MessageFormat

public class MessageFormat extends Format

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.

Patterns and Their Interpretation

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.

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.

FormatType FormatStyle 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")}

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.

See Also

Nested Class Summary

class MessageFormat.Field Defines constants that are used as attribute keys in the AttributedCharacterIterator returned from MessageFormat.formatToCharacterIterator. 

Public Constructor Summary

MessageFormat(String pattern)
Constructs a MessageFormat for the default FORMAT locale and the specified pattern.
MessageFormat(String pattern, Locale locale)
Constructs a MessageFormat for the specified locale and pattern.

Public Method Summary

void
applyPattern(String pattern)
Sets the pattern used by this message format.
Object
clone()
Creates and returns a copy of this object.
boolean
equals(Object obj)
Equality comparison between two message format objects
final StringBuffer
format(Object[] arguments, StringBuffer result, FieldPosition pos)
Formats an array of objects and appends the MessageFormat's pattern, with format elements replaced by the formatted objects, to the provided StringBuffer.
final StringBuffer
format(Object arguments, StringBuffer result, FieldPosition pos)
Formats an array of objects and appends the MessageFormat's pattern, with format elements replaced by the formatted objects, to the provided StringBuffer.
static String
format(String pattern, Object... arguments)
Creates a MessageFormat with the given pattern and uses it to format the given arguments.
AttributedCharacterIterator
formatToCharacterIterator(Object arguments)
Formats an array of objects and inserts them into the MessageFormat's pattern, producing an AttributedCharacterIterator.
Format[]
getFormats()
Gets the formats used for the format elements in the previously set pattern string.
Format[]
getFormatsByArgumentIndex()
Gets the formats used for the values passed into format methods or returned from parse methods.
Locale
getLocale()
Gets the locale that's used when creating or comparing subformats.
int
hashCode()
Generates a hash code for the message format object.
Object[]
parse(String source)
Parses text from the beginning of the given string to produce an object array.
Object[]
parse(String source, ParsePosition pos)
Parses the string.
Object
parseObject(String source, ParsePosition pos)
Parses text from a string to produce an object array.
void
setFormat(int formatElementIndex, Format newFormat)
Sets the format to use for the format element with the given format element index within the previously set pattern string.
void
setFormatByArgumentIndex(int argumentIndex, Format newFormat)
Sets the format to use for the format elements within the previously set pattern string that use the given argument index.
void
setFormats(Format[] newFormats)
Sets the formats to use for the format elements in the previously set pattern string.
void
setFormatsByArgumentIndex(Format[] newFormats)
Sets the formats to use for the values passed into format methods or returned from parse methods.
void
setLocale(Locale locale)
Sets the locale to be used when creating or comparing subformats.
String
toPattern()
Returns a pattern representing the current state of the message format.

Inherited Method Summary

Public Constructors

public MessageFormat (String pattern)

Constructs a MessageFormat for the default FORMAT locale and the specified pattern. The constructor first sets the locale, then parses the pattern and creates a list of subformats for the format elements contained in it. Patterns and their interpretation are specified in the class description.

Parameters
pattern the pattern for this message format
Throws
IllegalArgumentException if the pattern is invalid

public MessageFormat (String pattern, Locale locale)

Constructs a MessageFormat for the specified locale and pattern. The constructor first sets the locale, then parses the pattern and creates a list of subformats for the format elements contained in it. Patterns and their interpretation are specified in the class description.

Parameters
pattern the pattern for this message format
locale the locale for this message format
Throws
IllegalArgumentException if the pattern is invalid

Public Methods

public void applyPattern (String pattern)

Sets the pattern used by this message format. The method parses the pattern and creates a list of subformats for the format elements contained in it. Patterns and their interpretation are specified in the class description.

Parameters
pattern the pattern for this message format
Throws
IllegalArgumentException if the pattern is invalid

public Object clone ()

Creates and returns a copy of this object.

Returns
  • a clone of this instance.

public boolean equals (Object obj)

Equality comparison between two message format objects

Parameters
obj the object to compare this instance with.
Returns
  • true if the specified object is equal to this Object; false otherwise.

public final StringBuffer format (Object[] arguments, StringBuffer result, FieldPosition pos)

Formats an array of objects and appends the 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 ?
(new MessageFormat(subformat.format(argument), getLocale())).format(argument) : subformat.format(argument)
!= 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.

Parameters
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.
Returns
  • the string buffer passed in as result, with formatted text appended
Throws
IllegalArgumentException if an argument in the arguments array is not of the type expected by the format element(s) that use it.

public final StringBuffer format (Object arguments, StringBuffer result, FieldPosition pos)

Formats an array of objects and appends the MessageFormat's pattern, with format elements replaced by the formatted objects, to the provided StringBuffer. This is equivalent to

format((Object[]) arguments, result, pos)

Parameters
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.
Returns
  • the string buffer passed in as toAppendTo, with formatted text appended
Throws
IllegalArgumentException if an argument in the arguments array is not of the type expected by the format element(s) that use it.

public static String format (String pattern, Object... arguments)

Creates a MessageFormat with the given pattern and uses it to format the given arguments. This is equivalent to

(new MessageFormat(pattern)).format(arguments, new StringBuffer(), null).toString()

Parameters
pattern the pattern string
arguments object(s) to format
Returns
  • the formatted string
Throws
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.

public AttributedCharacterIterator formatToCharacterIterator (Object arguments)

Formats an array of objects and inserts them into the 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.

Parameters
arguments an array of objects to be formatted and substituted.
Returns
  • AttributedCharacterIterator describing the formatted value.
Throws
NullPointerException if arguments is null.
IllegalArgumentException if an argument in the arguments array is not of the type expected by the format element(s) that use it.

public Format[] getFormats ()

Gets the formats used for the format elements in the previously set pattern string. The order of formats in the returned array corresponds to the order of format elements in the pattern string.

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.

Returns
  • the formats used for the format elements in the pattern

public Format[] getFormatsByArgumentIndex ()

Gets the formats used for the values passed into 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.

Returns
  • the formats used for the arguments within the pattern

public Locale getLocale ()

Gets the locale that's used when creating or comparing subformats.

Returns
  • the locale used when creating or comparing subformats

public int hashCode ()

Generates a hash code for the message format object.

Returns
  • this object's hash code.

public Object[] parse (String source)

Parses text from the beginning of the given string to produce an object array. The method may not use the entire text of the given string.

See the parse(String, ParsePosition) method for more information on message parsing.

Parameters
source A String whose beginning should be parsed.
Returns
  • An Object array parsed from the string.
Throws
ParseException if the beginning of the specified string cannot be parsed.

public Object[] parse (String source, ParsePosition pos)

Parses the string.

Caveats: The parse may fail in a number of circumstances. For example:

  • If one of the arguments does not occur in the pattern.
  • If the format of an argument loses information, such as with a choice format where a large number formats to "many".
  • Does not yet handle recursion (where the substituted strings contain {n} references.)
  • Will not always find a match (or the correct match) if some part of the parse is ambiguous. For example, if the pattern "{1},{2}" is used with the string arguments {"a,b", "c"}, it will format as "a,b,c". When the result is parsed, it will return {"a", "b,c"}.
  • If a single argument is parsed more than once in the string, then the later parse wins.
When the parse fails, use ParsePosition.getErrorIndex() to find out where in the string the parsing failed. The returned error index is the starting offset of the sub-patterns that the string is comparing with. For example, if the parsing string "AAA {0} BBB" is comparing against the pattern "AAD {0} BBB", the error index is 0. When an error occurs, the call to this method will return null. If the source is null, return an empty array.

Parameters
source the string to parse
pos the parse position
Returns
  • an array of parsed objects

public Object parseObject (String source, ParsePosition pos)

Parses text from a string to produce an object array.

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(String, ParsePosition) method for more information on message parsing.

Parameters
source A String, part of which should be parsed.
pos A ParsePosition object with index and error index information as described above.
Returns
  • An Object array parsed from the string. In case of error, returns null.
Throws
NullPointerException if pos is null.

public void setFormat (int formatElementIndex, Format newFormat)

Sets the format to use for the format element with the given format element index within the previously set pattern string. The format element index is the zero-based number of the format element counting from the start of the pattern string.

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.

Parameters
formatElementIndex the index of a format element within the pattern
newFormat the format to use for the specified format element
Throws
ArrayIndexOutOfBoundsException if formatElementIndex is equal to or larger than the number of format elements in the pattern string

public void setFormatByArgumentIndex (int argumentIndex, Format newFormat)

Sets the format to use for the format elements within the previously set pattern string that use the given argument index. The argument index is part of the format element definition and represents an index into the 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.

Parameters
argumentIndex the argument index for which to use the new format
newFormat the new format to use

public void setFormats (Format[] newFormats)

Sets the formats to use for the format elements in the previously set pattern string. The order of formats in 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.

Parameters
newFormats the new formats to use
Throws
NullPointerException if newFormats is null

public void setFormatsByArgumentIndex (Format[] newFormats)

Sets the formats to use for the values passed into 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.

Parameters
newFormats the new formats to use
Throws
NullPointerException if newFormats is null

public void setLocale (Locale locale)

Sets the locale to be used when creating or comparing subformats. This affects subsequent calls

  • to the applyPattern and toPattern methods if format elements specify a format type and therefore have the subformats created in the applyPattern method, as well as
  • to the format and formatToCharacterIterator methods if format elements do not specify a format type and therefore have the subformats created in the formatting methods.
Subformats that have already been created are not affected.

Parameters
locale the locale to be used when creating or comparing subformats

public String toPattern ()

Returns a pattern representing the current state of the message format. The string is constructed from internal information and therefore does not necessarily equal the previously applied pattern.

Returns
  • a pattern representing the current state of the message format