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
String:
StringPartopt
String StringPart
StringPart:
''
' QuotedString '
UnquotedString
SubformatPattern:
SubformatPatternPartopt
SubformatPattern SubformatPatternPart
SubFormatPatternPart:
' QuotedPattern '
UnquotedPattern
Within a String, "''"
represents a single
quote. A QuotedString can contain arbitrary characters
except single quotes; the surrounding single quotes are removed.
An UnquotedString can contain arbitrary characters
except single quotes and left curly brackets. Thus, a string that
should result in the formatted message "'{0}'" can be written as
"'''{'0}''"
or "'''{0}'''"
.
Within a SubformatPattern, different rules apply.
A QuotedPattern can contain arbitrary characters
except single quotes; but the surrounding single quotes are
not removed, so they may be interpreted by the
subformat. For example, "{1,number,$'#',##}"
will
produce a number format with the pound-sign quoted, with a result
such as: "$#31,45".
An UnquotedPattern can contain arbitrary characters
except single quotes, but curly braces within it must be balanced.
For example, "ab {0} de"
and "ab '}' de"
are valid subformat patterns, but "ab {0'}' de"
and
"ab } de"
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.
Format Type
| Format Style
| 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(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())
|
SubformatPattern
| new SimpleDateFormat(subformatPattern, 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())
|
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")}
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.