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

  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 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:

• 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.

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.

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.

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 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 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 and sequences of bytes.