MessageFormat provides a means to produce concatenated
messages in 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, new DecimalFormatSymbols(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())
|
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,integer} files}.");
Note: As we see above, the string produced
by a ChoiceFormat in MessageFormat is treated specially;
occurences of '{' are used to indicated 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 occurences 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.
format
(obj,
new StringBuffer(), new FieldPosition(0)).toString();
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.
MessageFormat's
pattern, with format elements replaced by the formatted objects, to the
provided StringBuffer.
This is equivalent to
format
((Object[]) arguments, result, pos)
(new MessageFormat
(pattern)).format
(arguments, new StringBuffer(), null).toString()
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.
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.
format 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.
wait methods.
The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:
synchronized statement
that synchronizes on the object.
Class, by executing a
synchronized static method of that class.
Only one thread at a time can own an object's monitor.
wait methods.
The awakened threads will not be able to proceed until the current thread relinquishes the lock on this object. The awakened threads will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened threads enjoy no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
Caveats: The parse may fail in a number of circumstances. For example:
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.
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.
arguments 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.
newFormats 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.
format 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.
format and
formatToCharacterIterator
methods.toString method returns a string that
"textually represents" this object. The result should
be a concise but informative representation that is easy for a
person to read.
It is recommended that all subclasses override this method.
The toString method for class Object
returns a string consisting of the name of the class of which the
object is an instance, the at-sign character `@', and
the unsigned hexadecimal representation of the hash code of the
object. In other words, this method returns a string equal to the
value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up
either through a call to the notify method or the
notifyAll method. The thread then waits until it can
re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait();
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.The current thread must own this object's monitor.
This method causes the current thread (call it T) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. Thread T becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:
A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout);
... // Perform action appropriate to condition
}
(For more information on this topic, see Section 3.2.3 in Doug Lea's
"Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
2000), or Item 50 in Joshua Bloch's "Effective Java Programming
Language Guide" (Addison-Wesley, 2001).
If the current thread is interrupted by another thread while it is waiting, then an InterruptedException is thrown. This exception is not thrown until the lock status of this object has been restored as described above.
Note that the wait method, as it places the current thread into the wait set for this object, unlocks only this object; any other objects on which the current thread may be synchronized remain locked while the thread waits.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
This method is similar to the wait method of one
argument, but it allows finer control over the amount of time to
wait for a notification before giving up. The amount of real time,
measured in nanoseconds, is given by:
1000000*timeout+nanos
In all other respects, this method does the same thing as the method of one argument. In particular, wait(0, 0) means the same thing as wait(0).
The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until either of the following two conditions has occurred:
notify method
or the notifyAll method.
timeout
milliseconds plus nanos nanoseconds arguments, has
elapsed.
The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout, nanos);
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.