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abstract public class: Format [javadoc | source]

All Implemented Interfaces:
    Cloneable, Serializable

Direct Known Subclasses:
    DateFormat, NumberFormat, ChoiceFormat, DecimalFormat, MessageFormat, SimpleDateFormat

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:
public 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
interface  Format.FieldDelegate  FieldDelegate is notified by the various Format implementations as they are formatting the Objects. This allows for storage of the individual sections of the formatted String for later use, such as in a FieldPosition or for an AttributedCharacterIterator.

Delegates should NOT assume that the Format will notify the delegate of fields in any particular order. 

 protected Format() 
Method from java.text.Format Summary:
clone,   createAttributedCharacterIterator,   createAttributedCharacterIterator,   createAttributedCharacterIterator,   createAttributedCharacterIterator,   format,   format,   formatToCharacterIterator,   parseObject,   parseObject
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from java.text.Format Detail:
 public Object clone() 
    Creates and returns a copy of this object.
 AttributedCharacterIterator createAttributedCharacterIterator(String s) 
    Creates an AttributedCharacterIterator for the String s.
 AttributedCharacterIterator createAttributedCharacterIterator(AttributedCharacterIterator[] iterators) 
    Creates an AttributedCharacterIterator containg the concatenated contents of the passed in AttributedCharacterIterators.
 AttributedCharacterIterator createAttributedCharacterIterator(String string,
    Attribute key,
    Object value) 
    Returns an AttributedCharacterIterator with the String string and additional key/value pair key, value.
 AttributedCharacterIterator createAttributedCharacterIterator(AttributedCharacterIterator iterator,
    Attribute key,
    Object value) 
    Creates an AttributedCharacterIterator with the contents of iterator and the additional attribute key value.
 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();
 abstract public 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.
 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.

    The default implementation creates an AttributedCharacterIterator with no attributes. Subclasses that support fields should override this and create an AttributedCharacterIterator with meaningful attributes.

 public Object parseObject(String source) throws ParseException 
    Parses text from the beginning of the given string to produce an object. The method may not use the entire text of the given string.
 abstract public Object parseObject(String source,
    ParsePosition pos)
    Parses text from a string to produce an object.

    The 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.