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JavaTM 2 Platform Std. Ed. v1.4.2 | ||||||||||
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java.lang.Objectjava.nio.charset.Charset
A named mapping between sequences of sixteen-bit Unicode characters and sequences of bytes. This class defines methods for creating decoders and encoders and for retrieving the various names associated with a charset. Instances of this class are immutable.
This class also defines static methods for testing whether a particular
charset is supported, for locating charset instances by name, and for
constructing a map that contains every charset for which support is available in
the current Java virtual machine. Support for new charsets can be added via the
service-provider interface defined in the CharsetProvider
class.
All of the methods defined in this class are safe for use by multiple
concurrent threads.
Charsets are named by strings composed of the following characters:
Charset names
A charset name
must begin with either a letter or a digit. The empty string is not a legal
charset name. Charset names are not case-sensitive; that is, case is always
ignored when comparing charset names. Charset names generally follow the
conventions documented in RFC 2278: IANA Charset
Registration Procedures.
Every charset has a canonical name and may also have one or more
aliases. The canonical name is returned by Some charsets have an historical name that is defined for
compatibility with previous versions of the Java platform. A charset's
historical name is either its canonical name or one of its aliases. The
historical name is returned by the getEncoding() methods of the If a charset listed in the IANA Charset
Registry is supported by an implementation of the Java platform then its
canonical name must be the name listed in the registry. Many charsets are given
more than one name in the registry, in which case the registry identifies one of
the names as MIME-preferred. If a charset has more than one registry name
then its canonical name must be the MIME-preferred name and the other names in
the registry must be valid aliases. If a supported charset is not listed in the
IANA registry then its canonical name must begin with one of the strings
"X-" or "x-".
The IANA charset registry does change over time, and so the canonical name
and the aliases of a particular charset may also change over time. To ensure
compatibility it is recommended that no alias ever be removed from a charset,
and that if the canonical name of a charset is changed then its previous
canonical name be made into an alias.
Every implementation of the Java platform is required to support the
following standard charsets. Consult the release documentation for your
implementation to see if any other charsets are supported.
Charset Description The UTF-8 charset is specified by RFC 2279; the
transformation format upon which it is based is specified in Amendment 2 of
ISO 10646-1 and is also described in § 3.8 of The Unicode
Standard, Version 3.0 (amended).
The UTF-16 charsets are specified by RFC 2781; the
transformation formats upon which they are based are specified in
Amendment 1 of ISO 10646-1 and are also described in § 3.8 of The Unicode
Standard, Version 3.0.
The UTF-16 charsets use sixteen-bit quantities and are therefore
sensitive to byte order. In these encodings the byte order of a stream may be
indicated by an initial byte-order mark represented by the Unicode
character '\uFEFF'. Byte-order marks are handled as follows:
When decoding, the UTF-16BE and UTF-16LE charsets ignore
byte-order marks; when encoding, they do not write byte-order marks. When decoding, the UTF-16 charset interprets a byte-order mark to
indicate the byte order of the stream but defaults to big-endian if there is
no byte-order mark; when encoding, it uses big-endian byte order and writes a
big-endian byte-order mark. Every instance of the Java virtual machine has a default charset, which may
or may not be one of the standard charsets. The default charset is determined
during virtual-machine startup and typically depends upon the locale and charset
being used by the underlying operating system. The name of this class is taken from the terms used in RFC 2278. In that
document a charset is defined as the combination of a coded character set
and a character-encoding scheme.
A coded character set is a mapping between a set of abstract
characters and a set of integers. US-ASCII, ISO 8859-1,
JIS X 0201, and full Unicode, which is the same as ISO 10646-1,
are examples of coded character sets.
A character-encoding scheme is a mapping between a coded character set
and a set of octet (eight-bit byte) sequences. UTF-8, UCS-2, UTF-16,
ISO 2022, and EUC are examples of character-encoding schemes. Encoding
schemes are often associated with a particular coded character set; UTF-8, for
example, is used only to encode Unicode. Some schemes, however, are associated
with multiple character sets; EUC, for example, can be used to encode characters
in a variety of Asian character sets.
When a coded character set is used exclusively with a single
character-encoding scheme then the corresponding charset is usually named for
the character set; otherwise a charset is usually named for the encoding scheme
and, possibly, the locale of the character sets that it supports. Hence
US-ASCII is the name of the charset for US-ASCII while EUC-JP
is the name of the charset that encodes the JIS X 0201,
JIS X 0208, and JIS X 0212 character sets.
The native coded character set of the Java programming language is that of
the first seventeen planes of the Unicode version 3.0 character set; that
is, it consists in the basic multilingual plane (BMP) of Unicode
version 1 plus the next sixteen planes of Unicode version 3. This is
because the language's internal representation of characters uses the UTF-16
encoding, which encodes the BMP directly and uses surrogate pairs, a
simple escape mechanism, to encode the other planes. Hence a charset in the Java
platform defines a mapping between sequences of sixteen-bit values in UTF-16 and
sequences of bytes.
name
method of this class. Canonical names are, by convention, usually in upper case.
The aliases of a charset are returned by the aliases
method.
InputStreamReader
and OutputStreamWriter
classes.
Standard charsets
US-ASCII
Seven-bit ASCII, a.k.a. ISO646-US, a.k.a. the Basic Latin
block of the Unicode character set
ISO-8859-1
ISO Latin Alphabet No. 1, a.k.a. ISO-LATIN-1
UTF-8
Eight-bit UCS Transformation Format
UTF-16BE
Sixteen-bit UCS Transformation Format, big-endian
byte order
UTF-16LE
Sixteen-bit UCS Transformation Format, little-endian
byte order
UTF-16
Sixteen-bit UCS Transformation Format, byte order identified by
an optional byte-order mark
In any case, when a byte-order mark is
read at the beginning of a decoding operation it is omitted from the resulting
sequence of characters. Byte order marks occuring after the first element of an
input sequence are not omitted since the same code is used to represent
ZERO-WIDTH NON-BREAKING SPACE.
Terminology
CharsetDecoder
,
CharsetEncoder
,
CharsetProvider
Constructor Summary | |
protected
|
Charset(String canonicalName,
String[] aliases)
Initializes a new charset with the given canonical name and alias set. |
Method Summary | |
Set |
aliases()
Returns a set containing this charset's aliases. |
static SortedMap |
availableCharsets()
Constructs a sorted map from canonical charset names to charset objects. |
boolean |
canEncode()
Tells whether or not this charset supports encoding. |
int |
compareTo(Object ob)
Compares this charset to another object. |
abstract
boolean |
contains(Charset cs)
Tells whether or not this charset contains the given charset. |
CharBuffer |
decode(ByteBuffer bb)
Convenience method that decodes bytes in this charset into Unicode characters. |
String |
displayName()
Returns this charset's human-readable name for the default locale. |
String |
displayName(Locale locale)
Returns this charset's human-readable name for the given locale. |
ByteBuffer |
encode(CharBuffer cb)
Convenience method that encodes Unicode characters into bytes in this charset. |
ByteBuffer |
encode(String str)
Convenience method that encodes a string into bytes in this charset. |
boolean |
equals(Object ob)
Tells whether or not this object is equal to another. |
static Charset |
forName(String charsetName)
Returns a charset object for the named charset. |
int |
hashCode()
Computes a hashcode for this charset. |
boolean |
isRegistered()
Tells whether or not this charset is registered in the IANA Charset Registry. |
static boolean |
isSupported(String charsetName)
Tells whether the named charset is supported. |
String |
name()
Returns this charset's canonical name. |
abstract
CharsetDecoder |
newDecoder()
Constructs a new decoder for this charset. |
abstract
CharsetEncoder |
newEncoder()
Constructs a new encoder for this charset. |
String |
toString()
Returns a string describing this charset. |
Methods inherited from class java.lang.Object |
clone,
finalize,
getClass,
notify,
notifyAll,
wait,
wait,
wait |
Constructor Detail |
protected Charset(String canonicalName, String[] aliases)
canonicalName
- The canonical name of this charset
aliases
- An array of this charset's aliases, or null if it
has no aliases
IllegalCharsetNameException
- If the canonical name or any of the aliases are illegalMethod Detail |
public static boolean isSupported(String charsetName)
charsetName
- The name of the requested charset; may be
either a canonical name or an alias
IllegalCharsetNameException
- If the given charset name is illegalpublic static Charset forName(String charsetName)
charsetName
- The name of the requested charset; may be
either a canonical name or an alias
IllegalCharsetNameException
- If the given charset name is illegal
UnsupportedCharsetException
- If no support for the named charset is available in this instance of the
Java virtual machinepublic static SortedMap availableCharsets()
The map returned by this method will have one entry for each charset for which support is available in the current Java virtual machine. If two or more supported charsets have the same canonical name then the resulting map will contain just one of them; which one it will contain is not specified.
The invocation of this method, and the subsequent use of the resulting map,
may cause time-consuming disk or network I/O operations to occur. This method
is provided for applications that need to enumerate all of the available
charsets, for example to allow user charset selection. This method is not used
by the forName
method, which instead employs an efficient incremental lookup algorithm.
This method may return different results at different times if new charset
providers are dynamically made available to the current Java virtual machine.
In the absence of such changes, the charsets returned by this method are
exactly those that can be retrieved via the forName
method.
public final String name()
public final Set aliases()
public String displayName()
The default implementation of this method simply returns this charset's canonical name. Concrete subclasses of this class may override this method in order to provide a localized display name.
public final boolean isRegistered()
public String displayName(Locale locale)
The default implementation of this method simply returns this charset's canonical name. Concrete subclasses of this class may override this method in order to provide a localized display name.
locale
- The locale for which the display name is to be
retrieved
public abstract boolean contains(Charset cs)
A charset C is said to contain a charset D if, and only if, every character representable in D is also representable in C. If this relationship holds then it is guaranteed that every string that can be encoded in D can also be encoded in C without performing any replacements.
That C contains D does not imply that each character representable in C by a particular byte sequence is represented in D by the same byte sequence, although sometimes this is the case.
Every charset contains itself.
This method computes an approximation of the containment relation: If it returns true then the given charset is known to be contained by this charset; if it returns false, however, then it is not necessarily the case that the given charset is not contained in this charset.
public abstract CharsetDecoder newDecoder()
public abstract CharsetEncoder newEncoder()
UnsupportedOperationException
- If this charset does not support encodingpublic boolean canEncode()
Nearly all charsets support encoding. The primary exceptions are special-purpose auto-detect charsets whose decoders can determine which of several possible encoding schemes is in use by examining the input byte sequence. Such charsets do not support encoding because there is no way to determine which encoding should be used on output. Implementations of such charsets should override this method to return false.
public final CharBuffer decode(ByteBuffer bb)
An invocation of this method upon a charset cs returns the same result as the expression
cs.newDecoder() .onMalformedInput(CodingErrorAction.REPLACE) .onUnmappableCharacter(CodingErrorAction.REPLACE) .decode(bb);except that it is potentially more efficient because it can cache decoders between successive invocations.
This method always replaces malformed-input and unmappable-character
sequences with this charset's default replacement byte array. In order to
detect such sequences, use the CharsetDecoder.decode(java.nio.ByteBuffer)
method directly.
bb
- The byte buffer to be decoded
public final ByteBuffer encode(CharBuffer cb)
An invocation of this method upon a charset cs returns the same result as the expression
cs.newEncoder() .onMalformedInput(CodingErrorAction.REPLACE) .onUnmappableCharacter(CodingErrorAction.REPLACE) .encode(bb);except that it is potentially more efficient because it can cache encoders between successive invocations.
This method always replaces malformed-input and unmappable-character
sequences with this charset's default replacement string. In order to detect
such sequences, use the CharsetEncoder.encode(java.nio.CharBuffer)
method directly.
cb
- The char buffer to be encoded
public final ByteBuffer encode(String str)
An invocation of this method upon a charset cs returns the same result as the expression
cs.encode(CharBuffer.wrap(s));
str
- The string to be encoded
public final int compareTo(Object ob)
Charsets are ordered by their canonical names, without regard to case.
compareTo
in interface Comparable
ob
- The object to which this object is to be compared
public final int hashCode()
hashCode
in class Object
Object.equals(java.lang.Object)
,
Hashtable
public final boolean equals(Object ob)
Two charsets are equal if, and only if, they have the same canonical names. A charset is never equal to any other type of object.
equals
in class Object
ob
- the reference object with which to compare.
Object.hashCode()
,
Hashtable
public final String toString()
toString
in class Object
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Copyright 2003 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.