Package java.text

Class MessageFormat

java.lang.Object
java.text.Format
java.text.MessageFormat
All Implemented Interfaces:
Serializable, Cloneable
public class MessageFormat
extends Format
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

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:
         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.

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 = {
         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.
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 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:
Formatter, Serialized Form

  • Constructor Details

    • MessageFormat

      public MessageFormat​(String template, Locale locale)
      Constructs a new MessageFormat using the specified pattern and locale.
      Parameters:
      template - the pattern.
      locale - the locale.
      Throws:
      IllegalArgumentException - if the pattern cannot be parsed.

    • MessageFormat

      public MessageFormat​(String template)
      Constructs a new MessageFormat using the specified pattern and the user's default locale. See "Be wary of the default locale".
      Parameters:
      template - the pattern.
      Throws:
      IllegalArgumentException - if the pattern cannot be parsed.

  • Method Details

    • applyPattern

      public void applyPattern​(String template)
      Changes this MessageFormat to use the specified pattern.
      Parameters:
      template - the new pattern.
      Throws:
      IllegalArgumentException - if the pattern cannot be parsed.

    • clone

      public Object clone()
      Returns a new instance of MessageFormat with the same pattern and formats as this MessageFormat.
      Overrides:
      clone in class Format
      Returns:
      a shallow copy of this MessageFormat.
      See Also:
      Cloneable

    • equals

      public boolean equals​(Object object)
      Compares the specified object to this MessageFormat and indicates if they are equal. In order to be equal, object must be an instance of MessageFormat and have the same pattern.
      Overrides:
      equals in class Object
      Parameters:
      object - the object to compare with this object.
      Returns:
      true if the specified object is equal to this MessageFormat; false otherwise.
      See Also:
      hashCode()

    • formatToCharacterIterator

      public AttributedCharacterIterator formatToCharacterIterator​(Object object)
      Formats the specified object using the rules of this message format and returns an AttributedCharacterIterator with the formatted message and attributes. The AttributedCharacterIterator returned also includes the attributes from the formats of this message format.
      Overrides:
      formatToCharacterIterator in class Format
      Parameters:
      object - the object to format.
      Returns:
      an AttributedCharacterIterator with the formatted message and attributes.
      Throws:
      IllegalArgumentException - if the arguments in the object array cannot be formatted by this message format.

    • format

      public final StringBuffer format​(Object[] objects, StringBuffer buffer, FieldPosition field)
      Converts the specified objects into a string which it appends to the specified string buffer using the pattern of this message format.

      If the field member of the specified FieldPosition is MessageFormat.Field.ARGUMENT, then the begin and end index of this field position is set to the location of the first occurrence of a message format argument. Otherwise, the FieldPosition is ignored.

      Parameters:
      objects - the array of objects to format.
      buffer - the target string buffer to append the formatted message to.
      field - on input: an optional alignment field; on output: the offsets of the alignment field in the formatted text.
      Returns:
      the string buffer.

    • format

      public final StringBuffer format​(Object object, StringBuffer buffer, FieldPosition field)
      Converts the specified objects into a string which it appends to the specified string buffer using the pattern of this message format.

      If the field member of the specified FieldPosition is MessageFormat.Field.ARGUMENT, then the begin and end index of this field position is set to the location of the first occurrence of a message format argument. Otherwise, the FieldPosition is ignored.

      Calling this method is equivalent to calling 

       format((Object[])object, buffer, field)
      Specified by:
      format in class Format
      Parameters:
      object - the object to format, must be an array of Object.
      buffer - the target string buffer to append the formatted message to.
      field - on input: an optional alignment field; on output: the offsets of the alignment field in the formatted text.
      Returns:
      the string buffer.
      Throws:
      ClassCastException - if object is not an array of Object.

    • format

      public static String format​(String format, Object... args)
      Formats the supplied objects using the specified message format pattern.
      Parameters:
      format - the format string (see Formatter.format(java.lang.String, java.lang.Object...))
      args - the list of arguments passed to the formatter. If there are more arguments than required by format, additional arguments are ignored.
      Returns:
      the formatted result.
      Throws:
      IllegalArgumentException - if the pattern cannot be parsed.

    • getFormats

      public Format[] getFormats()
      Returns the Format instances used by this message format.
      Returns:
      an array of Format instances.

    • getFormatsByArgumentIndex

      public Format[] getFormatsByArgumentIndex()
      Returns the formats used for each argument index. If an argument is placed more than once in the pattern string, then this returns the format of the last one.
      Returns:
      an array of formats, ordered by argument index.

    • setFormatByArgumentIndex

      public void setFormatByArgumentIndex​(int argIndex, Format format)
      Sets the format used for the argument at index argIndex to format.
      Parameters:
      argIndex - the index of the format to set.
      format - the format that will be set at index argIndex.

    • setFormatsByArgumentIndex

      public void setFormatsByArgumentIndex​(Format[] formats)
      Sets the formats used for each argument. The formats array elements should be in the order of the argument indices.
      Parameters:
      formats - the formats in an array.

    • getLocale

      public Locale getLocale()
      Returns the locale used when creating formats.
      Returns:
      the locale used to create formats.

    • hashCode

      public int hashCode()
      Description copied from class: Object
      Returns an integer hash code for this object. By contract, any two objects for which Object.equals(java.lang.Object) returns true must return the same hash code value. This means that subclasses of Object usually override both methods or neither method.

      Note that hash values must not change over time unless information used in equals comparisons also changes.

      See Writing a correct hashCode method if you intend implementing your own hashCode method.

      Overrides:
      hashCode in class Object
      Returns:
      this object's hash code.
      See Also:
      Object.equals(java.lang.Object)

    • parse

      public Object[] parse​(String string) throws ParseException
      Parses the message arguments from the specified string using the rules of this message format.
      Parameters:
      string - the string to parse.
      Returns:
      the array of Object arguments resulting from the parse.
      Throws:
      ParseException - if an error occurs during parsing.

    • parse

      public Object[] parse​(String string, ParsePosition position)
      Parses the message argument from the specified string starting at the index specified by position. If the string is successfully parsed then the index of the ParsePosition is updated to the index following the parsed text. On error, the index is unchanged and the error index of ParsePosition is set to the index where the error occurred.
      Parameters:
      string - the string to parse.
      position - input/output parameter, specifies the start index in string from where to start parsing. If parsing is successful, it is updated with the index following the parsed text; on error, the index is unchanged and the error index is set to the index where the error occurred.
      Returns:
      the array of objects resulting from the parse, or null if there is an error.

    • parseObject

      public Object parseObject​(String string, ParsePosition position)
      Parses the message argument from the specified string starting at the index specified by position. If the string is successfully parsed then the index of the ParsePosition is updated to the index following the parsed text. On error, the index is unchanged and the error index of ParsePosition is set to the index where the error occurred.
      Specified by:
      parseObject in class Format
      Parameters:
      string - the string to parse.
      position - input/output parameter, specifies the start index in string from where to start parsing. If parsing is successful, it is updated with the index following the parsed text; on error, the index is unchanged and the error index is set to the index where the error occurred.
      Returns:
      the array of objects resulting from the parse, or null if there is an error.

    • setFormat

      public void setFormat​(int offset, Format format)
      Sets the specified format used by this message format.
      Parameters:
      offset - the index of the format to change.
      format - the Format that replaces the old format.

    • setFormats

      public void setFormats​(Format[] formats)
      Sets the formats used by this message format.
      Parameters:
      formats - an array of Format.

    • setLocale

      public void setLocale​(Locale locale)
      Sets the locale to use when creating Format instances. Changing the locale may change the behavior of applyPattern, toPattern, format and formatToCharacterIterator.
      Parameters:
      locale - the new locale.

    • toPattern

      public String toPattern()
      Returns the pattern of this message format.
      Returns:
      the pattern.