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

# Class Format

java.lang.Object
java.text.Format

All Implemented Interfaces:
  `Serializable, Cloneable`

Direct Known Subclasses:
  `DateFormat, ListFormat, MessageFormat, NumberFormat`

public abstract class Format extends Object implements Serializable, Cloneable
`Format` is an abstract base class for formatting locale-sensitive information such as dates, messages, numbers, and lists.

`Format` defines the programming interface for formatting locale-sensitive objects into `String`s (the `format` method) and for parsing `String`s back into objects (the `parseObject` method).

Generally, a format's `parseObject` method must be able to parse any string formatted by its `format` method. However, there may be exceptional cases where this is not possible. For example, a `format` method might create two adjacent integer numbers with no separator in between, and in this case the `parseObject` could not tell which digits belong to which number.

## Subclassing

The Java Platform provides specialized subclasses of `Format`-- `DateFormat`, `MessageFormat`, `NumberFormat`, and `ListFormat`--for formatting dates, messages, numbers, and lists respectively.

Concrete subclasses must implement three methods:

1. `format(Object obj, StringBuffer toAppendTo, FieldPosition pos)`
2. `formatToCharacterIterator(Object obj)`
3. `parseObject(String source, ParsePosition pos)`

These general methods allow polymorphic parsing and formatting of objects and are used, for example, by `MessageFormat`. Subclasses often also provide additional `format` methods for specific input types as well as `parse` methods for specific result types. Any `parse` method that does not take a `ParsePosition` argument should throw `ParseException` when no text in the required format is at the beginning of the input text.

Most subclasses will also implement the following factory methods:

1. `getInstance` for getting a useful format object appropriate for the current locale
2. `getInstance(Locale)` for getting a useful format object appropriate for the specified locale

In addition, some subclasses may also implement other `getXxxxInstance` methods for more specialized control. For example, the `NumberFormat` class provides `getPercentInstance` and `getCurrencyInstance` methods for getting specialized number formatters.

Subclasses of `Format` that allow programmers to create objects for locales (with `getInstance(Locale)` for example) must also implement the following class method:

>     public static Locale[] getAvailableLocales()

Subclasses may also consider implementing leniency when parsing. The definition of leniency should be delegated to the subclass.

And finally subclasses may define a set of constants to identify the various fields in the formatted output. These constants are used to create a FieldPosition object which identifies what information is contained in the field and its position in the formatted result. These constants should be named `item_FIELD` where `item` identifies the field. For examples of these constants, see `ERA_FIELD` and its friends in `DateFormat`.

### Synchronization

Formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

Since:
  1.1
See Also:
  - `ParsePosition`
  - `FieldPosition`
  - `NumberFormat`
  - `DateFormat`
  - `MessageFormat`
  - `ListFormat`
  - Serialized Form

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `static class`
  `Format.Field`
  Defines constants that are used as attribute keys in the `AttributedCharacterIterator` returned from `Format.formatToCharacterIterator` and as field identifiers in `FieldPosition`.

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `Format()`
  Sole constructor.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `Object`
  `clone()`
  Creates and returns a copy of this object.
  `final String`
  `format(Object obj)`
  Formats an object to produce a string.
  `abstract StringBuffer`
  `format(Object obj, StringBuffer toAppendTo, FieldPosition pos)`
  Formats an object and appends the resulting text to a given string buffer.
  `AttributedCharacterIterator`
  `formatToCharacterIterator(Object obj)`
  Formats an Object producing an `AttributedCharacterIterator`.
  `Object`
  `parseObject(String source)`
  Parses text from the beginning of the given string to produce an object.
  `abstract Object`
  `parseObject(String source, ParsePosition pos)`
  Parses text from the given string to produce an object.

  ### Methods declared in class Object

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

- ## Constructor Details

  - ### Format

    protected Format()
    Sole constructor. (For invocation by subclass constructors, typically implicit.)

- ## Method Details

  - ### format

    public final String format(Object obj)
    Formats an object to produce a string.
    Implementation Requirements:
      This method returns a string that would be equal to the string returned by

      > `format``(obj, new StringBuffer(), new FieldPosition(0)).toString();`

    Parameters:
      `obj` - The object to format
    Returns:
      Formatted string.
    Throws:
      `IllegalArgumentException` - if the Format cannot format the given object

  - ### format

    public abstract StringBuffer format(Object obj, StringBuffer toAppendTo, FieldPosition pos)
    Formats an object and appends the resulting text to a given string buffer. If the `pos` argument identifies a field used by the format, then its indices are set to the beginning and end of the first such field encountered.
    Parameters:
      `obj` - The object to format
      `toAppendTo` - where the text is to be appended
      `pos` - A `FieldPosition` identifying a field in the formatted text
    Returns:
      the string buffer passed in as `toAppendTo`, with formatted text appended
    Throws:
      `NullPointerException` - if `toAppendTo` or `pos` is null
      `IllegalArgumentException` - if the Format cannot format the given object

  - ### formatToCharacterIterator

    public AttributedCharacterIterator formatToCharacterIterator(Object obj)
    Formats an Object producing an `AttributedCharacterIterator`. You can use the returned `AttributedCharacterIterator` to build the resulting String, as well as to determine information about the resulting String.

    Each attribute key of the AttributedCharacterIterator will be of type `Field`. It is up to each `Format` implementation to define what the legal values are for each attribute in the `AttributedCharacterIterator`, but typically the attribute key is also used as the attribute value.

    API Note:
      Subclasses that support fields should override this and create an `AttributedCharacterIterator` with meaningful attributes.
    Implementation Requirements:
      The default implementation creates an `AttributedCharacterIterator` with no attributes.
    Parameters:
      `obj` - The object to format
    Returns:
      AttributedCharacterIterator describing the formatted value.
    Throws:
      `NullPointerException` - if obj is null.
      `IllegalArgumentException` - when the Format cannot format the given object.
    Since:
      1.4

  - ### parseObject

    public abstract Object parseObject(String source, ParsePosition pos)
    Parses text from the given string to produce an object.

    This method attempts to parse text starting at the index given by `pos`. If parsing succeeds, then the index of `pos` 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 `pos` can be used to indicate the starting point for the next call to this method. If an error occurs, then the index of `pos` is not changed, the error index of `pos` is set to the index of the character where the error occurred, and `null` is returned.

    Parameters:
      `source` - the `String` to parse
      `pos` - A `ParsePosition` object with index and error index information as described above.
    Returns:
      An `Object` parsed from the string. In case of error, returns `null`.
    Throws:
      `NullPointerException` - if `source` or `pos` is `null`.

  - ### parseObject

    public Object parseObject(String source) throws ParseException
    Parses text from the beginning of the given string to produce an object. This method may not use the entire text of the given string.
    Parameters:
      `source` - A `String`, to be parsed from the beginning.
    Returns:
      An `Object` parsed from the string.
    Throws:
      `ParseException` - if parsing fails
      `NullPointerException` - if `source` is `null`.

  - ### clone

    public Object clone()
    Creates and returns a copy of this object.
    Overrides:
      `clone` in class `Object`
    Returns:
      a clone of this instance.
    See Also:
      - `Cloneable`