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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 charset is a named mapping between Unicode characters and byte sequences. Every Charset can decode, converting a byte sequence into a sequence of characters, and some can also encode, converting a sequence of characters into a byte sequence. Use the method canEncode() to find out whether a charset supports both.

Characters

In the context of this class, character always refers to a Java character: a Unicode code point in the range U+0000 to U+FFFF. (Java represents supplementary characters using surrogates.) Not all byte sequences will represent a character, and not all characters can necessarily be represented by a given charset. The method contains(Charset) can be used to determine whether every character representable by one charset can also be represented by another (meaning that a lossless transformation is possible from the contained to the container).

Encodings

There are many possible ways to represent Unicode characters as byte sequences. See UTR#17: Unicode Character Encoding Model for detailed discussion.

The most important mappings capable of representing every character are the Unicode Transformation Format (UTF) charsets. Of those, UTF-8 and the UTF-16 family are the most common. UTF-8 (described in RFC 3629) encodes a character using 1 to 4 bytes. UTF-16 uses exactly 2 bytes per character (potentially wasting space, but allowing efficient random access into BMP text), and UTF-32 uses exactly 4 bytes per character (trading off even more space for efficient random access into text that includes supplementary characters).

UTF-16 and UTF-32 encode characters directly, using their code point as a two- or four-byte integer. This means that any given UTF-16 or UTF-32 byte sequence is either big- or little-endian. To assist decoders, Unicode includes a special byte order mark (BOM) character U+FEFF used to determine the endianness of a sequence. The corresponding byte-swapped code point U+FFFE is guaranteed never to be assigned. If a UTF-16 decoder sees 0xfe, 0xff, for example, it knows it's reading a big-endian byte sequence, while 0xff, 0xfe, would indicate a little-endian byte sequence.

UTF-8 can contain a BOM, but since the UTF-8 encoding of a character always uses the same byte sequence, there is no information about endianness to convey. Seeing the bytes corresponding to the UTF-8 encoding of U+FEFF (0xef, 0xbb, 0xbf) would only serve to suggest that you're reading UTF-8. Note that BOMs are decoded as the U+FEFF character, and will appear in the output character sequence. This means that a disadvantage to including a BOM in UTF-8 is that most applications that use UTF-8 do not expect to see a BOM. (This is also a reason to prefer UTF-8: it's one less complication to worry about.)

Because a BOM indicates how the data that follows should be interpreted, a BOM should occur as the first character in a character sequence.

See the Byte Order Mark (BOM) FAQ for more about dealing with BOMs.

Endianness and BOM behavior

The following tables show the endianness and BOM behavior of the UTF-16 variants.

This table shows what the encoder writes. "BE" means that the byte sequence is big-endian, "LE" means little-endian. "BE BOM" means a big-endian BOM (that is, 0xfe, 0xff).

Charset Encoder writes
UTF-16BE BE, no BOM
UTF-16LE LE, no BOM
UTF-16 BE, with BE BOM

The next table shows how each variant's decoder behaves when reading a byte sequence. The exact meaning of "failure" in the table is dependent on the CodingErrorAction supplied to malformedInputAction(), so "BE, failure" means "the byte sequence is treated as big-endian, and a little-endian BOM triggers the malformedInputAction".

The phrase "includes BOM" means that the output includes the U+FEFF byte order mark character.

Charset BE BOM LE BOM No BOM
UTF-16BE BE, includes BOM BE, failure BE
UTF-16LE LE, failure LE, includes BOM LE
UTF-16 BE LE BE

Charset names

A charset has a canonical name, returned by name(). Most charsets will also have one or more aliases, returned by aliases(). A charset can be looked up by canonical name or any of its aliases using forName(String).

Guaranteed-available charsets

The following charsets are available on every Java implementation:

All of these charsets support both decoding and encoding. The charsets whose names begin "UTF" can represent all characters, as mentioned above. The "ISO-8859-1" and "US-ASCII" charsets can only represent small subsets of these characters. Except when required to do otherwise for compatibility, new code should use one of the UTF charsets listed above. The platform's default charset is UTF-8. (This is in contrast to some older implementations, where the default charset depended on the user's locale.)

Most implementations will support hundreds of charsets. Use availableCharsets() or isSupported(String) to see what's available. If you intend to use the charset if it's available, just call forName(String) and catch the exceptions it throws if the charset isn't available.

Additional charsets can be made available by configuring one or more charset providers through provider configuration files. Such files are always named as "java.nio.charset.spi.CharsetProvider" and located in the "META-INF/services" directory of one or more classpaths. The files should be encoded in "UTF-8". Each line of their content specifies the class name of a charset provider which extends CharsetProvider. A line should end with '\r', '\n' or '\r\n'. Leading and trailing whitespace is trimmed. Blank lines, and lines (after trimming) starting with "#" which are regarded as comments, are both ignored. Duplicates of names already found are also ignored. Both the configuration files and the provider classes will be loaded using the thread context class loader.

Although class is thread-safe, the CharsetDecoder and CharsetEncoder instances it returns are inherently stateful.

Summary

Protected constructors

Charset(String canonicalName, String[] aliases)

Constructs a Charset object.

Public methods

final Set<String> aliases()

Returns an unmodifiable set of this charset's aliases.

static SortedMap<StringCharset> availableCharsets()

Returns an immutable case-insensitive map from canonical names to Charset instances.

boolean canEncode()

Returns true if this charset supports encoding, false otherwise.

final int compareTo(Charset charset)

Compares this charset with the given charset.

abstract boolean contains(Charset charset)

Determines whether this charset is a superset of the given charset.

final CharBuffer decode(ByteBuffer buffer)

Returns a new CharBuffer containing the characters decoded from buffer.

static Charset defaultCharset()

Returns the system's default charset.

String displayName()

Returns the name of this charset for the default locale.

String displayName(Locale l)

Returns the name of this charset for the specified locale.

final ByteBuffer encode(CharBuffer buffer)

Returns a new ByteBuffer containing the bytes encoding the characters from buffer.

final ByteBuffer encode(String s)

Returns a new ByteBuffer containing the bytes encoding the characters from s.

final boolean equals(Object obj)

Determines whether this charset equals to the given object.

static Charset forName(String charsetName)

Returns a Charset instance for the named charset.

final int hashCode()

Gets the hash code of this charset.

final boolean isRegistered()

Returns true if this charset is known to be registered in the IANA Charset Registry.

static boolean isSupported(String charsetName)

Determines whether the specified charset is supported by this runtime.

final String name()

Returns the canonical name of this charset.

abstract CharsetDecoder newDecoder()

Returns a new instance of a decoder for this charset.

abstract CharsetEncoder newEncoder()

Returns a new instance of an encoder for this charset.

final String toString()

Gets a string representation of 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)

Constructs a Charset object. Duplicated aliases are ignored.

Parameters
canonicalName String: the canonical name of the charset.
aliases String: an array containing all aliases of the charset. May be null.
Throws
IllegalCharsetNameException on an illegal value being supplied for either canonicalName or for any element of aliases.

Public methods

aliases

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

Returns an unmodifiable set of this charset's aliases.

Returns
Set<String>

availableCharsets

Added in API level 1
SortedMap<StringCharset> availableCharsets ()

Returns an immutable case-insensitive map from canonical names to Charset instances. If multiple charsets have the same canonical name, it is unspecified which is returned in the map. This method may be slow. If you know which charset you're looking for, use forName(String).

Returns
SortedMap<StringCharset>

canEncode

Added in API level 1
boolean canEncode ()

Returns true if this charset supports encoding, false otherwise.

Returns
boolean

compareTo

Added in API level 1
int compareTo (Charset charset)

Compares this charset with the given charset. This comparison is based on the case insensitive canonical names of the charsets.

Parameters
charset Charset: the given object to be compared with.
Returns
int a negative integer if less than the given object, a positive integer if larger than it, or 0 if equal to it.

contains

Added in API level 1
boolean contains (Charset charset)

Determines whether this charset is a superset of the given charset. A charset C1 contains charset C2 if every character representable by C2 is also representable by C1. This means that lossless conversion is possible from C2 to C1 (but not necessarily the other way round). It does not imply that the two charsets use the same byte sequences for the characters they share.

Note that this method is allowed to be conservative, and some implementations may return false when this charset does contain the other charset. Android's implementation is precise, and will always return true in such cases.

Parameters
charset Charset: a given charset.
Returns
boolean true if this charset is a super set of the given charset, false if it's unknown or this charset is not a superset of the given charset.

decode

Added in API level 1
CharBuffer decode (ByteBuffer buffer)

Returns a new CharBuffer containing the characters decoded from buffer. This method uses CodingErrorAction.REPLACE.

Applications should generally create a CharsetDecoder using newDecoder() for performance.

Parameters
buffer ByteBuffer: the byte buffer containing the content to be decoded.
Returns
CharBuffer a character buffer containing the output of the decoding.

defaultCharset

Added in API level 1
Charset defaultCharset ()

Returns the system's default charset. This is determined during VM startup, and will not change thereafter. On Android, the default charset is UTF-8.

Returns
Charset

displayName

Added in API level 1
String displayName ()

Returns the name of this charset for the default locale.

The default implementation returns the canonical name of this charset. Subclasses may return a localized display name.

Returns
String

displayName

Added in API level 1
String displayName (Locale l)

Returns the name of this charset for the specified locale.

The default implementation returns the canonical name of this charset. Subclasses may return a localized display name.

Parameters
l Locale
Returns
String

encode

Added in API level 1
ByteBuffer encode (CharBuffer buffer)

Returns a new ByteBuffer containing the bytes encoding the characters from buffer. This method uses CodingErrorAction.REPLACE.

Applications should generally create a CharsetEncoder using newEncoder() for performance.

Parameters
buffer CharBuffer: the character buffer containing the content to be encoded.
Returns
ByteBuffer the result of the encoding.

encode

Added in API level 1
ByteBuffer encode (String s)

Returns a new ByteBuffer containing the bytes encoding the characters from s. This method uses CodingErrorAction.REPLACE.

Applications should generally create a CharsetEncoder using newEncoder() for performance.

Parameters
s String: the string to be encoded.
Returns
ByteBuffer the result of the encoding.

equals

Added in API level 1
boolean equals (Object obj)

Determines whether this charset equals to the given object. They are considered to be equal if they have the same canonical name.

Parameters
obj Object: the given object to be compared with.
Returns
boolean true if they have the same canonical name, otherwise false.

forName

Added in API level 1
Charset forName (String charsetName)

Returns a Charset instance for the named charset.

Parameters
charsetName String: a charset name (either canonical or an alias)
Returns
Charset
Throws
IllegalCharsetNameException if the specified charset name is illegal.
UnsupportedCharsetException if the desired charset is not supported by this runtime.

hashCode

Added in API level 1
int hashCode ()

Gets the hash code of this charset.

Returns
int the hash code of this charset.

isRegistered

Added in API level 1
boolean isRegistered ()

Returns true if this charset is known to be registered in the IANA Charset Registry.

Returns
boolean

isSupported

Added in API level 1
boolean isSupported (String charsetName)

Determines whether the specified charset is supported by this runtime.

Parameters
charsetName String: the name of the charset.
Returns
boolean true if the specified charset is supported, otherwise false.
Throws
IllegalCharsetNameException if the specified charset name is illegal.

name

Added in API level 1
String name ()

Returns the canonical name of this charset.

If a charset is in the IANA registry, this will be the MIME-preferred name (a charset may have multiple IANA-registered names). Otherwise the canonical name will begin with "x-" or "X-".

Returns
String

newDecoder

Added in API level 1
CharsetDecoder newDecoder ()

Returns a new instance of a decoder for this charset.

Returns
CharsetDecoder

newEncoder

Added in API level 1
CharsetEncoder newEncoder ()

Returns a new instance of an encoder for this charset.

Returns
CharsetEncoder

toString

Added in API level 1
String toString ()

Gets a string representation of this charset. Usually this contains the canonical name of the charset.

Returns
String a string representation of this charset.
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