Class PluralFormat

java.lang.Object
java.text.Format
org.graalvm.shadowed.com.ibm.icu.text.UFormat
org.graalvm.shadowed.com.ibm.icu.text.PluralFormat
All Implemented Interfaces:
Serializable, Cloneable
public class PluralFormat extends UFormat
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 https://unicode-org.github.io/cldr-staging/charts/latest/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.

See Also:

  • Constructor Details

    • PluralFormat

      public PluralFormat()
      Creates a new cardinal-number PluralFormat for the default FORMAT locale. This locale will be used to get the set of plural rules and for standard number formatting.
      See Also:

    • PluralFormat

      public PluralFormat(ULocale ulocale)
      Creates a new cardinal-number PluralFormat for a given locale.
      Parameters:
      ulocale - the PluralFormat will be configured with rules for this locale. This locale will also be used for standard number formatting.

    • PluralFormat

      public PluralFormat(Locale locale)
      Creates a new cardinal-number PluralFormat for a given Locale.
      Parameters:
      locale - the PluralFormat will be configured with rules for this locale. This locale will also be used for standard number formatting.

    • PluralFormat

      public PluralFormat(PluralRules rules)
      Creates a new cardinal-number PluralFormat for a given set of rules. The standard number formatting will be done using the default FORMAT locale.
      Parameters:
      rules - defines the behavior of the PluralFormat object.
      See Also:

    • PluralFormat

      public PluralFormat(ULocale ulocale, PluralRules rules)
      Creates a new cardinal-number PluralFormat for a given set of rules. The standard number formatting will be done using the given locale.
      Parameters:
      ulocale - the default number formatting will be done using this locale.
      rules - defines the behavior of the PluralFormat object.

    • PluralFormat

      public PluralFormat(Locale locale, PluralRules rules)
      Creates a new cardinal-number PluralFormat for a given set of rules. The standard number formatting will be done using the given locale.
      Parameters:
      locale - the default number formatting will be done using this locale.
      rules - defines the behavior of the PluralFormat object.

    • PluralFormat

      public PluralFormat(ULocale ulocale, PluralRules.PluralType type)
      Creates a new PluralFormat for the plural type. The standard number formatting will be done using the given locale.
      Parameters:
      ulocale - the default number formatting will be done using this locale.
      type - The plural type (e.g., cardinal or ordinal).

    • PluralFormat

      public PluralFormat(Locale locale, PluralRules.PluralType type)
      Creates a new PluralFormat for the plural type. The standard number formatting will be done using the given Locale.
      Parameters:
      locale - the default number formatting will be done using this locale.
      type - The plural type (e.g., cardinal or ordinal).

    • PluralFormat

      public PluralFormat(String pattern)
      Creates a new cardinal-number PluralFormat for a given pattern string. The default FORMAT locale will be used to get the set of plural rules and for standard number formatting.
      Parameters:
      pattern - the pattern for this PluralFormat.
      Throws:
      IllegalArgumentException - if the pattern is invalid.
      See Also:

    • PluralFormat

      public PluralFormat(ULocale ulocale, String pattern)
      Creates a new cardinal-number PluralFormat for a given pattern string and locale. The locale will be used to get the set of plural rules and for standard number formatting.

      Example code:invalid input: '{@'.jcite org.graalvm.shadowed.com.ibm.icu.samples.text.pluralformat.PluralFormatSample:---PluralFormatExample}

      Parameters:
      ulocale - the PluralFormat will be configured with rules for this locale. This locale will also be used for standard number formatting.
      pattern - the pattern for this PluralFormat.
      Throws:
      IllegalArgumentException - if the pattern is invalid.

    • PluralFormat

      public PluralFormat(PluralRules rules, String pattern)
      Creates a new cardinal-number PluralFormat for a given set of rules and a pattern. The standard number formatting will be done using the default FORMAT locale.
      Parameters:
      rules - defines the behavior of the PluralFormat object.
      pattern - the pattern for this PluralFormat.
      Throws:
      IllegalArgumentException - if the pattern is invalid.
      See Also:

    • PluralFormat

      public PluralFormat(ULocale ulocale, PluralRules rules, String pattern)
      Creates a new cardinal-number PluralFormat for a given set of rules, a pattern and a locale.
      Parameters:
      ulocale - the PluralFormat will be configured with rules for this locale. This locale will also be used for standard number formatting.
      rules - defines the behavior of the PluralFormat object.
      pattern - the pattern for this PluralFormat.
      Throws:
      IllegalArgumentException - if the pattern is invalid.

    • PluralFormat

      public PluralFormat(ULocale ulocale, PluralRules.PluralType type, String pattern)
      Creates a new PluralFormat for a plural type, a pattern and a locale.
      Parameters:
      ulocale - the PluralFormat will be configured with rules for this locale. This locale will also be used for standard number formatting.
      type - The plural type (e.g., cardinal or ordinal).
      pattern - the pattern for this PluralFormat.
      Throws:
      IllegalArgumentException - if the pattern is invalid.

  • Method Details

    • applyPattern

      public void applyPattern(String pattern)
      Sets the pattern used by this plural format. The method parses the pattern and creates a map of format strings for the plural rules. Patterns and their interpretation are specified in the class description.
      Parameters:
      pattern - the pattern for this plural format.
      Throws:
      IllegalArgumentException - if the pattern is invalid.

    • toPattern

      public String toPattern()
      Returns the pattern for this PluralFormat.
      Returns:
      the pattern string

    • format

      public final String format(double number)
      Formats a plural message for a given number.
      Parameters:
      number - a number for which the plural message should be formatted. If no pattern has been applied to this PluralFormat object yet, the formatted number will be returned.
      Returns:
      the string containing the formatted plural message.

    • format

      public StringBuffer format(Object number, StringBuffer toAppendTo, FieldPosition pos)
      Formats a plural message for a given number and appends the formatted message to the given StringBuffer.
      Specified by:
      format in class Format
      Parameters:
      number - a number object (instance of Number for which the plural message should be formatted. If no pattern has been applied to this PluralFormat object yet, the formatted number will be returned. Note: If this object is not an instance of Number, the toAppendTo will not be modified.
      toAppendTo - the formatted message will be appended to this StringBuffer.
      pos - will be ignored by this method.
      Returns:
      the string buffer passed in as toAppendTo, with formatted text appended.
      Throws:
      IllegalArgumentException - if number is not an instance of Number

    • parse

      public Number parse(String text, ParsePosition parsePosition)
      This method is not yet supported by PluralFormat.
      Parameters:
      text - the string to be parsed.
      parsePosition - defines the position where parsing is to begin, and upon return, the position where parsing left off. If the position has not changed upon return, then parsing failed.
      Returns:
      nothing because this method is not yet implemented.
      Throws:
      UnsupportedOperationException - will always be thrown by this method.

    • parseObject

      public Object parseObject(String source, ParsePosition pos)
      This method is not yet supported by PluralFormat.
      Specified by:
      parseObject in class Format
      Parameters:
      source - the string to be parsed.
      pos - defines the position where parsing is to begin, and upon return, the position where parsing left off. If the position has not changed upon return, then parsing failed.
      Returns:
      nothing because this method is not yet implemented.
      Throws:
      UnsupportedOperationException - will always be thrown by this method.

    • setLocale

      @Deprecated public void setLocale(ULocale ulocale)
      Deprecated.
      ICU 50 This method clears the pattern and might create a different kind of PluralRules instance; use one of the constructors to create a new instance instead.
      Sets the locale used by this PluraFormat object. Note: Calling this method resets this PluraFormat object, i.e., a pattern that was applied previously will be removed, and the NumberFormat is set to the default number format for the locale. The resulting format behaves the same as one constructed from PluralFormat(ULocale, PluralRules.PluralType) with PluralType.CARDINAL.
      Parameters:
      ulocale - the ULocale used to configure the formatter. If ulocale is null, the default FORMAT locale will be used.
      See Also:

    • setNumberFormat

      public void setNumberFormat(NumberFormat format)
      Sets the number format used by this formatter. You only need to call this if you want a different number format than the default formatter for the locale.
      Parameters:
      format - the number format to use.

    • equals

      public boolean equals(Object rhs)
      Overrides:
      equals in class Object

    • equals

      public boolean equals(PluralFormat rhs)
      Returns true if this equals the provided PluralFormat.
      Parameters:
      rhs - the PluralFormat to compare against
      Returns:
      true if this equals rhs

    • hashCode

      public int hashCode()
      Overrides:
      hashCode in class Object

    • toString

      public String toString()
      Overrides:
      toString in class Object