MessageFormat

public class MessageFormat extends Format

MessageFormat provides a means to produce concatenated messages in a 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 itself doesn't implement locale specific behavior. Any locale specific behavior is defined by the pattern that you provide as well as the subformats used for inserted arguments.

Patterns and Their Interpretation

MessageFormat uses patterns of the following form:
 MessageFormatPattern:
         String
         MessageFormatPattern FormatElement String

 FormatElement:
         { ArgumentIndex }
         { ArgumentIndex , FormatType }
         { ArgumentIndex , FormatType , FormatStyle }

 FormatType: one of 
         number date time choice

 FormatStyle:
         short
         medium
         long
         full
         integer
         currency
         percent
         SubformatPattern
 

Within a String, a pair of single quotes can be used to quote any arbitrary characters except single quotes. For example, pattern string "'{0}'" represents string "{0}", not a FormatElement. A single quote itself must be represented by doubled single quotes '' throughout a String. For example, pattern string "'{''}'" is interpreted as a sequence of '{ (start of quoting and a left curly brace), '' (a single quote), and }' (a right curly brace and end of quoting), not '{' and '}' (quoted left and right curly braces): representing string "{'}", not "{}".

A SubformatPattern is interpreted by its corresponding subformat, and subformat-dependent pattern rules apply. For example, pattern string "{1,number,$'#',##}" (SubformatPattern with underline) will produce a number format with the pound-sign quoted, with a result such as: "$#31,45". Refer to each Format subclass documentation for details.

Any unmatched quote is treated as closed at the end of the given pattern. For example, pattern string "'{0}" is treated as pattern "'{0}'".

Any curly braces within an unquoted pattern must be balanced. For example, "ab {0} de" and "ab '}' de" are valid patterns, but "ab {0'}' de", "ab } de" and "''{''" are not.

Warning:
The rules for using quotes within message format patterns unfortunately have shown to be somewhat confusing. In particular, it isn't always obvious to localizers whether single quotes need to be doubled or not. Make sure to inform localizers about the rules, and tell them (for example, by using comments in resource bundle source files) which strings will be processed by MessageFormat. Note that localizers may need to use single quotes in translated strings where the original version doesn't have them.

The ArgumentIndex value is a non-negative integer written using the digits '0' through '9', and represents an index into the arguments array passed to the format methods or the result array returned by the parse methods.

The FormatType and FormatStyle values are used to create a Format instance for the format element. The following table shows how the values map to Format instances. Combinations not shown in the table are illegal. A SubformatPattern must be a valid pattern string for the Format subclass used.

FormatType FormatStyle Subformat Created
(none) (none) null
number (none) NumberFormat.getInstance(getLocale())
integer NumberFormat.getIntegerInstance(getLocale())
currency NumberFormat.getCurrencyInstance(getLocale())
percent NumberFormat.getPercentInstance(getLocale())
SubformatPattern new DecimalFormat(subformatPattern, DecimalFormatSymbols.getInstance(getLocale()))
date (none) DateFormat.getDateInstance(DEFAULT, getLocale())
short DateFormat.getDateInstance(SHORT, getLocale())
medium DateFormat.getDateInstance(DEFAULT, getLocale())
long DateFormat.getDateInstance(LONG, getLocale())
full DateFormat.getDateInstance(FULL, getLocale())
SubformatPattern new SimpleDateFormat(subformatPattern, getLocale())
time (none) DateFormat.getTimeInstance(DEFAULT, getLocale())
short DateFormat.getTimeInstance(SHORT, getLocale())
medium DateFormat.getTimeInstance(DEFAULT, getLocale())
long DateFormat.getTimeInstance(LONG, getLocale())
full DateFormat.getTimeInstance(FULL, getLocale())
SubformatPattern new SimpleDateFormat(subformatPattern, getLocale())
choice SubformatPattern new ChoiceFormat(subformatPattern)

Usage Information

Here are some examples of usage. In real internationalized programs, the message format pattern and other static strings will, of course, be obtained from resource bundles. Other parameters will be dynamically determined at runtime.

The first example uses the static method MessageFormat.format, which internally creates a MessageFormat for one-time use:

 int planet = 7;
 String event = "a disturbance in the Force";

 String result = MessageFormat.format(
     "At {1,time} on {1,date}, there was {2} on planet {0,number,integer}.",
     planet, new Date(), event);
 
The output is:
 At 12:30 PM on Jul 3, 2053, there was a disturbance in the Force on planet 7.
 

The following example creates a MessageFormat instance that can be used repeatedly:

 int fileCount = 1273;
 String diskName = "MyDisk";
 Object[] testArgs = {new Long(fileCount), diskName};

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

 System.out.println(form.format(testArgs));
 
The output with different values for fileCount:
 The disk "MyDisk" contains 0 file(s).
 The disk "MyDisk" contains 1 file(s).
 The disk "MyDisk" contains 1,273 file(s).
 

For more sophisticated patterns, you can use a ChoiceFormat to produce correct forms for singular and plural:

 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.setFormatByArgumentIndex(0, fileform);

 int fileCount = 1273;
 String diskName = "MyDisk";
 Object[] testArgs = {new Long(fileCount), diskName};

 System.out.println(form.format(testArgs));
 
The output with different values for fileCount:
 The disk "MyDisk" contains no files.
 The disk "MyDisk" contains one file.
 The disk "MyDisk" contains 1,273 files.
 

You can create the ChoiceFormat programmatically, as in the above example, or by using a pattern. See ChoiceFormat for more information.

 form.applyPattern(
    "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 as special; occurrences of '{' are used to indicate 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.

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 occurrences 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")}
 

Synchronization

Message formats are 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

class MessageFormat.Field Defines constants that are used as attribute keys in the AttributedCharacterIterator returned from MessageFormat.formatToCharacterIterator

Public Constructor Summary

MessageFormat(String pattern)
Constructs a MessageFormat for the default locale and the specified pattern.
MessageFormat(String pattern, Locale locale)
Constructs a MessageFormat for the specified locale and pattern.

Public Method Summary

void
applyPattern(String pattern)
Sets the pattern used by this message format.
Object
clone()
Creates and returns a copy of this object.
boolean
equals(Object obj)
Equality comparison between two message format objects
final StringBuffer
format(Object[] arguments, StringBuffer result, FieldPosition pos)
Formats an array of objects and appends the MessageFormat's pattern, with format elements replaced by the formatted objects, to the provided StringBuffer.
final StringBuffer
format(Object arguments, StringBuffer result, FieldPosition pos)
Formats an array of objects and appends the MessageFormat's pattern, with format elements replaced by the formatted objects, to the provided StringBuffer.
static String
format(String pattern, Object... arguments)
Creates a MessageFormat with the given pattern and uses it to format the given arguments.
AttributedCharacterIterator
formatToCharacterIterator(Object arguments)
Formats an array of objects and inserts them into the MessageFormat's pattern, producing an AttributedCharacterIterator.
Format[]
getFormats()
Gets the formats used for the format elements in the previously set pattern string.
Format[]
getFormatsByArgumentIndex()
Gets the formats used for the values passed into format methods or returned from parse methods.
Locale
getLocale()
Gets the locale that's used when creating or comparing subformats.
int
hashCode()
Generates a hash code for the message format object.
Object[]
parse(String source)
Parses text from the beginning of the given string to produce an object array.