PluralFormat
supports the creation of internationalized
messages with plural inflection. It is based on
plural
selection, i.e. the caller specifies messages for each
plural case that can appear in the user's language and the
PluralFormat
selects the appropriate message based on
the number.
The Problem of Plural Forms in Internationalized Messages
Different languages have different ways to inflect
plurals. Creating internationalized messages that include plural
forms is only feasible when the framework is able to handle plural
forms of all languages correctly. ChoiceFormat
doesn't handle this well, because it attaches a number interval to
each message and selects the message whose interval contains a
given number. This can only handle a finite number of
intervals. But in some languages, like Polish, one plural case
applies to infinitely many intervals (e.g., the paucal case applies to
numbers ending with 2, 3, or 4 except those ending with 12, 13, or
14). Thus ChoiceFormat
is not adequate.
PluralFormat
deals with this by breaking the problem
into two parts:
- It uses
PluralRules
that can define more complex
conditions for a plural case than just a single interval. These plural
rules define both what plural cases exist in a language, and to
which numbers these cases apply.
- It provides predefined plural rules for many languages. Thus, the programmer
need not worry about the plural cases of a language and
does not have to define the plural cases; they can simply
use the predefined keywords. The whole plural formatting of messages can
be done using localized patterns from resource bundles. For predefined plural
rules, see the CLDR Language Plural Rules page at
http://unicode.org/repos/cldr-tmp/trunk/diff/supplemental/language_plural_rules.html
Usage of PluralFormat
Note: Typically, plural formatting is done via MessageFormat
with a plural
argument type,
rather than using a stand-alone PluralFormat
.
This discussion assumes that you use PluralFormat
with
a predefined set of plural rules. You can create one using one of
the constructors that takes a ULocale
object. To
specify the message pattern, you can either pass it to the
constructor or set it explicitly using the
applyPattern()
method. The format()
method takes a number object and selects the message of the
matching plural case. This message will be returned.
Patterns and Their Interpretation
The pattern text defines the message output for each plural case of the
specified locale. Syntax:
pluralStyle = [offsetValue] (selector '{' message '}')+
offsetValue = "offset:" number
selector = explicitValue | keyword
explicitValue = '=' number // adjacent, no white space in between
keyword = [^[[:Pattern_Syntax:][:Pattern_White_Space:]]]+
message: see
MessageFormat
Pattern_White_Space between syntax elements is ignored, except
between the {curly braces} and their sub-message,
and between the '=' and the number of an explicitValue.
There are 6 predefined case keywords in CLDR/ICU - 'zero', 'one', 'two', 'few', 'many' and
'other'. You always have to define a message text for the default plural case
"other
" which is contained in every rule set.
If you do not specify a message text for a particular plural case, the
message text of the plural case "other
" gets assigned to this
plural case.
When formatting, the input number is first matched against the explicitValue clauses.
If there is no exact-number match, then a keyword is selected by calling
the PluralRules
with the input number minus the offset.
(The offset defaults to 0 if it is omitted from the pattern string.)
If there is no clause with that keyword, then the "other" clauses is returned.
An unquoted pound sign (#
) in the selected sub-message
itself (i.e., outside of arguments nested in the sub-message)
is replaced by the input number minus the offset.
The number-minus-offset value is formatted using a
NumberFormat
for the PluralFormat
's locale. If you
need special number formatting, you have to use a MessageFormat
and explicitly specify a NumberFormat
argument.
Note: That argument is formatting without subtracting the offset!
If you need a custom format and have a non-zero offset, then you need to pass the
number-minus-offset value as a separate parameter.
For a usage example, see the
MessageFormat class documentation.
Defining Custom Plural Rules
If you need to use PluralFormat
with custom rules, you can
create a PluralRules
object and pass it to
PluralFormat
's constructor. If you also specify a locale in this
constructor, this locale will be used to format the number in the message
texts.
For more information about PluralRules
, see
PluralRules.