Class BreakIterator
- All Implemented Interfaces:
Cloneable
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 getWordInstance
, getLineInstance
,
getSentenceInstance
, and getCharacterInstance
to create BreakIterator
s 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.
- Implementation Requirements:
- The default implementation of the character boundary analysis conforms to the Unicode Consortium's Extended Grapheme Cluster breaks. For more detail, refer to Grapheme Cluster Boundaries section in the Unicode Standard Annex #29.
- Implementation Note:
- The default implementations of
BreakIterator
will perform the equivalent of callingsetText("")
if the text hasn't been set by eithersetText(String)
orsetText(CharacterIterator)
and a boundary searching operation is called by theBreakIterator
instance. TheBreakIterator
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); } }
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)); } }
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)); } }
public static void printFirst(BreakIterator boundary, String source) { int start = boundary.first(); int end = boundary.next(); System.out.println(source.substring(start,end)); }
public static void printLast(BreakIterator boundary, String source) { int end = boundary.last(); int start = boundary.previous(); System.out.println(source.substring(start,end)); }
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)); }
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; }
- Since:
- 1.1
- See Also:
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Field Summary
Modifier and TypeFieldDescriptionstatic final int
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
-
Method Summary
Modifier and TypeMethodDescriptionclone()
Create a copy of this iteratorabstract 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[]
Returns an array of all locales for which theget*Instance
methods of this class can return localized instances.static BreakIterator
static BreakIterator
getCharacterInstance
(Locale locale) Returns a newBreakIterator
instance for character breaks for the given locale.static BreakIterator
static BreakIterator
getLineInstance
(Locale locale) Returns a newBreakIterator
instance for line breaks for the given locale.static BreakIterator
static BreakIterator
getSentenceInstance
(Locale locale) Returns a newBreakIterator
instance for sentence breaks for the given locale.abstract CharacterIterator
getText()
Get the text being scannedstatic BreakIterator
static BreakIterator
getWordInstance
(Locale locale) Returns a newBreakIterator
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.void
Set a new text string to be scanned.abstract void
setText
(CharacterIterator newText) Set a new text for scanning.
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Field Details
-
DONE
public static final int DONEDONE 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:
-
-
Constructor Details
-
BreakIterator
protected BreakIterator()Constructor. BreakIterator is stateless and has no default behavior.
-
-
Method Details
-
clone
-
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 returnsBreakIterator.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 returnBreakIterator.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 returnsBreakIterator.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:
-
previous
public abstract int previous()Returns the boundary preceding the current boundary. If the current boundary is the first text boundary, it returnsBreakIterator.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.
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following
public abstract int following(int offset) Returns the first boundary following the specified character offset. If the specified offset is equal to the last text boundary, it returnsBreakIterator.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 valueBreakIterator.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 is equal to the first text boundary, it returnsBreakIterator.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 valueBreakIterator.DONE
.- Parameters:
offset
- the character 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
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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.- Throws:
IllegalArgumentException
- if the specified offset is less than the first text boundary or greater than the last text boundary.- Since:
- 1.2
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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 returnsBreakIterator.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:
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getText
Get the text being scanned- Returns:
- the text being scanned
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setText
Set a new text string to be scanned. The current scan position is reset to first().- Parameters:
newText
- new text to scan.
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setText
Set a new text for scanning. The current scan position is reset to first().- Parameters:
newText
- new text to scan.
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getWordInstance
- Returns:
- A break iterator for word breaks
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getWordInstance
Returns a newBreakIterator
instance for word breaks for the given locale.- Parameters:
locale
- the desired locale- Returns:
- A break iterator for word breaks
- Throws:
NullPointerException
- iflocale
is null
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getLineInstance
- Returns:
- A break iterator for line breaks
-
getLineInstance
Returns a newBreakIterator
instance for line breaks for the given locale.- Parameters:
locale
- the desired locale- Returns:
- A break iterator for line breaks
- Throws:
NullPointerException
- iflocale
is null
-
getCharacterInstance
- Returns:
- A break iterator for character breaks
-
getCharacterInstance
Returns a newBreakIterator
instance for character breaks for the given locale.- Parameters:
locale
- the desired locale- Returns:
- A break iterator for character breaks
- Throws:
NullPointerException
- iflocale
is null
-
getSentenceInstance
- Returns:
- A break iterator for sentence breaks
-
getSentenceInstance
Returns a newBreakIterator
instance for sentence breaks for the given locale.- Parameters:
locale
- the desired locale- Returns:
- A break iterator for sentence breaks
- Throws:
NullPointerException
- iflocale
is null
-
getAvailableLocales
Returns an array of all locales for which theget*Instance
methods of this class can return localized instances. The returned array represents the union of locales supported by the Java runtime and by installedBreakIteratorProvider
implementations. At a minimum, the returned array must contain aLocale
instance equal toLocale.ROOT
and aLocale
instance equal toLocale.US
.- Returns:
- An array of locales for which localized
BreakIterator
instances are available.
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