public Class MessageFormat

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

Patterns and Their Interpretation

MessageFormatPattern:
        String
        MessageFormatPattern FormatElement String

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

FormatType:
        number
        dtf_date
        dtf_time
        dtf_datetime
        pre-defined DateTimeFormatter(s)
        date
        time
        choice
        list

FormatStyle:
        short
        medium
        long
        full
        integer
        currency
        percent
        compact_short
        compact_long
        or
        unit
        SubformatPattern

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.

Any constructor or method that takes a String pattern parameter will throw an IllegalArgumentException if the pattern contains an ArgumentIndex value that is equal to or exceeds an implementation limit.

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. These values are case-insensitive when passed to applyPattern(String). Combinations not shown in the table are illegal. A SubformatPattern must be a valid pattern string for the Format subclass used.

Quoting Rules in Patterns

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.

Usage Information

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 GregorianCalendar(2053, Calendar.JULY, 3, 12, 30).getTime(), event);

At 12:30:00 PM on Jul 3, 2053, there was a disturbance in the Force on planet 7.

For more sophisticated patterns, ChoiceFormat can be used with MessageFormat to produce accurate forms for singular and plural:

MessageFormat msgFmt = new MessageFormat("The disk \"{0}\" contains {1,choice,0#no files|1#one file|1< {1,number,integer} files}.");
Object[] args = {"MyDisk", fileCount};
String result = msgFmt.format(args);

The disk "MyDisk" contains no files.
The disk "MyDisk" contains one file.
The disk "MyDisk" contains 1,273 files.

Formatting Date and Time

1) a date FormatType with a full FormatStyle,

Object[] arg = {new GregorianCalendar(2023, Calendar.NOVEMBER, 16).getTime()};
var fmt = new MessageFormat("The date was {0,date,full}");
fmt.format(arg); // returns "The date was Thursday, November 16, 2023"

2) a dtf_date FormatType with a full FormatStyle,

Object[] arg = {LocalDate.of(2023, 11, 16)};
var fmt = new MessageFormat("The date was {0,dtf_date,full}");
fmt.format(arg); // returns "The date was Thursday, November 16, 2023"

3) an ISO_LOCAL_DATE FormatType,

Object[] arg = {LocalDate.of(2023, 11, 16)};
var fmt = new MessageFormat("The date was {0,ISO_LOCAL_DATE}");
fmt.format(arg); // returns "The date was 2023-11-16"

Parsing

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 = {Double.valueOf(3.1415)};
String result = mf.format( objs );
// result now equals "3.14, 3.1"
objs = mf.parse(result, new ParsePosition(0));
// objs now equals {Double.valueOf(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));
// objs 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.