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.
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.
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.
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);
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));
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));
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,integerfiles}."); }
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")}
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.
extends
java.util.Locale, Format, NumberFormat, DecimalFormat, DecimalFormatSymbols, ChoiceFormat, DateFormat, SimpleDateFormat