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 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 three specialized subclasses of Format
--
DateFormat
, MessageFormat
, and
NumberFormat
--for formatting dates, messages, and numbers,
respectively.
Concrete subclasses must implement three methods:
-
format(Object obj, StringBuffer toAppendTo, FieldPosition pos)
-
formatToCharacterIterator(Object obj)
-
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:
-
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
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.