public abstract class Format extends Object
implements Serializable Cloneable
Known Direct Subclasses
Known Indirect Subclasses

Format is an abstract base class for formatting locale-sensitive information such as dates, messages, and numbers.

Format defines the programming interface for formatting locale-sensitive objects into Strings (the format method) and for parsing Strings 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.


The Java Platform provides three specialized subclasses of Format-- DateFormat, MessageFormat, and NumberFormat--for formatting dates, messages, and numbers, 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()

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.


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.

Nested Class Summary

class Format.Field Defines constants that are used as attribute keys in the AttributedCharacterIterator returned from Format.formatToCharacterIterator and as field identifiers in FieldPosition

Protected Constructor Summary

Sole constructor.

Public Method Summary

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.
formatToCharacterIterator(Object obj)
Formats an Object producing an AttributedCharacterIterator.
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 a string to produce an object.

Inherited Method Summary

Protected Constructors

protected Format ()

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

Public Methods

public Object clone ()

Creates and returns a copy of this object.

  • a clone of this instance.

public final String format (Object obj)

Formats an object to produce a string. This is equivalent to

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

obj The object to format
  • Formatted string.
IllegalArgumentException if the Format cannot format the given object

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.

obj The object to format
toAppendTo where the text is to be appended
pos A FieldPosition identifying a field in the formatted text