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
format method) and for parsing
Strings back into objects (the
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
NumberFormat--for formatting dates, messages, and numbers, respectively.
Concrete subclasses must implement three methods:
format(Object obj, StringBuffer toAppendTo, FieldPosition pos)
parseObject(String source, ParsePosition pos)
These general methods allow polymorphic parsing and formatting of objects and are used, for example, by
. Subclasses often also provide additional
methods for specific input types as well as
methods for specific result types. Any
method that does not take a
argument should throw
when no text in the required format is at the beginning of the input text.
Most subclasses will also implement the following factory methods:
getInstance for getting a useful format object appropriate for the current locale
getInstance(Locale) for getting a useful format object appropriate for the specified locale
In addition, some subclasses may also implement other
methods for more specialized control. For example, the
methods for getting specialized number formatters.
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 identifies the field. For examples of these constants, see
ERA_FIELD and its friends in
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.