Java Platform 1.2
Beta 4

Class java.text.MessageFormat


public class MessageFormat
extends Format
MessageFormat provides a means to produce concatenated messages in language-neutral way. Use this to construct messages displayed for end users.

MessageFormat takes a set of objects, formats them, then inserts the formatted strings into the pattern at the appropriate places.

Note: MessageFormat differs from the other Format classes in that you create a MessageFormat object with one of its constructors (not with a getInstance style factory method). The factory methods aren't necessary because MessageFormat doesn't require any complex setup for a given locale. In fact, MessageFormat doesn't implement any locale specific behavior at all. It just needs to be set up on a sentence by sentence basis.

Here are some examples of usage:

 Object[] arguments = {
     new Integer(7),
     new Date(System.currentTimeMillis()),
     "a disturbance in the Force"

 String result = MessageFormat.format(
     "At {1,time} on {1,date}, there was {2} on planet {0,number,integer}.",

 output: At 12:30 PM on Jul 3, 2053, there was a disturbance
           in the Force on planet 7.

Typically, the message format will come from resources, and the arguments will be dynamically set at runtime.

Example 2:

 Object[] testArgs = {new Long(3), "MyDisk"};

 MessageFormat form = new MessageFormat(
     "The disk \"{1}\" contains {0} file(s).");


 // output, with different testArgs
 output: The disk "MyDisk" contains 0 file(s).
 output: The disk "MyDisk" contains 1 file(s).
 output: The disk "MyDisk" contains 1,273 file(s).

The pattern is of the form:

 messageFormatPattern := string ( "{" messageFormatElement "}" string )*

 messageFormatElement := argument { "," elementFormat }

 elementFormat := "time" { "," datetimeStyle }
                | "date" { "," datetimeStyle }
                | "number" { "," numberStyle }
                | "choice" { "," choiceStyle }

 datetimeStyle := "short"
                  | "medium"
                  | "long"
                  | "full"
                  | dateFormatPattern

 numberStyle := "currency"
               | "percent"
               | "integer"
               | numberFormatPattern

 choiceStyle := choiceFormatPattern
If there is no elementFormat, then the argument must be a string, which is substituted. If there is no dateTimeStyle or numberStyle, then the default format is used (for example, NumberFormat.getInstance, DateFormat.getTimeInstance, or DateFormat.getInstance).

In strings, single quotes can be used to quote the "{" (curly brace) if necessary. A real single quote is represented by ''. Inside a messageFormatElement, quotes are not removed. For example, {1,number,$'#',##} will produce a number format with the pound-sign quoted, with a result such as: "$#31,45".

If a pattern is used, then unquoted braces in the pattern, if any, must match: that is, "ab {0} de" and "ab '}' de" are ok, but "ab {0'}' de" and "ab } de" are not.

The argument is a number from 0 to 9, which corresponds to the arguments presented in an array to be formatted.

It is ok to have unused arguments in the array. With missing arguments or arguments that are not of the right class for the specified format, a ParseException is thrown. First, format checks to see if a Format object has been specified for the argument with the setFormats method. If so, then format uses that Format object to format the argument. Otherwise, the argument is formatted based on the object's type. If the argument is a Number, then format uses NumberFormat.getInstance to format the argument; if the argument is a Date, then format uses DateFormat.getDateTimeInstance to format the argument. Otherwise, it uses the toString method.

For more sophisticated patterns, you can use a ChoiceFormat to get output such as:

 MessageFormat form = new MessageFormat("The disk \"{1}\" contains {0}.");
 double[] filelimits = {0,1,2};
 String[] filepart = {"no files","one file","{0,number} files"};
 ChoiceFormat fileform = new ChoiceFormat(filelimits, filepart);
 form.setFormat(1,fileform); // NOT zero, see below

 Object[] testArgs = {new Long(12373), "MyDisk"};


 // output, with different testArgs
 output: The disk "MyDisk" contains no files.
 output: The disk "MyDisk" contains one file.
 output: The disk "MyDisk" contains 1,273 files.
You can either do this programmatically, as in the above example, or by using a pattern (see ChoiceFormat for more information) as in:
    "There {0,choice,0#are no files|1#is one file|1#are {0,number,integer} files}.");

Note: As we see above, the string produced by a ChoiceFormat in MessageFormat is treated specially; occurances of '{' are used to indicated subformats, and cause recursion. If you create both a MessageFormat and ChoiceFormat programmatically (instead of using the string patterns), then be careful not to produce a format that recurses on itself, which will cause an infinite loop.

Note: formats are numbered by order of variable in the string. This is not the same as the argument numbering! For example: with "abc{2}def{3}ghi{0}...",

When a single argument is parsed more than once in the string, the last match will be the final result of the parsing. For example,

 MessageFormat mf = new MessageFormat("{0,number,#.##}, {0,number,#.#}");
 Object[] objs = {new Double(3.1415)};
 String result = mf.format( objs );
 // result now equals "3.14, 3.1"
 objs = null;
 objs = mf.parse(result, new ParsePosition(0));
 // objs now equals {new Double(3.1)}

Likewise, parsing with a MessageFormat object using patterns containing multiple occurances of the same argument would return the last match. For example,

 MessageFormat mf = new MessageFormat("{0}, {0}, {0}");
 String forParsing = "x, y, z";
 Object[] objs = mf.parse(forParsing, new ParsePosition(0));
 // result now equals {new String("z")}

You can use setLocale followed by applyPattern (and then possibly setFormat) to re-initialize a MessageFormat with a different locale.

See Also:
Locale, Format, NumberFormat, DecimalFormat, ChoiceFormat, Serialized Form

Constructor Summary
MessageFormat(String pattern)
          Constructs with the specified pattern.
Method Summary
 void applyPattern(String newPattern)
          Sets the pattern.
 Object clone()
          Overrides Cloneable
 boolean equals(Object obj)
          Equality comparision between two message format objects
 StringBuffer format(Object source, StringBuffer result, FieldPosition ignore)
 StringBuffer format(Object[] source, StringBuffer result, FieldPosition ignore)
          Returns pattern with formatted objects.
static String format(String pattern, Object[] arguments)
          Convenience routine.
 Format[] getFormats()
          Gets formats that were set with setFormats.
 Locale getLocale()
          Gets the locale.
 int hashCode()
          Generates a hash code for the message format object.
 Object[] parse(String source, ParsePosition status)
          Parses the string.
 Object[] parse(String source)
          Parses the string.
 Object parseObject(String text, ParsePosition status)
          Parses the string.
 void setFormat(int variable, Format newFormat)
          Set a format to be used on a variable in the pattern.
 void setFormats(Format[] newFormats)
          Sets formats to use on parameters.
 void setLocale(Locale theLocale)
          Constructs with the specified pattern and formats for the arguments in that pattern.
 String toPattern()
          Gets the pattern.
Methods inherited from class java.text.Format
format , parseObject
Methods inherited from class java.lang.Object
finalize , getClass , notify , notifyAll , toString , wait , wait , wait

Constructor Detail


public MessageFormat(String pattern)
Constructs with the specified pattern.
See Also:
Method Detail


public void setLocale(Locale theLocale)
Constructs with the specified pattern and formats for the arguments in that pattern.


public Locale getLocale()
Gets the locale. This locale is used for fetching default number or date format information.


public void applyPattern(String newPattern)
Sets the pattern. See the class description.


public String toPattern()
Gets the pattern. See the class description.


public void setFormats(Format[] newFormats)
Sets formats to use on parameters. See the class description about format numbering.


public void setFormat(int variable,
                      Format newFormat)
Set a format to be used on a variable in the pattern.
variable - the zero-based number of the variable in the format. This is not the argument number. If variable is out of range, an ArrayIndexOutOfBoundsException is thrown.
newFormat - the format to use for the specified variable


public Format[] getFormats()
Gets formats that were set with setFormats. See the class description about format numbering.


public final StringBuffer format(Object[] source,
                                 StringBuffer result,
                                 FieldPosition ignore)
Returns pattern with formatted objects. If source is null, the original pattern is returned, if source contains null objects, the formatted result will substitute each argument with the string "null".
source - an array of objects to be formatted & substituted.
result - where text is appended.
ignore - no useful status is returned.


public static String format(String pattern,
                            Object[] arguments)
Convenience routine. Avoids explicit creation of MessageFormat, but doesn't allow future optimizations.


public final StringBuffer format(Object source,
                                 StringBuffer result,
                                 FieldPosition ignore)
format in class Format


public Object[] parse(String source,
                      ParsePosition status)
Parses the string.

Caveats: The parse may fail in a number of circumstances. For example:

When the parse fails, use ParsePosition.getErrorIndex() to find out where in the string did the parsing failed. The returned error index is the starting offset of the sub-patterns that the string is comparing with. For example, if the parsing string "AAA {0} BBB" is comparing against the pattern "AAD {0} BBB", the error index is 0. When an error occurs, the call to this method will return null. If the soruce is null, return an empty array.


public Object[] parse(String source)
               throws ParseException
Parses the string. Does not yet handle recursion (where the substituted strings contain {n} references.)
ParseException - if the string can't be parsed.


public Object parseObject(String text,
                          ParsePosition status)
Parses the string. Does not yet handle recursion (where the substituted strings contain %n references.)
parseObject in class Format


public Object clone()
Overrides Cloneable
clone in class Format


public boolean equals(Object obj)
Equality comparision between two message format objects
equals in class Object


public int hashCode()
Generates a hash code for the message format object.
hashCode in class Object

Java Platform 1.2
Beta 4

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