ListFormat (Java SE 25 & JDK 25)
Description: declaration: module: java.base, package: java.text, class: ListFormat
Keywords: java.text.ListFormat class

# Class ListFormat

java.lang.Object
java.text.Format
java.text.ListFormat

All Implemented Interfaces:
  `Serializable, Cloneable`

public final class ListFormat extends Format
`ListFormat` formats or parses a list of strings in a locale-sensitive way. Use `ListFormat` to construct a list of strings displayed for end users. For example, displaying a list of 3 weekdays, e.g. "Monday", "Wednesday", "Friday" as "Monday, Wednesday, and Friday" in an inclusive list type. This class provides the functionality defined in Unicode Consortium's LDML specification for List Patterns.

Three formatting types are provided: `STANDARD`, `OR`, and `UNIT`, which determines the punctuation between the strings and the connecting words if any. Also, three formatting styles for each type are provided: `FULL`, `SHORT`, and `NARROW`, suitable for how the strings are abbreviated (or not). The following snippet is an example of formatting the list of Strings `"Foo", "Bar", "Baz"` in US English with `STANDARD` type and `FULL` style:

    ListFormat.getInstance(Locale.US, ListFormat.Type.STANDARD, ListFormat.Style.FULL)
        .format(List.of("Foo", "Bar", "Baz"))

This will produce the concatenated list string, "Foo, Bar, and Baz" as seen in the following:

Formatting examples
| FULL | SHORT | NARROW
--- | --- | --- | ---
STANDARD | Foo, Bar, and Baz | Foo, Bar, & Baz | Foo, Bar, Baz
OR | Foo, Bar, or Baz | Foo, Bar, or Baz | Foo, Bar, or Baz
UNIT | Foo, Bar, Baz | Foo, Bar, Baz | Foo Bar Baz

Note: these examples are from CLDR, there could be different results from other locale providers.

Alternatively, Locale, Type, and/or Style independent instances can be created with `getInstance(String[])`. The String array to the method specifies the delimiting patterns for the start/middle/end portion of the formatted string, as well as optional specialized patterns for two or three elements. Refer to the method description for more detail.

On parsing, if some ambiguity is found in the input string, such as delimiting sequences in the input string, the result, when formatted with the same formatting, does not re-produce the input string. For example, a two element String list "a, b,", "c" will be formatted as "a, b, and c", but may be parsed as three elements "a", "b", "c".

Implementation Requirements:
  This class is immutable and thread-safe
Since:
  22
External Specifications
  - Unicode Locale Data Markup Language (LDML)
See Also:
  - Serialized Form

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `static enum`
  `ListFormat.Style`
  A ListFormat style - `FULL`, `SHORT`, and `NARROW`.
  `static enum`
  `ListFormat.Type`
  A ListFormat type - `STANDARD`, `OR`, and `UNIT`.

  ### Nested classes/interfaces declared in class Format

  `Format.Field`

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `equals(Object obj)`
  Compares the specified object with this `ListFormat` for equality.
  `StringBuffer`
  `format(Object obj, StringBuffer toAppendTo, FieldPosition pos)`
  Formats an object and appends the resulting text to a given string buffer.
  `String`
  `format(List<String> input)`
  Returns the string that consists of the input strings, concatenated with the patterns of this `ListFormat`.
  `static Locale[]`
  `getAvailableLocales()`
  Returns the available locales that support ListFormat.
  `static ListFormat`
  `getInstance()`
  Returns the ListFormat object for the default `FORMAT Locale`, `STANDARD` type, and `FULL` style.
  `static ListFormat`
  `getInstance(String[] patterns)`
  Returns the ListFormat object for the specified patterns.
  `static ListFormat`
  `getInstance(Locale locale, ListFormat.Type type, ListFormat.Style style)`
  Returns the ListFormat object for the specified `Locale`, `Type`, and `Style`.
  `Locale`
  `getLocale()`
  Returns the `Locale` of this ListFormat.
  `String[]`
  `getPatterns()`
  Returns the patterns used in this ListFormat.
  `List<String>`
  `parse(String source)`
  Returns the parsed list of strings from the `source` string.
  `Object`
  `parseObject(String source, ParsePosition parsePos)`
  Parses text from a string to produce a list of strings.
  `String`
  `toString()`
  Returns a string identifying this `ListFormat`, for debugging.

  ### Methods declared in class Format

  `clone, format, formatToCharacterIterator, parseObject`

  ### Methods declared in class Object

  `finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- ## Method Details

  - ### getAvailableLocales

    public static Locale[] getAvailableLocales()
    Returns the available locales that support ListFormat.
    Returns:
      the available locales that support ListFormat

  - ### getInstance

    public static ListFormat getInstance()
    Returns the ListFormat object for the default `FORMAT Locale`, `STANDARD` type, and `FULL` style.
    Returns:
      the ListFormat object for the default `FORMAT Locale`, `STANDARD` type, and `FULL` style

  - ### getInstance

    public static ListFormat getInstance(Locale locale, ListFormat.Type type, ListFormat.Style style)
    Returns the ListFormat object for the specified `Locale`, `Type`, and `Style`.
    Parameters:
      `locale` - `Locale` to be used, not null
      `type` - type of the ListFormat. One of `STANDARD`, `OR`, or `UNIT`, not null
      `style` - style of the ListFormat. One of `FULL`, `SHORT`, or `NARROW`, not null
    Returns:
      the ListFormat object for the specified `Locale`, `Type`, and `Style`
    Throws:
      `NullPointerException` - if any of the arguments are null

  - ### getInstance

    public static ListFormat getInstance(String[] patterns)
    Returns the ListFormat object for the specified patterns.

    This factory returns an instance based on the customized patterns array, instead of letting the runtime provide appropriate patterns for the `Locale`, `Type`, or `Style`.

    The patterns array should contain five String patterns, each corresponding to the Unicode LDML's `listPatternPart`, i.e., "start", "middle", "end", two element, and three element patterns in this order. Each pattern contains "{0}" and "{1}" (and "{2}" for the three element pattern) placeholders that are substituted with the passed input strings on formatting. If the length of the patterns array is not 5, an `IllegalArgumentException` is thrown.

    Each pattern string is first parsed as follows. Literals in parentheses, such as "start_before", are optional:

    >     start := (start_before){0}start_between{1}
    >     middle := {0}middle_between{1}
    >     end := {0}end_between{1}(end_after)
    >     two := (two_before){0}two_between{1}(two_after)
    >     three := (three_before){0}three_between1{1}three_between2{2}(three_after)

    If two or three pattern string is empty, it falls back to `"(start_before){0}end_between{1}(end_after)"`, `"(start_before){0}start_between{1}end_between{2}(end_after)"` respectively. If parsing of any pattern string for start, middle, end, two, or three fails, it throws an `IllegalArgumentException`.

    On formatting, the input string list with `n` elements substitutes above placeholders based on the number of elements:

    >     n = 1: {0}
    >     n = 2: parsed pattern for "two"
    >     n = 3: parsed pattern for "three"
    >     n > 3: (start_before){0}start_between{1}middle_between{2} ... middle_between{m}end_between{n}(end_after)

    As an example, the following table shows a pattern array which is equivalent to `STANDARD` type, `FULL` style in US English:

    Standard/Full Patterns in US English
    Pattern Kind | Pattern String
    --- | ---
    start | "{0}, {1}"
    middle | "{0}, {1}"
    end | "{0}, and {1}"
    two | "{0} and {1}"
    three | ""

    Here are the resulting formatted strings with the above pattern array.

    Formatting examples
    Input String List | Formatted String
    --- | ---
    "Foo", "Bar", "Baz", "Qux" | "Foo, Bar, Baz, and Qux"
    "Foo", "Bar", "Baz" | "Foo, Bar, and Baz"
    "Foo", "Bar" | "Foo and Bar"
    "Foo" | "Foo"

    Parameters:
      `patterns` - array of patterns, not null
    Returns:
      the ListFormat object for the specified patterns
    Throws:
      `IllegalArgumentException` - if the length `patterns` array is not 5, or any of `start`, `middle`, `end`, `two`, or `three` patterns cannot be parsed.
      `NullPointerException` - if `patterns` is null.

  - ### getLocale

    public Locale getLocale()
    Returns the `Locale` of this ListFormat. The `locale` is defined by `getInstance(Locale, Type, Style)` or `getInstance(String[])`.
    Returns:
      the `Locale` of this ListFormat

  - ### getPatterns

    public String[] getPatterns()
    Returns the patterns used in this ListFormat. The `patterns` are defined by `getInstance(Locale, Type, Style)` or `getInstance(String[])`.
    Returns:
      the patterns used in this ListFormat

  - ### format

    public String format(List<String> input)
    Returns the string that consists of the input strings, concatenated with the patterns of this `ListFormat`.
    API Note:
      Formatting the string from an excessively long list may exceed memory or string sizes.
    Parameters:
      `input` - The list of input strings to format. There should at least one String element in this list, otherwise an `IllegalArgumentException` is thrown.
    Returns:
      the string that consists of the input strings, concatenated with the patterns of this `ListFormat`
    Throws:
      `IllegalArgumentException` - if the length of `input` is zero.
      `NullPointerException` - if `input` is null.

  - ### format

    public StringBuffer format(Object obj, StringBuffer toAppendTo, FieldPosition pos)
    Formats an object and appends the resulting text to a given string buffer. The object should either be a List or an array of Objects.
    Specified by:
      `format` in class `Format`
    API Note:
      Formatting the string from an excessively long list or array may exceed memory or string sizes.
    Parameters:
      `obj` - The object to format. Must be a List or an array of Object.
      `toAppendTo` - where the text is to be appended
      `pos` - Ignored. Not used in ListFormat. May be null
    Returns:
      the string buffer passed in as `toAppendTo`, with formatted text appended
    Throws:
      `NullPointerException` - if `obj` or `toAppendTo` is null
      `IllegalArgumentException` - if `obj` is neither a `List` nor an array of `Object`s, or its length is zero.

  - ### parse

    public List<String> parse(String source) throws ParseException
    Returns the parsed list of strings from the `source` string. Note that `format(List)` and this method may not guarantee a round-trip, if the input strings contain ambiguous delimiters. For example, a two element String list `"a, b,", "c"` will be formatted as `"a, b, and c"`, but may be parsed as three elements `"a", "b", "c"`.
    Parameters:
      `source` - the string to parse, not null.
    Returns:
      the parsed list of strings from the `source` string
    Throws:
      `ParseException` - if parse failed
      `NullPointerException` - if source is null

  - ### parseObject

    public Object parseObject(String source, ParsePosition parsePos)
    Parses text from a string to produce a list of strings.

    The method attempts to parse text starting at the index given by `parsePos`. If parsing succeeds, then the index of `parsePos` is updated to the index after the last character used (parsing does not necessarily use all characters up to the end of the string), and the parsed object is returned. The updated `parsePos` can be used to indicate the starting point for the next call to parse additional text. If an error occurs, then the index of `parsePos` is not changed, the error index of `parsePos` is set to the index of the character where the error occurred, and null is returned. See the `parse(String)` method for more information on list parsing.

    Specified by:
      `parseObject` in class `Format`
    Parameters:
      `source` - A string, part of which should be parsed.
      `parsePos` - A `ParsePosition` object with index and error index information as described above.
    Returns:
      A list of string parsed from the `source`. In case of error, returns null.
    Throws:
      `NullPointerException` - if `source` or `parsePos` is null.
      `IndexOutOfBoundsException` - if the starting index given by `parsePos` is outside `source`.

  - ### equals

    public boolean equals(Object obj)
    Compares the specified object with this `ListFormat` for equality. Returns `true` if the specified object is also a `ListFormat`, and `locale` and `patterns`, returned from `getLocale()` and `getPatterns()` respectively, are equal.
    Overrides:
      `equals` in class `Object`
    Parameters:
      `obj` - the object to be compared for equality.
    Returns:
      `true` if the specified object is equal to this `ListFormat`
    See Also:
      - `Object.hashCode()`
      - `HashMap`

  - ### toString

    public String toString()
    Returns a string identifying this `ListFormat`, for debugging.
    Overrides:
      `toString` in class `Object`
    Returns:
      a string identifying this `ListFormat`, for debugging