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Charset.javaAPI DocJava SE 6 API31552Tue Jun 10 00:25:42 BST 2008java.nio.charset

Charset

public abstract class Charset extends Object implements Comparable
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 {@link java.nio.charset.spi.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 {@link #name() name} method of this class. Canonical names are, by convention, usually in upper case. The aliases of a charset are returned by the {@link #aliases() 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 {@link java.io.InputStreamReader#getEncoding InputStreamReader} and {@link java.io.OutputStreamWriter#getEncoding 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.

author
Mark Reinhold
author
JSR-51 Expert Group
version
1.53, 06/06/28
since
1.4
see
CharsetDecoder
see
CharsetEncoder
see
java.nio.charset.spi.CharsetProvider
see
java.lang.Character

Fields Summary
private static String
bugLevel
private static CharsetProvider
standardProvider
private static volatile Object[]
cache1
private static volatile Object[]
cache2
private static ThreadLocal
gate
private static Object
extendedProviderLock
private static boolean
extendedProviderProbed
private static CharsetProvider
extendedProvider
private static volatile Charset
defaultCharset
private final String
name
private final String[]
aliases
private Set
aliasSet
Constructors Summary
protected Charset(String canonicalName, String[] aliases)
Initializes a new charset with the given canonical name and alias set.

param
canonicalName The canonical name of this charset
param
aliases 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


                                                                                    
         
	checkName(canonicalName);
	String[] as = (aliases == null) ? new String[0] : aliases;
	for (int i = 0; i < as.length; i++)
	    checkName(as[i]);
	this.name = canonicalName;
	this.aliases = as;
    
Methods Summary
public final java.util.Setaliases()
Returns a set containing this charset's aliases.

return
An immutable set of this charset's aliases

	if (aliasSet != null)
	    return aliasSet;
	int n = aliases.length;
	HashSet hs = new HashSet(n);
	for (int i = 0; i < n; i++)
	    hs.add(aliases[i]);
	aliasSet = Collections.unmodifiableSet(hs);
	return aliasSet;
    
static booleanatBugLevel(java.lang.String bl)


        		// package-private
	if (bugLevel == null) {
	    if (!sun.misc.VM.isBooted())
		return false;
	    java.security.PrivilegedAction pa =
		new GetPropertyAction("sun.nio.cs.bugLevel");
	    bugLevel = (String)AccessController.doPrivileged(pa);
	    if (bugLevel == null)
		bugLevel = "";
	}
	return (bugLevel != null) && bugLevel.equals(bl);
    
public static java.util.SortedMapavailableCharsets()
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 {@link #forName 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 {@link #forName forName} method.

return
An immutable, case-insensitive map from canonical charset names to charset objects

	return (SortedMap)AccessController
	    .doPrivileged(new PrivilegedAction() {
		public Object run() {
		    TreeMap m = new TreeMap(ASCIICaseInsensitiveComparator.CASE_INSENSITIVE_ORDER);
		    put(standardProvider.charsets(), m);
		    for (Iterator i = providers(); i.hasNext();) {
			CharsetProvider cp = (CharsetProvider)i.next();
			put(cp.charsets(), m);
		    }
		    return Collections.unmodifiableSortedMap(m);
		}
	    });
    
private static voidcache(java.lang.String charsetName, java.nio.charset.Charset cs)

 // "Level 2" cache

           
	cache2 = cache1;
	cache1 = new Object[] { charsetName, cs };
    
public booleancanEncode()
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.

return
true if, and only if, this charset supports encoding

	return true;
    
private static voidcheckName(java.lang.String s)
Checks that the given string is a legal charset name.

param
s A purported charset name
throws
IllegalCharsetNameException If the given name is not a legal charset name

	int n = s.length();
	if (!atBugLevel("1.4")) {
	    if (n == 0)
		throw new IllegalCharsetNameException(s);
	}
	for (int i = 0; i < n; i++) {
	    char c = s.charAt(i);
	    if (c >= 'A" && c <= 'Z") continue;
	    if (c >= 'a" && c <= 'z") continue;
	    if (c >= '0" && c <= '9") continue;
	    if (c == '-" && i != 0) continue;
	    if (c == ':" && i != 0) continue;
	    if (c == '_" && i != 0) continue;
	    if (c == '." && i != 0) continue;
	    throw new IllegalCharsetNameException(s);
	}
    
public final intcompareTo(java.nio.charset.Charset that)
Compares this charset to another.

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

param
that The charset to which this charset is to be compared
return
A negative integer, zero, or a positive integer as this charset is less than, equal to, or greater than the specified charset

	return (name().compareToIgnoreCase(that.name()));
    
public abstract booleancontains(java.nio.charset.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.

return
true if the given charset is contained in this charset

public final java.nio.CharBufferdecode(java.nio.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 {@link CharsetDecoder#decode(java.nio.ByteBuffer)} method directly.

param
bb The byte buffer to be decoded
return
A char buffer containing the decoded characters

	try {
	    return ThreadLocalCoders.decoderFor(this)
		.onMalformedInput(CodingErrorAction.REPLACE)
		.onUnmappableCharacter(CodingErrorAction.REPLACE)
		.decode(bb);
	} catch (CharacterCodingException x) {
	    throw new Error(x);		// Can't happen
	}
    
public static java.nio.charset.CharsetdefaultCharset()
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.

return
A charset object for the default charset
since
1.5

        if (defaultCharset == null) {
	    synchronized (Charset.class) {
		java.security.PrivilegedAction pa =
		    new GetPropertyAction("file.encoding");
		String csn = (String)AccessController.doPrivileged(pa);
		Charset cs = lookup(csn);
		if (cs != null)
		    defaultCharset = cs;
                else 
		    defaultCharset = forName("UTF-8");
            }
	}
	return defaultCharset;
    
public java.lang.StringdisplayName()
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.

return
The display name of this charset in the default locale

	return name;
    
public java.lang.StringdisplayName(java.util.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.

param
locale The locale for which the display name is to be retrieved
return
The display name of this charset in the given locale

	return name;
    
public final java.nio.ByteBufferencode(java.nio.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 {@link CharsetEncoder#encode(java.nio.CharBuffer)} method directly.

param
cb The char buffer to be encoded
return
A byte buffer containing the encoded characters

	try {
	    return ThreadLocalCoders.encoderFor(this)
		.onMalformedInput(CodingErrorAction.REPLACE)
		.onUnmappableCharacter(CodingErrorAction.REPLACE)
		.encode(cb);
	} catch (CharacterCodingException x) {
	    throw new Error(x);		// Can't happen
	}
    
public final java.nio.ByteBufferencode(java.lang.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)); 

param
str The string to be encoded
return
A byte buffer containing the encoded characters

	return encode(CharBuffer.wrap(str));
    
public final booleanequals(java.lang.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.

return
true if, and only if, this charset is equal to the given object

	if (!(ob instanceof Charset))
	    return false;
	if (this == ob)
	    return true;
	return name.equals(((Charset)ob).name());
    
public static java.nio.charset.CharsetforName(java.lang.String charsetName)
Returns a charset object for the named charset.

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

	Charset cs = lookup(charsetName);
	if (cs != null)
	    return cs;
	throw new UnsupportedCharsetException(charsetName);
    
public final inthashCode()
Computes a hashcode for this charset.

return
An integer hashcode

	return name().hashCode();
    
public final booleanisRegistered()
Tells whether or not this charset is registered in the IANA Charset Registry.

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

	return !name.startsWith("X-") && !name.startsWith("x-");
    
public static booleanisSupported(java.lang.String charsetName)
Tells whether the named charset is supported.

param
charsetName The name of the requested charset; may be either a canonical name or an alias
return
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
throws
IllegalArgumentException If the given charsetName is null

	return (lookup(charsetName) != null);
    
private static java.nio.charset.Charsetlookup(java.lang.String charsetName)

	if (charsetName == null)
	    throw new IllegalArgumentException("Null charset name");

	Object[] a;
	if ((a = cache1) != null && charsetName.equals(a[0]))
	    return (Charset)a[1];
	// We expect most programs to use one Charset repeatedly.
	// We convey a hint to this effect to the VM by putting the
	// level 1 cache miss code in a separate method.
	return lookup2(charsetName);
    
private static java.nio.charset.Charsetlookup2(java.lang.String charsetName)

	Object[] a;
	if ((a = cache2) != null && charsetName.equals(a[0])) {
	    cache2 = cache1;
	    cache1 = a;
	    return (Charset)a[1];
	}

	Charset cs;
	if ((cs = standardProvider.charsetForName(charsetName)) != null ||
	    (cs = lookupExtendedCharset(charsetName))           != null ||
	    (cs = lookupViaProviders(charsetName))              != null)
	{
	    cache(charsetName, cs);
	    return cs;
	}

	/* Only need to check the name if we didn't find a charset for it */
	checkName(charsetName);
	return null;
    
private static java.nio.charset.CharsetlookupExtendedCharset(java.lang.String charsetName)

	CharsetProvider ecp = null;
	synchronized (extendedProviderLock) {
	    if (!extendedProviderProbed) {
		probeExtendedProvider();
		extendedProviderProbed = true;
	    }
	    ecp = extendedProvider;
	}
	return (ecp != null) ? ecp.charsetForName(charsetName) : null;
    
private static java.nio.charset.CharsetlookupViaProviders(java.lang.String charsetName)


          

	// The runtime startup sequence looks up standard charsets as a
	// consequence of the VM's invocation of System.initializeSystemClass
	// in order to, e.g., set system properties and encode filenames.  At
	// that point the application class loader has not been initialized,
	// however, so we can't look for providers because doing so will cause
	// that loader to be prematurely initialized with incomplete
	// information.
	//
	if (!sun.misc.VM.isBooted())
	    return null;

	if (gate.get() != null)
	    // Avoid recursive provider lookups
	    return null;
	try {
	    gate.set(gate);

	    return (Charset)AccessController
		.doPrivileged(new PrivilegedAction() {
		    public Object run() {
			for (Iterator i = providers(); i.hasNext();) {
			    CharsetProvider cp = (CharsetProvider)i.next();
			    Charset cs = cp.charsetForName(charsetName);
			    if (cs != null)
				return cs;
			}
			return null;
		    }
		});

	} finally {
	    gate.set(null);
	}
    
public final java.lang.Stringname()
Returns this charset's canonical name.

return
The canonical name of this charset

	return name;
    
public abstract java.nio.charset.CharsetDecodernewDecoder()
Constructs a new decoder for this charset.

return
A new decoder for this charset

public abstract java.nio.charset.CharsetEncodernewEncoder()
Constructs a new encoder for this charset.

return
A new encoder for this charset
throws
UnsupportedOperationException If this charset does not support encoding

private static voidprobeExtendedProvider()


        
	AccessController.doPrivileged(new PrivilegedAction() {
		public Object run() {
		    try {
			Class epc
			    = Class.forName("sun.nio.cs.ext.ExtendedCharsets");
			extendedProvider = (CharsetProvider)epc.newInstance();
		    } catch (ClassNotFoundException x) {
			// Extended charsets not available
			// (charsets.jar not present)
		    } catch (InstantiationException x) {
			throw new Error(x);
		    } catch (IllegalAccessException x) {
			throw new Error(x);
		    }
		    return null;
		}
	    });
    
private static java.util.Iteratorproviders()

	return new Iterator() {

		Class c = java.nio.charset.spi.CharsetProvider.class;
		ClassLoader cl = ClassLoader.getSystemClassLoader();
		Iterator i = Service.providers(c, cl);
		Object next = null;

		private boolean getNext() {
		    while (next == null) {
			try {
			    if (!i.hasNext())
				return false;
			    next = i.next();
			} catch (ServiceConfigurationError sce) {
			    if (sce.getCause() instanceof SecurityException) {
				// Ignore security exceptions
				continue;
			    }
			    throw sce;
			}
		    }
		    return true;
		}

		public boolean hasNext() {
		    return getNext();
		}

		public Object next() {
		    if (!getNext())
			throw new NoSuchElementException();
		    Object n = next;
		    next = null;
		    return n;
		}

		public void remove() {
		    throw new UnsupportedOperationException();
		}

	    };
    
private static voidput(java.util.Iterator i, java.util.Map m)

	while (i.hasNext()) {
	    Charset cs = (Charset)i.next();
	    if (!m.containsKey(cs.name()))
		m.put(cs.name(), cs);
	}
    
public final java.lang.StringtoString()
Returns a string describing this charset.

return
A string describing this charset

	return name();