MessageFormat prepares strings for display to users, with optional arguments (variables/placeholders). The arguments can occur in any order, which is necessary for translation into languages with different grammars.

A MessageFormat is constructed from a pattern string with arguments in {curly braces} which will be replaced by formatted values.

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). 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 and the subformats used for inserted arguments.

Arguments can be named (using identifiers) or numbered (using small ASCII-digit integers). Some of the API methods work only with argument numbers and throw an exception if the pattern has named arguments (see usesNamedArguments()).

An argument might not specify any format type. In this case, a Number value is formatted with a default (for the locale) NumberFormat, a Date value is formatted with a default (for the locale) DateFormat, and for any other value its toString() value is used.

An argument might specify a "simple" type for which the specified Format object is created, cached and used.

An argument might have a "complex" type with nested MessageFormat sub-patterns. During formatting, one of these sub-messages is selected according to the argument value and recursively formatted.

After construction, a custom Format object can be set for a top-level argument, overriding the default formatting and parsing behavior for that argument. However, custom formatting can be achieved more simply by writing a typeless argument in the pattern string and supplying it with a preformatted string value.

When formatting, MessageFormat takes a collection of argument values and writes an output string. The argument values may be passed as an array (when the pattern contains only numbered arguments) or as a Map (which works for both named and numbered arguments).

Each argument is matched with one of the input values by array index or map key and formatted according to its pattern specification (or using a custom Format object if one was set). A numbered pattern argument is matched with a map key that contains that number as an ASCII-decimal-digit string (without leading zero).

Patterns and Their Interpretation

MessageFormat uses patterns of the following form:

message = messageText (argument messageText)*
argument = noneArg | simpleArg | complexArg
complexArg = choiceArg | pluralArg | selectArg | selectordinalArg

noneArg = '{' argNameOrNumber '}'
simpleArg = '{' argNameOrNumber ',' argType [',' argStyle] '}'
choiceArg = '{' argNameOrNumber ',' "choice" ',' choiceStyle '}'
pluralArg = '{' argNameOrNumber ',' "plural" ',' pluralStyle '}'
selectArg = '{' argNameOrNumber ',' "select" ',' selectStyle '}'
selectordinalArg = '{' argNameOrNumber ',' "selectordinal" ',' pluralStyle '}'

choiceStyle: see ChoiceFormat
pluralStyle: see PluralFormat
selectStyle: see SelectFormat

argNameOrNumber = argName | argNumber
argName = [^[[:Pattern_Syntax:][:Pattern_White_Space:]]]+
argNumber = '0' | ('1'..'9' ('0'..'9')*)

argType = "number" | "date" | "time" | "spellout" | "ordinal" | "duration"
argStyle = "short" | "medium" | "long" | "full" | "integer" | "currency" | "percent" | argStyleText | "::" argSkeletonText

• messageText can contain quoted literal strings including syntax characters. A quoted literal string begins with an ASCII apostrophe and a syntax character (usually a {curly brace}) and continues until the next single apostrophe. A double ASCII apostrophe inside or outside of a quoted string represents one literal apostrophe.
• Quotable syntax characters are the {curly braces} in all messageText parts, plus the '#' sign in a messageText immediately inside a pluralStyle, and the '|' symbol in a messageText immediately inside a choiceStyle.
• See also MessagePattern.ApostropheMode
• In argStyleText, every single ASCII apostrophe begins and ends quoted literal text, and unquoted {curly braces} must occur in matched pairs.

Recommendation: Use the real apostrophe (single quote) character \\u2019 for human-readable text, and use the ASCII apostrophe (\\u0027 ' ) only in program syntax, like quoting in MessageFormat. See the annotations for U+0027 Apostrophe in The Unicode Standard.

The choice argument type is deprecated. Use plural arguments for proper plural selection, and select arguments for simple selection among a fixed set of choices.

The argType and argStyle 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. Any argStyleText must be a valid pattern string for the Format subclass used.

argType	argStyle	resulting Format object
(none)		null
number	(none)	NumberFormat.getInstance(getLocale())
	integer	NumberFormat.getIntegerInstance(getLocale())
	currency	NumberFormat.getCurrencyInstance(getLocale())
	percent	NumberFormat.getPercentInstance(getLocale())
	argStyleText	new DecimalFormat(argStyleText, new DecimalFormatSymbols(getLocale()))
	argSkeletonText	NumberFormatter.forSkeleton(argSkeletonText).locale(getLocale()).toFormat()
date	(none)	DateFormat.getDateInstance(DateFormat.DEFAULT, getLocale())
	short	DateFormat.getDateInstance(DateFormat.SHORT, getLocale())
	medium	DateFormat.getDateInstance(DateFormat.DEFAULT, getLocale())
	long	DateFormat.getDateInstance(DateFormat.LONG, getLocale())
	full	DateFormat.getDateInstance(DateFormat.FULL, getLocale())
	argStyleText	new SimpleDateFormat(argStyleText, getLocale())
	argSkeletonText	DateFormat.getInstanceForSkeleton(argSkeletonText, getLocale())
time	(none)	DateFormat.getTimeInstance(DateFormat.DEFAULT, getLocale())
	short	DateFormat.getTimeInstance(DateFormat.SHORT, getLocale())
	medium	DateFormat.getTimeInstance(DateFormat.DEFAULT, getLocale())
	long	DateFormat.getTimeInstance(DateFormat.LONG, getLocale())
	full	DateFormat.getTimeInstance(DateFormat.FULL, getLocale())
	argStyleText	new SimpleDateFormat(argStyleText, getLocale())
spellout	argStyleText (optional)	new RuleBasedNumberFormat(getLocale(), RuleBasedNumberFormat.SPELLOUT)     .setDefaultRuleset(argStyleText);
ordinal	argStyleText (optional)	new RuleBasedNumberFormat(getLocale(), RuleBasedNumberFormat.ORDINAL)     .setDefaultRuleset(argStyleText);
duration	argStyleText (optional)	new RuleBasedNumberFormat(getLocale(), RuleBasedNumberFormat.DURATION)     .setDefaultRuleset(argStyleText);

Differences from java.text.MessageFormat

The ICU MessageFormat supports both named and numbered arguments, while the JDK MessageFormat only supports numbered arguments. Named arguments make patterns more readable.

ICU implements a more user-friendly apostrophe quoting syntax. In message text, an apostrophe only begins quoting literal text if it immediately precedes a syntax character (mostly {curly braces}).
In the JDK MessageFormat, an apostrophe always begins quoting, which requires common text like "don't" and "aujourd'hui" to be written with doubled apostrophes like "don''t" and "aujourd''hui". For more details see MessagePattern.ApostropheMode.

ICU does not create a ChoiceFormat object for a choiceArg, pluralArg or selectArg but rather handles such arguments itself. The JDK MessageFormat does create and use a ChoiceFormat object (new ChoiceFormat(argStyleText)). The JDK does not support plural and select arguments at all.

Both the ICU and the JDK MessageFormat can control the argument formats by using argStyle. But the JDK MessageFormat only supports predefined formats and number / date / time pattern strings (which would need to be localized).
ICU supports everything the JDK does, and also number / date / time skeletons using the :: prefix (which automatically yield output appropriate for the MessageFormat locale).

Argument formatting

Arguments are formatted according to their type, using the default ICU formatters for those types, unless otherwise specified. For unknown types, MessageFormat will call toString().

There are also several ways to control the formatting.

We recommend you use default styles, predefined style values, skeletons, or preformatted values, but not pattern strings or custom format objects.

For more details, see the ICU User Guide.

Usage Information

Here are some examples of usage:

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

String result = MessageFormat.format(
    "At {1,time,::jmm} on {1,date,::dMMMM}, there was {2} on planet {0,number,integer}.",
    arguments);

output: At 4:34 PM on March 23, 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 = { 3, "MyDisk" };

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

System.out.println(form.format(testArgs));

// 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).

For messages that include plural forms, you can use a plural argument:

MessageFormat msgFmt = new MessageFormat(
    "{num_files, plural, " +
    "=0{There are no files on disk \"{disk_name}\".}" +
    "=1{There is one file on disk \"{disk_name}\".}" +
    "other{There are # files on disk \"{disk_name}\".}}",
    ULocale.ENGLISH);
Map args = new HashMap();
args.put("num_files", 0);
args.put("disk_name", "MyDisk");
System.out.println(msgFmt.format(args));
args.put("num_files", 3);
System.out.println(msgFmt.format(args));

output:
There are no files on disk "MyDisk".
There are 3 files on "MyDisk".

See PluralFormat and PluralRules for details.

Synchronization

MessageFormats 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.