MessageFormat

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## java.text
Class MessageFormat

    java.lang.Object
      java.text.Format
          java.text.MessageFormat

All Implemented Interfaces:
  Serializable, Cloneable

    public class MessageFormat

extends Format

`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 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.

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())`
`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.

Version:
  1.38, 01/19/00
Author:
  Mark Davis
See Also:
  `Locale`, `Format`, `NumberFormat`, `DecimalFormat`, `ChoiceFormat`, Serialized Form

Nested Class Summary
`static class` | `MessageFormat.Field` Defines constants that are used as attribute keys in the `AttributedCharacterIterator` returned from `MessageFormat.formatToCharacterIterator`.

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.

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
`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.
`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.

Methods inherited from class java.text.Format
`format, parseObject`

Methods inherited from class java.lang.Object
`getClass, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### MessageFormat

    public MessageFormat(String pattern)

  Constructs a MessageFormat for the default 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

### MessageFormat

    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
  Since:
    1.4

Method Detail

### setLocale

    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 as well as to the `format` and `formatToCharacterIterator` methods.

  Parameters:
    `locale` - the locale to be used when creating or comparing subformats

### getLocale

    public Locale getLocale()

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

  Returns:
    the locale used when creating or comparing subformats

### applyPattern

    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

### 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

### 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
  Throws:
    `NullPointerException` - if `newFormats` is null
  Since:
    1.4

### setFormats

    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

### 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
  Since:
    1.4

### setFormat

    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

### 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
  Since:
    1.4

### getFormats

    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

### 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.
  Throws:
    `IllegalArgumentException` - if an argument in the `arguments` array is not of the type expected by the format element(s) that use it.

### format

    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()`

  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.

### 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)`

  Specified by:
    `format` in class `Format`
  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.

### formatToCharacterIterator

    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.

  Overrides:
    `formatToCharacterIterator` in class `Format`
  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.
  Since:
    1.4

### 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.

### parse

    public Object[] parse(String source)
                   throws ParseException

  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.

### parseObject

    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.

  Specified by:
    `parseObject` in class `Format`
  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.

### clone

    public Object clone()

  Creates and returns a copy of this object.

  Overrides:
    `clone` in class `Format`
  Returns:
    a clone of this instance.
  See Also:
    `Cloneable`

### equals

    public boolean equals(Object obj)

  Equality comparison between two message format objects

  Overrides:
    `equals` in class `Object`
  Parameters:
    `obj` - the reference object with which to compare.
  Returns:
    `true` if this object is the same as the obj argument; `false` otherwise.
  See Also:
    `Object.hashCode()`, `Hashtable`

### hashCode

    public int hashCode()

  Generates a hash code for the message format object.

  Overrides:
    `hashCode` in class `Object`
  Returns:
    a hash code value for this object.
  See Also:
    `Object.equals(java.lang.Object)`, `Hashtable`

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