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Added in API level 1

Charset

public abstract class Charset
extends Object implements Comparable<Charset>

java.lang.Object
   ↳ java.nio.charset.Charset


A named mapping between sequences of sixteen-bit Unicode code units 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.

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).
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 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 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

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:

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.

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 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.

See also:

Summary

Protected constructors

Charset(String canonicalName, String[] aliases)

Initializes a new charset with the given canonical name and alias set.

Public methods

final Set<String> aliases()

Returns a set containing this charset's aliases.

static SortedMap<StringCharset> availableCharsets()

Constructs a sorted map from canonical charset names to charset objects.

boolean canEncode()

Tells whether or not this charset supports encoding.

final int compareTo(Charset that)

Compares this charset to another.

abstract boolean contains(Charset cs)

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 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.

final ByteBuffer encode(String str)

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 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.

final int hashCode()

Computes a hashcode for this charset.

final 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.

final 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.

final String toString()

Returns a string describing this charset.

Inherited methods

From class java.lang.Object
From interface java.lang.Comparable

Protected constructors

Charset

Added in API level 1
Charset (String canonicalName, 
                String[] aliases)

Initializes a new charset with the given canonical name and alias set.

Parameters
canonicalName String: The canonical name of this charset
aliases String: 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

Public methods

aliases

Added in API level 1
Set<String> aliases ()

Returns a set containing this charset's aliases.

Returns
Set<String> An immutable set of this charset's aliases

availableCharsets

Added in API level 1
SortedMap<StringCharset> 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
SortedMap<StringCharset> An immutable, case-insensitive map from canonical charset names to charset objects

canEncode

Added in API level 1
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
boolean true if, and only if, this charset supports encoding

compareTo

Added in API level 1
int compareTo (Charset that)

Compares this charset to another.

Charsets are ordered by their canonical names, without regard to case.

Parameters
that Charset: The charset to which this charset is to be compared
Returns
int A negative integer, zero, or a positive integer as this charset is less than, equal to, or greater than the specified charset

contains

Added in API level 1
boolean contains (Charset cs)

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 returns false, however, then it is not necessarily the case that the given charset is not contained in this charset.

Parameters
cs Charset
Returns
boolean true if the given charset is contained in this charset

decode

Added in API level 1
CharBuffer decode (ByteBuffer bb)

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 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 decode(java.nio.ByteBuffer) method directly.

Parameters
bb ByteBuffer: The byte buffer to be decoded
Returns
CharBuffer A char buffer containing the decoded characters

defaultCharset

Added in API level 1
Charset defaultCharset ()

Returns the default charset of this Java virtual machine.

The default charset is determined during virtual-machine startup and typically depends upon the locale and charset of the underlying operating system.

Returns
Charset A charset object for the default charset

displayName

Added in API level 1
String 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
String The display name of this charset in the default locale

displayName

Added in API level 1
String displayName (Locale locale)

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 Locale: The locale for which the display name is to be retrieved
Returns
String The display name of this charset in the given locale

encode

Added in API level 1
ByteBuffer encode (String str)

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 expression

     cs.encode(CharBuffer.wrap(s)); 

Parameters
str String: The string to be encoded
Returns
ByteBuffer A byte buffer containing the encoded characters

encode

Added in API level 1
ByteBuffer encode (CharBuffer cb)

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 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 encode(java.nio.CharBuffer) method directly.

Parameters
cb CharBuffer: The char buffer to be encoded
Returns
ByteBuffer A byte buffer containing the encoded characters

equals

Added in API level 1
boolean equals (Object ob)

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.

Parameters
ob Object: the reference object with which to compare.
Returns
boolean true if, and only if, this charset is equal to the given object

forName

Added in API level 1
Charset forName (String charsetName)

Returns a charset object for the named charset.

Parameters
charsetName String: The name of the requested charset; may be either a canonical name or an alias
Returns
Charset A charset object for the named charset
Throws
IllegalCharsetNameException If the given charset name is illegal
IllegalArgumentException If the given charsetName is null
UnsupportedCharsetException If no support for the named charset is available in this instance of the Java virtual machine

hashCode

Added in API level 1
int hashCode ()

Computes a hashcode for this charset.

Returns
int An integer hashcode

isRegistered

Added in API level 1
boolean isRegistered ()

Tells whether or not this charset is registered in the IANA Charset Registry.

Returns
boolean true if, and only if, this charset is known by its implementor to be registered with the IANA

isSupported

Added in API level 1
boolean isSupported (String charsetName)

Tells whether the named charset is supported.

Parameters
charsetName String: The name of the requested charset; may be either a canonical name or an alias
Returns
boolean 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 illegal
IllegalArgumentException If the given charsetName is null

name

Added in API level 1
String name ()

Returns this charset's canonical name.

Returns
String The canonical name of this charset

newDecoder

Added in API level 1
CharsetDecoder newDecoder ()

Constructs a new decoder for this charset.

Returns
CharsetDecoder A new decoder for this charset

newEncoder

Added in API level 1
CharsetEncoder newEncoder ()

Constructs a new encoder for this charset.

Returns
CharsetEncoder A new encoder for this charset
Throws
UnsupportedOperationException If this charset does not support encoding

toString

Added in API level 1
String toString ()

Returns a string describing this charset.

Returns
String A string describing this charset
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