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Android Reference

Charset


java.nio.charset

Class Charset

  • All Implemented Interfaces:
    Comparable<Charset>


    public abstract class Charset
    extends Object
    implements Comparable<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(java.nio.charset.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 CharsetDecoder.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(java.lang.String).

    Guaranteed-available charsets

    The following charsets are available on every Java implementation:

    • ISO-8859-1
    • US-ASCII
    • UTF-16
    • UTF-16BE
    • UTF-16LE
    • UTF-8

    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(java.lang.String) to see what's available. If you intend to use the charset if it's available, just call forName(java.lang.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.

    • Constructor Detail

      • Charset

        protected Charset(String canonicalName,
               String[] aliases)
        Constructs a Charset object. Duplicated aliases are ignored.
        Parameters:
        canonicalName - the canonical name of the charset.
        aliases - 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.
    • Method Detail

      • availableCharsets

        public static SortedMap<String,Charset> 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(java.lang.String).
        Returns:
        an immutable case-insensitive map from canonical names to Charset instances
      • forName

        public static Charset forName(String charsetName)
        Returns a Charset instance for the named charset.
        Parameters:
        charsetName - a charset name (either canonical or an alias)
        Throws:
        IllegalCharsetNameException - if the specified charset name is illegal.
        UnsupportedCharsetException - if the desired charset is not supported by this runtime.
      • isSupported

        public static boolean isSupported(String charsetName)
        Determines whether the specified charset is supported by this runtime.
        Parameters:
        charsetName - the name of the charset.
        Returns:
        true if the specified charset is supported, otherwise false.
        Throws:
        IllegalCharsetNameException - if the specified charset name is illegal.
      • contains

        public abstract 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 - a given charset.
        Returns:
        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.
      • newEncoder

        public abstract CharsetEncoder newEncoder()
        Gets a new instance of an encoder for this charset.
        Returns:
        a new instance of an encoder for this charset.
      • newDecoder

        public abstract CharsetDecoder newDecoder()
        Gets a new instance of a decoder for this charset.
        Returns:
        a new instance of a decoder for this charset.
      • name

        public final String name()
        Gets the canonical name of this charset.
        Returns:
        this charset's name in canonical form.
      • aliases

        public final Set<String> aliases()
        Gets the set of this charset's aliases.
        Returns:
        an unmodifiable set of this charset's aliases.
      • displayName

        public String displayName()
        Gets 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:
        the name of this charset for the default locale.
      • displayName

        public String displayName(Locale l)
        Gets 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 - a certain locale
        Returns:
        the name of this charset for the specified locale
      • isRegistered

        public final boolean isRegistered()
        Indicates whether this charset is known to be registered in the IANA Charset Registry.
        Returns:
        true if the charset is known to be registered, otherwise returns false.
      • canEncode

        public boolean canEncode()
        Returns true if this charset supports encoding, false otherwise.
        Returns:
        true if this charset supports encoding, false otherwise.
      • encode

        public final 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 - the character buffer containing the content to be encoded.
        Returns:
        the result of the encoding.
      • encode

        public final 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 - the string to be encoded.
        Returns:
        the result of the encoding.
      • decode

        public final 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 - the byte buffer containing the content to be decoded.
        Returns:
        a character buffer containing the output of the decoding.
      • compareTo

        public final int compareTo(Charset charset)
        Compares this charset with the given charset. This comparison is based on the case insensitive canonical names of the charsets.
        Specified by:
        compareTo in interface Comparable<Charset>
        Parameters:
        charset - the given object to be compared with.
        Returns:
        a negative integer if less than the given object, a positive integer if larger than it, or 0 if equal to it.
      • equals

        public final 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.
        Overrides:
        equals in class Object
        Parameters:
        obj - the given object to be compared with.
        Returns:
        true if they have the same canonical name, otherwise false.
        See Also:
        Object.hashCode()
      • toString

        public final String toString()
        Gets a string representation of this charset. Usually this contains the canonical name of the charset.
        Overrides:
        toString in class Object
        Returns:
        a string representation of this charset.
      • defaultCharset

        public static 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.


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