com.ibm.icu.text

Class MessageFormat


public class MessageFormat
extends UFormat

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.

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

 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 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) 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())
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:

 Object[] arguments = {
     new Integer(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 = {new Long(3), "MyDisk"};

 MessageFormat form = new MessageFormat(
     "The disk \"{1}\" contains {0} file(s).");

 System.out.println(form.format(testArgs));

 // output, with different testArgs
 output: The disk "MyDisk" contains 0 file(s).
 output: The disk "MyDisk" contains 1 file(s).
 output: 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 = {new Long(12373), "MyDisk"};

 System.out.println(form.format(testArgs));

 // output, with different testArgs
 output: The disk "MyDisk" contains no files.
 output: The disk "MyDisk" contains one file.
 output: The disk "MyDisk" contains 1,273 files.
 
You can either do this programmatically, as in the above example, or by using a pattern (see ChoiceFormat for more information) as in:
 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 occurances 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.

Author:
Mark Davis
See Also:
Locale, Format, NumberFormat, DecimalFormat, ChoiceFormat

Constructor Summary

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

Method Summary

void
applyPattern(String pattern)
Sets the pattern used by this message format.
static String
autoQuoteApostrophe(String pattern)
Convert an 'apostrophe-friendly' pattern into a standard pattern.
Object
clone()
Creates and returns a copy of this object.
boolean
equals(Object obj)
Equality comparison between two message format objects
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.
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.
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.
ULocale
getULocale()
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.
void
setLocale(ULocale 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.

Methods inherited from class com.ibm.icu.text.UFormat

getLocale

Constructor Details

MessageFormat

public MessageFormat(String pattern)
Parameters:
pattern - the pattern for this message format

MessageFormat

public MessageFormat(String pattern,
                     Locale locale)
Parameters:
pattern - the pattern for this message format
locale - the locale for this message format

MessageFormat

public MessageFormat(String pattern,
                     ULocale 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

Method Details

applyPattern

public void applyPattern(String pattern)
Parameters:
pattern - the pattern for this message format

autoQuoteApostrophe

public static String autoQuoteApostrophe(String pattern)
Convert an 'apostrophe-friendly' pattern into a standard pattern. Standard patterns treat all apostrophes as quotes, which is problematic in some languages, e.g. French, where apostrophe is commonly used. This utility assumes that only an unpaired apostrophe immediately before a brace is a true quote. Other unpaired apostrophes are paired, and the resulting standard pattern string is returned.

Note it is not guaranteed that the returned pattern is indeed a valid pattern. The only effect is to convert between patterns having different quoting semantics.

Parameters:
pattern - the 'apostrophe-friendly' patttern to convert
Returns:
the standard equivalent of the original pattern

clone

public Object clone()
Creates and returns a copy of this object.
Returns:
a clone of this instance.

equals

public boolean equals(Object obj)
Equality comparison between two message format objects

format

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.

format

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.

format

public static String format(String pattern,
                            Object[] arguments)

getFormats

public Format[] getFormats()
Returns:
the formats used for the format elements in the pattern

getFormatsByArgumentIndex

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

getLocale

public Locale getLocale()
Gets the locale that's used when creating or comparing subformats.
Returns:
the locale used when creating or comparing subformats

getULocale

public ULocale getULocale()
Gets the locale that's used when creating or comparing subformats.
Returns:
the locale used when creating or comparing subformats

hashCode

public int hashCode()
Generates a hash code for the message format object.

parse

public Object[] parse(String source)
            throws ParseException
Parameters:
source - A String whose beginning should be parsed.
Returns:
An Object array parsed from the string.

parse

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 did 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.

parseObject

public Object parseObject(String source,
                          ParsePosition pos)
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.

setFormat

public void setFormat(int formatElementIndex,
                      Format newFormat)
Parameters:
formatElementIndex - the index of a format element within the pattern
newFormat - the format to use for the specified format element

setFormatByArgumentIndex

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

setFormats

public void setFormats(Format[] newFormats)
Parameters:
newFormats - the new formats to use

setFormatsByArgumentIndex

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

setLocale

public void setLocale(Locale locale)
Parameters:
locale - the locale to be used when creating or comparing subformats

setLocale

public void setLocale(ULocale locale)
Sets the locale to be used when creating or comparing subformats. This affects subsequent calls to the applyPattern and toPattern methods as well as to the format and formatToCharacterIterator methods.
Parameters:
locale - the locale to be used when creating or comparing subformats

toPattern

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

Copyright (c) 2006 IBM Corporation and others.