Class Charset
- All Implemented Interfaces:
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.
Charset names
Charsets are named by strings composed of the following characters:
- The uppercase letters
'A'
through'Z'
('\u0041'
through'\u005a'
), - The lowercase letters
'a'
through'z'
('\u0061'
through'\u007a'
), - The digits
'0'
through'9'
('\u0030'
through'\u0039'
), - The dash character
'-'
('\u002d'
, HYPHEN-MINUS), - The plus character
'+'
('\u002b'
, PLUS SIGN), - The period character
'.'
('\u002e'
, FULL STOP), - The colon character
':'
('\u003a'
, COLON), and - The underscore character
'_'
('\u005f'
, LOW LINE).
Every charset has a canonical name and may also have one or more
aliases. The canonical name is returned by the 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.
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
InputStreamReader
and OutputStreamWriter
classes.
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.
Standard charsets
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 US-ASCII
Seven-bit ASCII, a.k.a. ISO646-US
, a.k.a. the Basic Latin block of the Unicode character setISO-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 UTF-32BE
Thirty-two-bit UCS Transformation Format, big-endian byte order UTF-32LE
Thirty-two-bit UCS Transformation Format, little-endian byte order UTF-32
Thirty-two-bit UCS Transformation Format, byte order identified by an optional byte-order mark
The UTF-8
charset is specified by RFC 2279; the
transformation format upon which it is based is specified in
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
ISO 10646-1 and are also described in the Unicode
Standard.
The UTF-32
charsets are based upon transformation formats
which are specified in
ISO 10646-1 and are also described in the Unicode
Standard.
The UTF-16
and UTF-32
charsets use sixteen-bit and thirty-two-bit
quantities respectively, 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 U+FEFF
. Byte-order marks are handled
as follows:
When decoding, the
UTF-16BE
,UTF-16LE
,UTF-32BE
, andUTF-32LE
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
andUTF-32
charsets interpret 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
is UTF-8
unless changed in an implementation specific manner. Refer to
defaultCharset()
for more detail.
The StandardCharsets
class defines constants for each of the
standard charsets.
Terminology
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.
- Since:
- 1.4
- External Specifications
- See Also:
-
Constructor Summary
-
Method Summary
Modifier and TypeMethodDescriptionaliases()
Returns a set containing this charset's aliases.Constructs a sorted map from canonical charset names to charset objects.boolean
Tells whether or not this charset supports encoding.final int
Compares this charset to another.abstract boolean
Tells whether or not this charset contains the given charset.final CharBuffer
decode
(ByteBuffer bb) Convenience method that decodes bytes in this charset into Unicode characters.static Charset
Returns the default charset of this Java virtual machine.Returns this charset's human-readable name for the default locale.displayName
(Locale locale) Returns this charset's human-readable name for the given locale.final ByteBuffer
Convenience method that encodes a string into bytes in this charset.final ByteBuffer
encode
(CharBuffer cb) Convenience method that encodes Unicode characters into bytes in this charset.final boolean
Tells whether or not this object is equal to another.static Charset
Returns a charset object for the named charset.static Charset
Returns a charset object for the named charset.final int
hashCode()
Returns the hashcode for this charset.final boolean
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.final String
name()
Returns this charset's canonical name.abstract CharsetDecoder
Constructs a new decoder for this charset.abstract CharsetEncoder
Constructs a new encoder for this charset.final String
toString()
Returns a string describing this charset.
-
Constructor Details
-
Charset
Initializes a new charset with the given canonical name and alias set.- Parameters:
canonicalName
- The canonical name of this charsetaliases
- An array of this charset's aliases, or null if it has no aliases- Throws:
IllegalCharsetNameException
- If the canonical name or any of the aliases are illegal
-
-
Method Details
-
isSupported
Tells whether the named charset is supported.- Parameters:
charsetName
- The name of the requested charset; may be either a canonical name or an alias- Returns:
true
if, and only if, support for the named charset is available in the current Java virtual machine- Throws:
IllegalCharsetNameException
- If the given charset name is illegalIllegalArgumentException
- If the givencharsetName
is null
-
forName
Returns a charset object for the named charset.- Parameters:
charsetName
- The name of the requested charset; may be either a canonical name or an alias- Returns:
- A charset object for the named charset
- Throws:
IllegalCharsetNameException
- If the given charset name is illegalIllegalArgumentException
- If the givencharsetName
is nullUnsupportedCharsetException
- If no support for the named charset is available in this instance of the Java virtual machine
-
forName
Returns a charset object for the named charset. If the charset object for the named charset is not available orcharsetName
is not a legal charset name, thenfallback
is returned.- Parameters:
charsetName
- The name of the requested charset; may be either a canonical name or an aliasfallback
- fallback charset in case the charset object for the named charset is not available orcharsetName
is not a legal charset name. May benull
- Returns:
- A charset object for the named charset, or
fallback
in case the charset object for the named charset is not available orcharsetName
is not a legal charset name - Throws:
IllegalArgumentException
- If the givencharsetName
isnull
- Since:
- 18
-
availableCharsets
Constructs a sorted map from canonical charset names to charset objects.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.- Returns:
- An immutable, case-insensitive map from canonical charset names to charset objects
-
defaultCharset
Returns the default charset of this Java virtual machine.The default charset is
UTF-8
, unless changed in an implementation specific manner.- Implementation Note:
- An implementation may override the default charset with
the system property
file.encoding
on the command line. If the value isCOMPAT
, the default charset is derived from thenative.encoding
system property, which typically depends upon the locale and charset of the underlying operating system. - Returns:
- A charset object for the default charset
- Since:
- 1.5
- See Also:
-
name
Returns this charset's canonical name.- Returns:
- The canonical name of this charset
-
aliases
-
displayName
Returns this charset's human-readable name for the default 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.
- Returns:
- The display name of this charset in the default locale
-
isRegistered
public final boolean isRegistered()Tells whether or not this charset is registered in the IANA Charset Registry.- Returns:
true
if, and only if, this charset is known by its implementor to be registered with the IANA- External Specifications
-
displayName
Returns this charset's human-readable name for the given 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.
- Parameters:
locale
- The locale for which the display name is to be retrieved- Returns:
- The display name of this charset in the given locale
-
contains
Tells whether or not this charset contains the given charset.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 returnsfalse
, however, then it is not necessarily the case that the given charset is not contained in this charset.- Parameters:
cs
- The given charset- Returns:
true
if the given charset is contained in this charset
-
newDecoder
Constructs a new decoder for this charset.- Returns:
- A new decoder for this charset
-
newEncoder
Constructs a new encoder for this charset.- Returns:
- A new encoder for this charset
- Throws:
UnsupportedOperationException
- If this charset does not support encoding
-
canEncode
public boolean canEncode()Tells whether or not this charset supports encoding.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
.- Returns:
true
if, and only if, this charset supports encoding
-
decode
Convenience method that decodes bytes in this charset into Unicode characters.An invocation of this method upon a charset
cs
returns the same result as the expressioncs.newDecoder() .onMalformedInput(CodingErrorAction.REPLACE) .onUnmappableCharacter(CodingErrorAction.REPLACE) .decode(bb);
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.- Parameters:
bb
- The byte buffer to be decoded- Returns:
- A char buffer containing the decoded characters
-
encode
Convenience method that encodes Unicode characters into bytes in this charset.An invocation of this method upon a charset
cs
returns the same result as the expressioncs.newEncoder() .onMalformedInput(CodingErrorAction.REPLACE) .onUnmappableCharacter(CodingErrorAction.REPLACE) .encode(bb);
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.- Parameters:
cb
- The char buffer to be encoded- Returns:
- A byte buffer containing the encoded characters
-
encode
Convenience method that encodes a string into bytes in this charset.An invocation of this method upon a charset
cs
returns the same result as the expressioncs.encode(CharBuffer.wrap(s));
- Parameters:
str
- The string to be encoded- Returns:
- A byte buffer containing the encoded characters
-
compareTo
Compares this charset to another.Charsets are ordered by their canonical names, without regard to case.
- Specified by:
compareTo
in interfaceComparable<Charset>
- Parameters:
that
- The charset to which this charset is to be compared- Returns:
- A negative integer, zero, or a positive integer as this charset is less than, equal to, or greater than the specified charset
-
hashCode
-
equals
Tells whether or not this object is equal to another.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.
-
toString
-