MessageFormat

Class Overview

Produces concatenated messages in language-neutral way. New code should probably use Formatter instead.

MessageFormat takes a set of objects, formats them and 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.

Patterns and their interpretation

 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:
         StringPart<sub>opt</sub>
         String StringPart
 StringPart:
         ''
         ' QuotedString '
         UnquotedString
 SubformatPattern:
         SubformatPatternPart<sub>opt</sub>
         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,$'#',##"} will produce a number format with the hash-sign quoted, with a result such as: "$#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.
Note also that the simplest way to avoid the problem is to use the real apostrophe (single quote) character ’ (') for human-readable text, and to use the ASCII apostrophe (' ' ) only in program syntax, like quoting in MessageFormat. See the annotations for U+0027 Apostrophe in The Unicode Standard.

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.

Usage Information

Here are some examples of usage:

 Object[] arguments = {
         Integer.valueOf(7), new Date(System.currentTimeMillis()),
         "a disturbance in the Force"};
 String result = MessageFormat.format(
         "At {1,time} on {1,date}, there was {2} on planet {0,number,integer}.",
         arguments);
 
 Output:
 
 At 12:30 PM on Jul 3, 2053, there was a disturbance in the Force on planet 7.
 

Typically, the message format will come from resources, and the arguments will be dynamically set at runtime.

Example 2:

 Object[] testArgs = {Long.valueOf(3), "MyDisk"};
 MessageFormat form = new MessageFormat("The disk \"{1}\" contains {0} file(s).");
 System.out.println(form.format(testArgs));
 
 Output with different testArgs:
 
 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 get output such as:

 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);
 Object[] testArgs = {Long.valueOf(12373), "MyDisk"};
 System.out.println(form.format(testArgs));
 
 Output (with different testArgs):
 
 The disk "MyDisk" contains no files.
 The disk "MyDisk" contains one file.
 The disk "MyDisk" contains 1,273 files.
 

 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; occurances 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 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")}
 

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.

Summary

[Expand] Inherited Methods
From class java.text.Format Object clone() Returns a copy of this Format instance. final String format( Object object) Formats the specified object using the rules of this format. abstract StringBuffer format( Object object, StringBuffer buffer, FieldPosition field) Appends the specified object to the specified string buffer using the rules of this format. AttributedCharacterIterator formatToCharacterIterator( Object object) Formats the specified object using the rules of this format and returns an AttributedCharacterIterator with the formatted string and no attributes. Object parseObject( String string) Parses the specified string using the rules of this format. abstract Object parseObject( String string, ParsePosition position) Parses the specified string starting at the index specified by position.
From class java.lang.Object Object clone() Creates and returns a copy of this Object. boolean equals( Object o) Compares this instance with the specified object and indicates if they are equal. void finalize() Invoked when the garbage collector has detected that this instance is no longer reachable. final Class<?> getClass() Returns the unique instance of Class that represents this object's class. int hashCode() Returns an integer hash code for this object. final void notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. final void notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. String toString() Returns a string containing a concise, human-readable description of this object. final void wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object. final void wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires. final void wait(long millis) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.