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.
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)
|
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
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.
java.util.LocaleFormatNumberFormatDecimalFormatChoiceFormatpattern the pattern for this message formatjava.lang.IllegalArgumentException if the pattern is invalidpattern the pattern for this message formatlocale the locale for this message formatjava.lang.IllegalArgumentException if the pattern is invalidapplyPattern
and toPattern methods if format elements specify
a format type and therefore have the subformats created in the
applyPattern method, as well as
format and
formatToCharacterIterator methods
if format elements do not specify a format type and therefore have
the subformats created in the formatting methods.
locale the locale to be used when creating or comparing subformatspattern the pattern for this message formatjava.lang.IllegalArgumentException if the pattern is invalidformat methods or returned from parse
methods. The indices of elements in newFormats
correspond to the argument indices used in the previously set
pattern string.
The order of formats in newFormats thus corresponds to
the order of elements in the arguments array passed
to the format methods or the result array returned
by the parse methods.
If an argument index is used for more than one format element
in the pattern string, then the corresponding new format is used
for all such format elements. If an argument index is not used
for any format element in the pattern string, then the
corresponding new format is ignored. If fewer formats are provided
than needed, then only the formats for argument indices less
than newFormats.length are replaced.
newFormats the new formats to usejava.lang.NullPointerException if newFormats is nullnewFormats corresponds to
the order of format elements in the pattern string.
If more formats are provided than needed by the pattern string,
the remaining ones are ignored. If fewer formats are provided
than needed, then only the first newFormats.length
formats are replaced.
Since the order of format elements in a pattern string often
changes during localization, it is generally better to use the
setFormatsByArgumentIndex
method, which assumes an order of formats corresponding to the
order of elements in the arguments array passed to
the format methods or the result array returned by
the parse methods.
newFormats the new formats to usejava.lang.NullPointerException if newFormats is nullarguments array passed
to the format methods or the result array returned
by the parse methods.
If the argument index is used for more than one format element in the pattern string, then the new format is used for all such format elements. If the argument index is not used for any format element in the pattern string, then the new format is ignored.
argumentIndex the argument index for which to use the new formatnewFormat the new format to use
Since the order of format elements in a pattern string often
changes during localization, it is generally better to use the
setFormatByArgumentIndex
method, which accesses format elements based on the argument
index they specify.
formatElementIndex the index of a format element within the patternnewFormat the format to use for the specified format elementjava.lang.ArrayIndexOutOfBoundsException if formatElementIndex is equal to or
larger than the number of format elements in the pattern stringformat methods or returned from parse
methods. The indices of elements in the returned array
correspond to the argument indices used in the previously set
pattern string.
The order of formats in the returned array thus corresponds to
the order of elements in the arguments array passed
to the format methods or the result array returned
by the parse methods.
If an argument index is used for more than one format element in the pattern string, then the format used for the last such format element is returned in the array. If an argument index is not used for any format element in the pattern string, then null is returned in the array.
Since the order of format elements in a pattern string often
changes during localization, it's generally better to use the
getFormatsByArgumentIndex
method, which assumes an order of formats corresponding to the
order of elements in the arguments array passed to
the format methods or the result array returned by
the parse methods.
MessageFormat's
pattern, with format elements replaced by the formatted objects, to the
provided StringBuffer.
The text substituted for the individual format elements is derived from
the current subformat of the format element and the
arguments element at the format element's argument index
as indicated by the first matching line of the following table. An
argument is unavailable if arguments is
null or has fewer than argumentIndex+1 elements.
| Subformat | Argument | Formatted Text |
|---|---|---|
| any | unavailable | "{" + argumentIndex + "}"
|
| any | null
| "null"
|
instanceof ChoiceFormat
| any | subformat.format(argument).indexOf('{') >= 0 ?
|
!= null
| any | subformat.format(argument)
|
null
| instanceof Number
| NumberFormat.getInstance(getLocale()).format(argument)
|
null
| instanceof Date
| DateFormat.getDateTimeInstance(DateFormat.SHORT, DateFormat.SHORT, getLocale()).format(argument)
|
null
| instanceof String
| argument
|
null
| any | argument.toString()
|
If pos is non-null, and refers to
Field.ARGUMENT, the location of the first formatted
string will be returned.
arguments an array of objects to be formatted and substituted.result where text is appended.pos On input: an alignment field, if desired.
On output: the offsets of the alignment field.java.lang.IllegalArgumentException if an argument in the
arguments array is not of the type
expected by the format element(s) that use it.
(new MessageFormat(pattern)).format(arguments, new StringBuffer(), null).toString()
java.lang.IllegalArgumentException if the pattern is invalid,
or if an argument in the arguments array
is not of the type expected by the format element(s)
that use it.MessageFormat's
pattern, with format elements replaced by the formatted objects, to the
provided StringBuffer.
This is equivalent to
format((Object[]) arguments, result, pos)
arguments an array of objects to be formatted and substituted.result where text is appended.pos On input: an alignment field, if desired.
On output: the offsets of the alignment field.java.lang.IllegalArgumentException if an argument in the
arguments array is not of the type
expected by the format element(s) that use it.MessageFormat's pattern, producing an
AttributedCharacterIterator.
You can use the returned AttributedCharacterIterator
to build the resulting String, as well as to determine information
about the resulting String.
The text of the returned AttributedCharacterIterator is
the same that would be returned by
format(arguments, new StringBuffer(), null).toString()
In addition, the AttributedCharacterIterator contains at
least attributes indicating where text was generated from an
argument in the arguments array. The keys of these attributes are of
type MessageFormat.Field, their values are
Integer objects indicating the index in the arguments
array of the argument from which the text was generated.
The attributes/value from the underlying Format
instances that MessageFormat uses will also be
placed in the resulting AttributedCharacterIterator.
This allows you to not only find where an argument is placed in the
resulting String, but also which fields it contains in turn.
arguments an array of objects to be formatted and substituted.java.lang.NullPointerException if arguments is null.java.lang.IllegalArgumentException if an argument in the
arguments array is not of the type
expected by the format element(s) that use it.Caveats: The parse may fail in a number of circumstances. For example:
See the parse(java.lang.String,java.text.ParsePosition)
source A String whose beginning should be parsed.Object array parsed from the string.ParseException if the beginning of the specified string
cannot be parsed.
The method attempts to parse text starting at the index given by
pos.
If parsing succeeds, then the index of pos is updated
to the index after the last character used (parsing does not necessarily
use all characters up to the end of the string), and the parsed
object array is returned. The updated pos can be used to
indicate the starting point for the next call to this method.
If an error occurs, then the index of pos is not
changed, the error index of pos is set to the index of
the character where the error occurred, and null is returned.
See the parse(java.lang.String,java.text.ParsePosition)
source A String, part of which should be parsed.pos A ParsePosition object with index and error
index information as described above.Object array parsed from the string. In case of
error, returns null.java.lang.NullPointerException if pos is null.java.io.InvalidObjectException if the constant could not be
resolved.offsets. Can also be thought of
as the index of the highest-numbered element in offsets that is being used.
All of these arrays should have the same number of elements being used as offsets
does, and so this variable suffices to tell us how many entries are in all of them.
characterIterators is
non-null, AttributedCharacterIterator will be created from the
subformats as necessary. If characterIterators is null
and fp is non-null and identifies
Field.MESSAGE_ARGUMENT, the location of
the first replaced argument will be set in it.
java.lang.IllegalArgumentException if an argument in the
arguments array is not of the type
expected by the format element(s) that use it.java.io.InvalidObjectException if the objects read from the stream is invalid.