JavaTM 2 Platform
Std. Ed. v1.3

Class Format

All Implemented Interfaces:
Cloneable, Serializable
Direct Known Subclasses:
DateFormat, 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, 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). Any String formatted by format is guaranteed to be parseable by parseObject.

If formatting is unsuccessful because the Format object cannot format the type of object specified, format throws an IllegalArgumentException. Otherwise, if there is something illformed about the object, format returns the Unicode replacement character \\uFFFD.

If there is no match when parsing, parseObject(String) throws a ParseException, and parseObject(String, ParsePosition) leaves the ParsePosition index member unchanged and returns null.

Subclassing: The Java 2 platform provides three concrete subclasses of Format-- DateFormat, MessageFormat, and NumberFormat--for formatting dates, messages, and numbers, respectively.

Concrete subclasses must implement these two methods:

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

Most subclasses will also implement the following two 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 choose to 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.

See Also:
ParsePosition, FieldPosition, NumberFormat, DateFormat, MessageFormat, Serialized Form

Constructor Summary
Method Summary
 Object clone()
          Overrides Cloneable
 String format(Object obj)
          Formats an object to produce a string.
abstract  StringBuffer format(Object obj, StringBuffer toAppendTo, FieldPosition pos)
          Formats an object to produce a string.
 Object parseObject(String source)
          Parses a string to produce an object.
abstract  Object parseObject(String source, ParsePosition status)
          Parses a string to produce an object.
Methods inherited from class java.lang.Object
equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail


public Format()
Method Detail


public final String format(Object obj)
Formats an object to produce a string.

Subclasses will override the StringBuffer version of format.

obj - The object to format
Formatted string.
IllegalArgumentException - when the Format cannot format the type of object.
See Also:
MessageFormat, format(Object, StringBuffer, FieldPosition)


public abstract StringBuffer format(Object obj,
                                    StringBuffer toAppendTo,
                                    FieldPosition pos)
Formats an object to produce a string. Subclasses will implement for particular object, such as:
 StringBuffer format (Number obj, StringBuffer toAppendTo)
 Number parse (String str)
These general routines allow polymorphic parsing and formatting for objects such as the MessageFormat.
obj - The object to format
toAppendTo - where the text is to be appended
pos - On input: an alignment field, if desired. On output: the offsets of the alignment field.
the value passed in as toAppendTo (this allows chaining, as with StringBuffer.append())
IllegalArgumentException - when the Format cannot format the given object.
See Also:
MessageFormat, FieldPosition


public abstract Object parseObject(String source,
                                   ParsePosition status)
Parses a string to produce an object. Subclasses will typically implement for particular object, such as:
       String format (Number obj);
       String format (long obj);
       String format (double obj);
       Number parse (String str);
status - Input-Output parameter.

Before calling, set status.index to the offset you want to start parsing at in the source. After calling, status.index is the end of the text you parsed. If error occurs, index is unchanged.

When parsing, leading whitespace is discarded (with successful parse), while trailing whitespace is left as is.

Example: Parsing "_12_xy" (where _ represents a space) for a number, with index == 0 will result in the number 12, with status.index updated to 3 (just before the second space). Parsing a second time will result in a ParseException since "xy" is not a number, and leave index at 3.

Subclasses will typically supply specific parse methods that return different types of values. Since methods can't overload on return types, these will typically be named "parse", while this polymorphic method will always be called parseObject. Any parse method that does not take a status should throw ParseException when no text in the required format is at the start position.

Object parsed from string. In case of error, returns null.
See Also:


public Object parseObject(String source)
                   throws ParseException
Parses a string to produce an object.
ParseException - if the specified string is invalid.


public Object clone()
Overrides Cloneable
clone in class Object
Following copied from class: java.lang.Object
a clone of this instance.
CloneNotSupportedException - if the object's class does not support the Cloneable interface. Subclasses that override the clone method can also throw this exception to indicate that an instance cannot be cloned.
OutOfMemoryError - if there is not enough memory.
See Also:

JavaTM 2 Platform
Std. Ed. v1.3

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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