Class MessageFormat

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

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.

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);

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 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 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.

Constructors

MessageFormat

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

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

Methods

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 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.

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)

Parameters:

newFormats - the new formats to use

Throws:

NullPointerException - if newFormats is null

Since:

1.4

setFormats

public void setFormats(Format[] newFormats)

Parameters:

newFormats - the new formats to use

Throws:

NullPointerException - if newFormats is null

setFormatByArgumentIndex

public void setFormatByArgumentIndex(int argumentIndex,
                                     Format newFormat)

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)

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()

Returns:

the formats used for the arguments within the pattern

Since:

1.4

getFormats

public Format[] getFormats()

Returns:

the formats used for the format elements in the pattern

format

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

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()

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.

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)

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

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)

Parameters:

source - the string to parse

pos - the parse position

Returns:

an array of parsed objects

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.

Throws:

ParseException - if the beginning of the specified string cannot be parsed.

parseObject

public Object parseObject(String source,
                          ParsePosition pos)

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(), HashMap

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), System.identityHashCode(java.lang.Object)