public abstract class Charset extends Object implements Comparable<Charset>
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 the 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 behavior
of such optional charsets may differ between implementations.
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 the Unicode
Standard.
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 the Unicode
Standard.
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 interpret the initial byte-order marks as a ZERO-WIDTH
NON-BREAKING SPACE; when encoding, they do not write
byte-order marks. When decoding, the UTF-16 charset interprets the
byte-order mark at the beginning of the input stream 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 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
one or more coded character sets and a character-encoding scheme.
(This definition is confusing; some other software systems define
charset as a synonym for coded character set.)
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 Unicode are examples of coded character sets.
Some standards have defined a character set to be simply a
set of abstract characters without an associated assigned numbering.
An alphabet is an example of such a character set. However, the subtle
distinction between character set and coded character set
is rarely used in practice; the former has become a short form for the
latter, including in the Java API specification.
A character-encoding scheme is a mapping between one or more
coded character sets and a set of octet (eight-bit byte) sequences.
UTF-8, 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
coded character sets; EUC, for example, can be used to encode
characters in a variety of Asian coded 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 coded character set; otherwise a charset is usually named
for the encoding scheme and, possibly, the locale of the coded
character sets that it supports. Hence US-ASCII is both the
name of a coded character set and of the charset that encodes it, while
EUC-JP is the name of the charset that encodes the
JIS X 0201, JIS X 0208, and JIS X 0212
coded character sets for the Japanese language.
The native character encoding of the Java programming language is
UTF-16. A charset in the Java platform therefore defines a mapping
between sequences of sixteen-bit UTF-16 code units (that is, sequences
of chars) 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, 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.
StandardCharsets class defines constants for each of the
standard charsets.
Terminology
CharsetDecoder,
CharsetEncoder,
CharsetProvider,
Character| Modifier | Constructor and Description |
|---|---|
protected |
Charset(String canonicalName,
String[] aliases)
Initializes a new charset with the given canonical name and alias
set.
|
| Modifier and Type | Method and Description |
|---|---|
Set<String> |
aliases()
Returns a set containing this charset's aliases.
|
static SortedMap<String,Charset> |
availableCharsets()
Constructs a sorted map from canonical charset names to charset objects.
|
boolean |
canEncode()
Tells whether or not this charset supports encoding.
|
int |
compareTo(Charset that)
Compares this charset to another.
|
abstract boolean |
contains(Charset cs)
Tells whether or not this charset contains the given charset.
|
static Charset |
defaultCharset()
Returns the default charset of this Java virtual machine.
|
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.
|
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.
|
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 aliasesIllegalCharsetNameException - If the canonical name or any of the aliases are illegalpublic static boolean isSupported(String charsetName)
charsetName - The name of the requested charset; may be either
a canonical name or an aliasIllegalCharsetNameException - If the given charset name is illegalIllegalArgumentException - If the given charsetName is nullpublic static Charset forName(String charsetName)
charsetName - The name of the requested charset; may be either
a canonical name or an aliasIllegalCharsetNameException - If the given charset name is illegalIllegalArgumentException - If the given charsetName is nullUnsupportedCharsetException - If no support for the named charset is available
in this instance of the Java virtual machinepublic static SortedMap<String,Charset> 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 static Charset defaultCharset()
The default charset is determined during virtual-machine startup and typically depends upon the locale and charset of the underlying operating system.
public final String name()
public final Set<String> 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 retrievedpublic 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 int compareTo(Charset that)
Charsets are ordered by their canonical names, without regard to case.
compareTo in interface Comparable<Charset>that - The charset to which this charset is to be comparedpublic final int hashCode()
hashCode in class ObjectObject.equals(java.lang.Object),
System.identityHashCode(java.lang.Object)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 Objectob - the reference object with which to compare.Object.hashCode(),
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