current position is set to either the first or last text boundary depending on which one is reached. Otherwise, the iterator's current position is set to the new boundary. For example, if the iterator's current position is the mth text boundary and three more boundaries exist from the current boundary to the last text boundary, the next(2) call will return m + 2. The new text position is set to the (m + 2)th text boundary. A next(4) call would return BreakIterator.DONE and the last text boundary would become the new text position.

Parameters:
n - which boundary to return. A value of 0 does nothing. Negative values move to previous boundaries and positive values move to later boundaries.
Returns:
The character index of the nth boundary from the current position or BreakIterator.DONE if either first or last text boundary has been reached.

next

public abstract int next()
Returns the boundary following the current boundary. If the current boundary is the last text boundary, it returns BreakIterator.DONE and the iterator's current position is unchanged. Otherwise, the iterator's current position is set to the boundary following the current boundary.

Returns:
The character index of the next text boundary or BreakIterator.DONE if the current boundary is the last text boundary. Equivalent to next(1).
See Also:
next(int)

previous

public abstract int previous()
Returns the boundary preceding the current boundary. If the current boundary is the first text boundary, it returns BreakIterator.DONE and the iterator's current position is unchanged. Otherwise, the iterator's current position is set to the boundary preceding the current boundary.

Returns:
The character index of the previous text boundary or BreakIterator.DONE if the current boundary is the first text boundary.

following

public abstract int following(int offset)
Returns the first boundary following the specified character offset. If the specified offset equals to the last text boundary, it returns BreakIterator.DONE and the iterator's current position is unchanged. Otherwise, the iterator's current position is set to the returned boundary. The value returned is always greater than the offset or the value BreakIterator.DONE.

Parameters:
offset - the character offset to begin scanning.
Returns:
The first boundary after the specified offset or BreakIterator.DONE if the last text boundary is passed in as the offset.
Throws:
IllegalArgumentException - if the specified offset is less than the first text boundary or greater than the last text boundary.

preceding

public int preceding(int offset)
Returns the last boundary preceding the specified character offset. If the specified offset equals to the first text boundary, it returns BreakIterator.DONE and the iterator's current position is unchanged. Otherwise, the iterator's current position is set to the returned boundary. The value returned is always less than the offset or the value BreakIterator.DONE.

Parameters:
offset - the characater offset to begin scanning.
Returns:
The last boundary before the specified offset or BreakIterator.DONE if the first text boundary is passed in as the offset.
Throws:
IllegalArgumentException - if the specified offset is less than the first text boundary or greater than the last text boundary.
Since:
1.2

isBoundary

public boolean isBoundary(int offset)
Returns true if the specified character offset is a text boundary.

Parameters:
offset - the character offset to check.
Returns:
true if "offset" is a boundary position, false otherwise.
Since:
1.2

current

public abstract int current()
Returns character index of the text boundary that was most recently returned by next(), next(int), previous(), first(), last(), following(int) or preceding(int). If any of these methods returns BreakIterator.DONE because either first or last text boundary has been reached, it returns the first or last text boundary depending on which one is reached.

Returns:
The text boundary returned from the above methods, first or last text boundary.
See Also:
next(), next(int), previous(), first(), last(), following(int), preceding(int)

getText

public abstract CharacterIterator getText()
Get the text being scanned

Returns:
the text being scanned

setText

public void setText(String newText)
Set a new text string to be scanned. The current scan position is reset to first().

Parameters:
newText - new text to scan.

setText

public abstract void setText(CharacterIterator newText)
Set a new text for scanning. The current scan position is reset to first().

Parameters:
newText - new text to scan.

getWordInstance

public static BreakIterator getWordInstance()
Returns a new BreakIterator instance for word breaks for the default locale.

Returns:
A break iterator for word breaks

getWordInstance

public static BreakIterator getWordInstance(Locale locale)
Returns a new BreakIterator instance for word breaks for the given locale.

Parameters:
locale - the desired locale
Returns:
A break iterator for word breaks
Throws:
NullPointerException - if locale is null

getLineInstance

public static BreakIterator getLineInstance()
Returns a new BreakIterator instance for line breaks for the default locale.

Returns:
A break iterator for line breaks

getLineInstance

public static BreakIterator getLineInstance(Locale locale)
Returns a new BreakIterator instance for line breaks for the given locale.

Parameters:
locale - the desired locale
Returns:
A break iterator for line breaks
Throws:
NullPointerException - if locale is null

getCharacterInstance

public static BreakIterator getCharacterInstance()
Returns a new BreakIterator instance for character breaks for the default locale.

Returns:
A break iterator for character breaks

getCharacterInstance

public static BreakIterator getCharacterInstance(Locale locale)
Returns a new BreakIterator instance for character breaks for the given locale.

Parameters:
locale - the desired locale
Returns:
A break iterator for character breaks
Throws:
NullPointerException - if locale is null

getSentenceInstance

public static BreakIterator getSentenceInstance()
Returns a new BreakIterator instance for sentence breaks for the default locale.

Returns:
A break iterator for sentence breaks

getSentenceInstance

public static BreakIterator getSentenceInstance(Locale locale)
Returns a new BreakIterator instance for sentence breaks for the given locale.

Parameters:
locale - the desired locale
Returns:
A break iterator for sentence breaks
Throws:
NullPointerException - if locale is null

getAvailableLocales

public static Locale[] getAvailableLocales()
Returns an array of all locales for which the get*Instance methods of this class can return localized instances. The returned array represents the union of locales supported by the Java runtime and by installed BreakIteratorProvider implementations. It must contain at least a Locale instance equal to Locale.US.

Returns:
An array of locales for which localized BreakIterator instances are available.

Java™ Platform
Standard Ed. 6

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