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JavaTM 2 Platform Std. Ed. v1.4.1 |
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java.lang.Object | +--java.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.
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 charsetaliases - 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 illegal| Method 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.
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 Comparableob - The object to which this object is to be compared
public final int hashCode()
hashCode in class ObjectObject.equals(java.lang.Object),
Hashtablepublic 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 Objectob - the reference object with which to compare.
Object.hashCode(),
Hashtablepublic final String toString()
toString in class Object
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JavaTM 2 Platform Std. Ed. v1.4.1 |
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Copyright 2002 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms.