Charset

Class Overview

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).

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 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:

• 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(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

[Expand] Inherited Methods
From class java.lang.Object Object clone() Creates and returns a copy of this Object. boolean equals(Object o) Compares this instance with the specified object and indicates if they are equal. void finalize() Invoked when the garbage collector has detected that this instance is no longer reachable. final Class<?> getClass() Returns the unique instance of Class that represents this object's class. int hashCode() Returns an integer hash code for this object. final void notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. final void notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. String toString() Returns a string containing a concise, human-readable description of this object. final void wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object. final void wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires. final void wait(long millis) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
From interface java.lang.Comparable abstract int compareTo(Charset another) Compares this object to the specified object to determine their relative order.