https://openjdk.java.net/ 
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 * Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
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package java.nio.charset;

import jdk.internal.misc.VM;
import sun.nio.cs.ThreadLocalCoders;
import sun.security.action.GetPropertyAction;

import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.spi.CharsetProvider;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Locale;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;
import java.util.Set;
import java.util.SortedMap;
import java.util.TreeMap;



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 plus character '+' ('\u002b', PLUS SIGN),
  
  •  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.


Description of standard charsets


CharsetDescription




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 interpret the initial byte-order marks as a ZERO-WIDTH
  NON-BREAKING SPACE; when encoding, they do not write
  byte-order marks. 
    • 
  
  •  When decoding, the UTF-16 charset interprets the byte-order mark at the beginning of the input stream 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, byte order marks occurring 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. 


The StandardCharsets class defines constants for each of the standard charsets. 
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
one or more coded character sets and a character-encoding scheme.
(This definition is confusing; some other software systems define
charset as a synonym for coded character set.)

 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 Unicode are examples of coded character sets.

 Some standards have defined a character set to be simply a
set of abstract characters without an associated assigned numbering.
An alphabet is an example of such a character set.  However, the subtle
distinction between character set and coded character set
is rarely used in practice; the former has become a short form for the
latter, including in the Java API specification.

 A character-encoding scheme is a mapping between one or more
coded character sets and a set of octet (eight-bit byte) sequences.
UTF-8, 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
coded character sets; EUC, for example, can be used to encode
characters in a variety of Asian coded 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 coded character set; otherwise a charset is usually named for the encoding scheme and, possibly, the locale of the coded character sets that it supports. Hence US-ASCII is both the name of a coded character set and of the charset that encodes it, while EUC-JP is the name of the charset that encodes the JIS X 0201, JIS X 0208, and JIS X 0212 coded character sets for the Japanese language. 
 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 (that is, sequences
of chars) and sequences of bytes. 


Author:Mark Reinhold, JSR-51 Expert Group
See Also:
Since:1.4
/**
 * A named mapping between sequences of sixteen-bit Unicode <a
 * href="../../lang/Character.html#unicode">code units</a> 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.
 *
 * <p> 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.
 *
 * <p> All of the methods defined in this class are safe for use by multiple
 * concurrent threads.
 *
 *
 * <a id="names"></a><a id="charenc"></a>
 * <h2>Charset names</h2>
 *
 * <p> Charsets are named by strings composed of the following characters:
 *
 * <ul>
 *
 *   <li> The uppercase letters {@code 'A'} through {@code 'Z'}
 *        (<code>'&#92;u0041'</code>&nbsp;through&nbsp;<code>'&#92;u005a'</code>),
 *
 *   <li> The lowercase letters {@code 'a'} through {@code 'z'}
 *        (<code>'&#92;u0061'</code>&nbsp;through&nbsp;<code>'&#92;u007a'</code>),
 *
 *   <li> The digits {@code '0'} through {@code '9'}
 *        (<code>'&#92;u0030'</code>&nbsp;through&nbsp;<code>'&#92;u0039'</code>),
 *
 *   <li> The dash character {@code '-'}
 *        (<code>'&#92;u002d'</code>,&nbsp;<small>HYPHEN-MINUS</small>),
 *
 *   <li> The plus character {@code '+'}
 *        (<code>'&#92;u002b'</code>,&nbsp;<small>PLUS SIGN</small>),
 *
 *   <li> The period character {@code '.'}
 *        (<code>'&#92;u002e'</code>,&nbsp;<small>FULL STOP</small>),
 *
 *   <li> The colon character {@code ':'}
 *        (<code>'&#92;u003a'</code>,&nbsp;<small>COLON</small>), and
 *
 *   <li> The underscore character {@code '_'}
 *        (<code>'&#92;u005f'</code>,&nbsp;<small>LOW&nbsp;LINE</small>).
 *
 * </ul>
 *
 * 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 <a
 * href="http://www.ietf.org/rfc/rfc2278.txt"><i>RFC&nbsp;2278:&nbsp;IANA Charset
 * Registration Procedures</i></a>.
 *
 * <p> Every charset has a <i>canonical name</i> and may also have one or more
 * <i>aliases</i>.  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.
 *
 * <p><a id="hn">Some charsets have an <i>historical name</i> that is defined for
 * compatibility with previous versions of the Java platform.</a>  A charset's
 * historical name is either its canonical name or one of its aliases.  The
 * historical name is returned by the {@code getEncoding()} methods of the
 * {@link java.io.InputStreamReader#getEncoding InputStreamReader} and {@link
 * java.io.OutputStreamWriter#getEncoding OutputStreamWriter} classes.
 *
 * <p><a id="iana"> </a>If a charset listed in the <a
 * href="http://www.iana.org/assignments/character-sets"><i>IANA Charset
 * Registry</i></a> 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 <i>MIME-preferred</i>.  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 {@code "X-"} or {@code "x-"}.
 *
 * <p> 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.
 *
 *
 * <h2>Standard charsets</h2>
 *
 *
 *
 * <p><a id="standard">Every implementation of the Java platform is required to support the
 * following standard charsets.</a>  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.
 *
 * <blockquote><table class="striped" style="width:80%">
 * <caption style="display:none">Description of standard charsets</caption>
 * <thead>
 * <tr><th scope="col" style="text-align:left">Charset</th><th scope="col" style="text-align:left">Description</th></tr>
 * </thead>
 * <tbody>
 * <tr><th scope="row" style="vertical-align:top">{@code US-ASCII}</th>
 *     <td>Seven-bit ASCII, a.k.a. {@code ISO646-US},
 *         a.k.a. the Basic Latin block of the Unicode character set</td></tr>
 * <tr><th scope="row" style="vertical-align:top"><code>ISO-8859-1&nbsp;&nbsp;</code></th>
 *     <td>ISO Latin Alphabet No. 1, a.k.a. {@code ISO-LATIN-1}</td></tr>
 * <tr><th scope="row" style="vertical-align:top">{@code UTF-8}</th>
 *     <td>Eight-bit UCS Transformation Format</td></tr>
 * <tr><th scope="row" style="vertical-align:top">{@code UTF-16BE}</th>
 *     <td>Sixteen-bit UCS Transformation Format,
 *         big-endian byte&nbsp;order</td></tr>
 * <tr><th scope="row" style="vertical-align:top">{@code UTF-16LE}</th>
 *     <td>Sixteen-bit UCS Transformation Format,
 *         little-endian byte&nbsp;order</td></tr>
 * <tr><th scope="row" style="vertical-align:top">{@code UTF-16}</th>
 *     <td>Sixteen-bit UCS Transformation Format,
 *         byte&nbsp;order identified by an optional byte-order mark</td></tr>
 * </tbody>
 * </table></blockquote>
 *
 * <p> The {@code UTF-8} charset is specified by <a
 * href="http://www.ietf.org/rfc/rfc2279.txt"><i>RFC&nbsp;2279</i></a>; the
 * transformation format upon which it is based is specified in
 * Amendment&nbsp;2 of ISO&nbsp;10646-1 and is also described in the <a
 * href="http://www.unicode.org/unicode/standard/standard.html"><i>Unicode
 * Standard</i></a>.
 *
 * <p> The {@code UTF-16} charsets are specified by <a
 * href="http://www.ietf.org/rfc/rfc2781.txt"><i>RFC&nbsp;2781</i></a>; the
 * transformation formats upon which they are based are specified in
 * Amendment&nbsp;1 of ISO&nbsp;10646-1 and are also described in the <a
 * href="http://www.unicode.org/unicode/standard/standard.html"><i>Unicode
 * Standard</i></a>.
 *
 * <p> The {@code 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 <i>byte-order mark</i> represented by
 * the Unicode character <code>'&#92;uFEFF'</code>.  Byte-order marks are handled
 * as follows:
 *
 * <ul>
 *
 *   <li><p> When decoding, the {@code UTF-16BE} and {@code UTF-16LE}
 *   charsets interpret the initial byte-order marks as a <small>ZERO-WIDTH
 *   NON-BREAKING SPACE</small>; when encoding, they do not write
 *   byte-order marks. </p></li>

 *
 *   <li><p> When decoding, the {@code UTF-16} charset interprets the
 *   byte-order mark at the beginning of the input stream 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. </p></li>
 *
 * </ul>
 *
 * In any case, byte order marks occurring after the first element of an
 * input sequence are not omitted since the same code is used to represent
 * <small>ZERO-WIDTH NON-BREAKING SPACE</small>.
 *
 * <p> 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. </p>
 *
 * <p>The {@link StandardCharsets} class defines constants for each of the
 * standard charsets.
 *
 * <h2>Terminology</h2>
 *
 * <p> The name of this class is taken from the terms used in
 * <a href="http://www.ietf.org/rfc/rfc2278.txt"><i>RFC&nbsp;2278</i></a>.
 * In that document a <i>charset</i> is defined as the combination of
 * one or more coded character sets and a character-encoding scheme.
 * (This definition is confusing; some other software systems define
 * <i>charset</i> as a synonym for <i>coded character set</i>.)
 *
 * <p> A <i>coded character set</i> is a mapping between a set of abstract
 * characters and a set of integers.  US-ASCII, ISO&nbsp;8859-1,
 * JIS&nbsp;X&nbsp;0201, and Unicode are examples of coded character sets.
 *
 * <p> Some standards have defined a <i>character set</i> to be simply a
 * set of abstract characters without an associated assigned numbering.
 * An alphabet is an example of such a character set.  However, the subtle
 * distinction between <i>character set</i> and <i>coded character set</i>
 * is rarely used in practice; the former has become a short form for the
 * latter, including in the Java API specification.
 *
 * <p> A <i>character-encoding scheme</i> is a mapping between one or more
 * coded character sets and a set of octet (eight-bit byte) sequences.
 * UTF-8, UTF-16, ISO&nbsp;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
 * coded character sets; EUC, for example, can be used to encode
 * characters in a variety of Asian coded character sets.
 *
 * <p> When a coded character set is used exclusively with a single
 * character-encoding scheme then the corresponding charset is usually
 * named for the coded character set; otherwise a charset is usually named
 * for the encoding scheme and, possibly, the locale of the coded
 * character sets that it supports.  Hence {@code US-ASCII} is both the
 * name of a coded character set and of the charset that encodes it, while
 * {@code EUC-JP} is the name of the charset that encodes the
 * JIS&nbsp;X&nbsp;0201, JIS&nbsp;X&nbsp;0208, and JIS&nbsp;X&nbsp;0212
 * coded character sets for the Japanese language.
 *
 * <p> 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 (that is, sequences
 * of chars) and sequences of bytes. </p>
 *
 *
 * @author Mark Reinhold
 * @author JSR-51 Expert Group
 * @since 1.4
 *
 * @see CharsetDecoder
 * @see CharsetEncoder
 * @see java.nio.charset.spi.CharsetProvider
 * @see java.lang.Character
 */

public abstract class Charset
    implements Comparable<Charset>
{

    /* -- Static methods -- */

    
Checks that the given string is a legal charset name. 


Params:
  • s – 
        A purported charset name
Throws:
/**
     * Checks that the given string is a legal charset name. </p>
     *
     * @param  s
     *         A purported charset name
     *
     * @throws  IllegalCharsetNameException
     *          If the given name is not a legal charset name
     */
    private static void checkName(String s) {
        int n = s.length();
        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;
            if (c == '.' && i != 0) continue;
            throw new IllegalCharsetNameException(s);
        }
    }

    /* The standard set of charsets */
    private static final CharsetProvider standardProvider
        = new sun.nio.cs.StandardCharsets();

    private static final String[] zeroAliases = new String[0];

    // Cache of the most-recently-returned charsets,
    // along with the names that were used to find them
    //
    private static volatile Object[] cache1; // "Level 1" cache
    private static volatile Object[] cache2; // "Level 2" cache

    private static void cache(String charsetName, Charset cs) {
        cache2 = cache1;
        cache1 = new Object[] { charsetName, cs };
    }

    // Creates an iterator that walks over the available providers, ignoring
    // those whose lookup or instantiation causes a security exception to be
    // thrown.  Should be invoked with full privileges.
    //
    private static Iterator<CharsetProvider> providers() {
        return new Iterator<>() {
                ClassLoader cl = ClassLoader.getSystemClassLoader();
                ServiceLoader<CharsetProvider> sl =
                    ServiceLoader.load(CharsetProvider.class, cl);
                Iterator<CharsetProvider> i = sl.iterator();
                CharsetProvider 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 CharsetProvider next() {
                    if (!getNext())
                        throw new NoSuchElementException();
                    CharsetProvider n = next;
                    next = null;
                    return n;
                }

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

            };
    }

    // Thread-local gate to prevent recursive provider lookups
    private static ThreadLocal<ThreadLocal<?>> gate =
            new ThreadLocal<ThreadLocal<?>>();

    private static Charset lookupViaProviders(final 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 (!VM.isBooted())
            return null;

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

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

        } finally {
            gate.set(null);
        }
    }

    /* The extended set of charsets */
    private static class ExtendedProviderHolder {
        static final CharsetProvider[] extendedProviders = extendedProviders();
        // returns ExtendedProvider, if installed
        private static CharsetProvider[] extendedProviders() {
            return AccessController.doPrivileged(new PrivilegedAction<>() {
                    public CharsetProvider[] run() {
                        CharsetProvider[] cps = new CharsetProvider[1];
                        int n = 0;
                        ServiceLoader<CharsetProvider> sl =
                            ServiceLoader.loadInstalled(CharsetProvider.class);
                        for (CharsetProvider cp : sl) {
                            if (n + 1 > cps.length) {
                                cps = Arrays.copyOf(cps, cps.length << 1);
                            }
                            cps[n++] = cp;
                        }
                        return n == cps.length ? cps : Arrays.copyOf(cps, n);
                    }});
        }
    }

    private static Charset lookupExtendedCharset(String charsetName) {
        if (!VM.isBooted())  // see lookupViaProviders()
            return null;
        CharsetProvider[] ecps = ExtendedProviderHolder.extendedProviders;
        for (CharsetProvider cp : ecps) {
            Charset cs = cp.charsetForName(charsetName);
            if (cs != null)
                return cs;
        }
        return null;
    }

    private static Charset lookup(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 Charset lookup2(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;
    }

    
Tells whether the named charset is supported.

Params:
  • charsetName – 
        The name of the requested charset; may be either
        a canonical name or an alias
Throws:
Returns: true if, and only if, support for the named charset is available in the current Java virtual machine
/**
     * 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  {@code 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 {@code charsetName} is null
     */
    public static boolean isSupported(String charsetName) {
        return (lookup(charsetName) != null);
    }

    
Returns a charset object for the named charset.

Params:
  • charsetName – 
        The name of the requested charset; may be either
        a canonical name or an alias
Throws:
Returns: A charset object for the named charset
/**
     * 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 {@code charsetName} is null
     *
     * @throws  UnsupportedCharsetException
     *          If no support for the named charset is available
     *          in this instance of the Java virtual machine
     */
    public static Charset forName(String charsetName) {
        Charset cs = lookup(charsetName);
        if (cs != null)
            return cs;
        throw new UnsupportedCharsetException(charsetName);
    }

    // Fold charsets from the given iterator into the given map, ignoring
    // charsets whose names already have entries in the map.
    //
    private static void put(Iterator<Charset> i, Map<String,Charset> m) {
        while (i.hasNext()) {
            Charset cs = i.next();
            if (!m.containsKey(cs.name()))
                m.put(cs.name(), cs);
        }
    }

    
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 
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 forName method. 


Returns:An immutable, case-insensitive map from canonical charset names
        to charset objects
/**
     * Constructs a sorted map from canonical charset names to charset objects.
     *
     * <p> 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. </p>
     *
     * <p> 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.
     *
     * <p> 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.  </p>
     *
     * @return An immutable, case-insensitive map from canonical charset names
     *         to charset objects
     */
    public static SortedMap<String,Charset> availableCharsets() {
        return AccessController.doPrivileged(
            new PrivilegedAction<>() {
                public SortedMap<String,Charset> run() {
                    TreeMap<String,Charset> m =
                        new TreeMap<>(
                            String.CASE_INSENSITIVE_ORDER);
                    put(standardProvider.charsets(), m);
                    CharsetProvider[] ecps = ExtendedProviderHolder.extendedProviders;
                    for (CharsetProvider ecp :ecps) {
                        put(ecp.charsets(), m);
                    }
                    for (Iterator<CharsetProvider> i = providers(); i.hasNext();) {
                        CharsetProvider cp = i.next();
                        put(cp.charsets(), m);
                    }
                    return Collections.unmodifiableSortedMap(m);
                }
            });
    }

    private static volatile Charset defaultCharset;

    
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.

Returns: A charset object for the default charset
Since:1.5
/**
     * Returns the default charset of this Java virtual machine.
     *
     * <p> 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
     */
    public static Charset defaultCharset() {
        if (defaultCharset == null) {
            synchronized (Charset.class) {
                String csn = GetPropertyAction
                        .privilegedGetProperty("file.encoding");
                Charset cs = lookup(csn);
                if (cs != null)
                    defaultCharset = cs;
                else
                    defaultCharset = sun.nio.cs.UTF_8.INSTANCE;
            }
        }
        return defaultCharset;
    }


    /* -- Instance fields and methods -- */

    private final String name;          // tickles a bug in oldjavac
    private final String[] aliases;     // tickles a bug in oldjavac
    private Set<String> aliasSet = null;

    
Initializes a new charset with the given canonical name and alias
set.

Params:
  • canonicalName – 
        The canonical name of this charset
  • aliases – 
        An array of this charset's aliases, or null if it has no aliases
Throws:
/**
     * 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
     */
    protected Charset(String canonicalName, String[] aliases) {
        String[] as = Objects.requireNonNullElse(aliases, zeroAliases);

        // Skip checks for the standard, built-in Charsets we always load
        // during initialization.
        if (canonicalName != "ISO-8859-1"
                && canonicalName != "US-ASCII"
                && canonicalName != "UTF-8") {
            checkName(canonicalName);
            for (int i = 0; i < as.length; i++) {
                checkName(as[i]);
            }
        }
        this.name = canonicalName;
        this.aliases = as;
    }

    
Returns this charset's canonical name.

Returns: The canonical name of this charset
/**
     * Returns this charset's canonical name.
     *
     * @return  The canonical name of this charset
     */
    public final String name() {
        return name;
    }

    
Returns a set containing this charset's aliases.

Returns: An immutable set of this charset's aliases
/**
     * Returns a set containing this charset's aliases.
     *
     * @return  An immutable set of this charset's aliases
     */
    public final Set<String> aliases() {
        if (aliasSet != null)
            return aliasSet;
        int n = aliases.length;
        HashSet<String> hs = new HashSet<>(n);
        for (int i = 0; i < n; i++)
            hs.add(aliases[i]);
        aliasSet = Collections.unmodifiableSet(hs);
        return aliasSet;
    }

    
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. 


Returns: The display name of this charset in the default locale
/**
     * Returns this charset's human-readable name for the default locale.
     *
     * <p> 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. </p>
     *
     * @return  The display name of this charset in the default locale
     */
    public String displayName() {
        return name;
    }

    
Tells whether or not this charset is registered in the IANA Charset
Registry.

Returns: true if, and only if, this charset is known by its implementor to be registered with the IANA
/**
     * Tells whether or not this charset is registered in the <a
     * href="http://www.iana.org/assignments/character-sets">IANA Charset
     * Registry</a>.
     *
     * @return  {@code true} if, and only if, this charset is known by its
     *          implementor to be registered with the IANA
     */
    public final boolean isRegistered() {
        return !name.startsWith("X-") && !name.startsWith("x-");
    }

    
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. 


Params:
  • locale – 
        The locale for which the display name is to be retrieved
Returns: The display name of this charset in the given locale
/**
     * Returns this charset's human-readable name for the given locale.
     *
     * <p> 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. </p>
     *
     * @param  locale
     *         The locale for which the display name is to be retrieved
     *
     * @return  The display name of this charset in the given locale
     */
    public String displayName(Locale locale) {
        return name;
    }

    
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. 
Params:
  • cs – 
         The given charset
Returns: true if the given charset is contained in this charset
/**
     * Tells whether or not this charset contains the given charset.
     *
     * <p> A charset <i>C</i> is said to <i>contain</i> a charset <i>D</i> if,
     * and only if, every character representable in <i>D</i> is also
     * representable in <i>C</i>.  If this relationship holds then it is
     * guaranteed that every string that can be encoded in <i>D</i> can also be
     * encoded in <i>C</i> without performing any replacements.
     *
     * <p> That <i>C</i> contains <i>D</i> does not imply that each character
     * representable in <i>C</i> by a particular byte sequence is represented
     * in <i>D</i> by the same byte sequence, although sometimes this is the
     * case.
     *
     * <p> Every charset contains itself.
     *
     * <p> This method computes an approximation of the containment relation:
     * If it returns {@code true} then the given charset is known to be
     * contained by this charset; if it returns {@code false}, however, then
     * it is not necessarily the case that the given charset is not contained
     * in this charset.
     *
     * @param   cs
     *          The given charset
     *
     * @return  {@code true} if the given charset is contained in this charset
     */
    public abstract boolean contains(Charset cs);

    
Constructs a new decoder for this charset.

Returns: A new decoder for this charset
/**
     * Constructs a new decoder for this charset.
     *
     * @return  A new decoder for this charset
     */
    public abstract CharsetDecoder newDecoder();

    
Constructs a new encoder for this charset.

Throws:
Returns: A new encoder for this charset
/**
     * Constructs a new encoder for this charset.
     *
     * @return  A new encoder for this charset
     *
     * @throws  UnsupportedOperationException
     *          If this charset does not support encoding
     */
    public abstract CharsetEncoder newEncoder();

    
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. 


Returns: true if, and only if, this charset supports encoding
/**
     * Tells whether or not this charset supports encoding.
     *
     * <p> Nearly all charsets support encoding.  The primary exceptions are
     * special-purpose <i>auto-detect</i> 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
     * {@code false}. </p>
     *
     * @return  {@code true} if, and only if, this charset supports encoding
     */
    public boolean canEncode() {
        return true;
    }

    
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 CharsetDecoder.decode(ByteBuffer) method directly. 


Params:
  • bb –  The byte buffer to be decoded
Returns: A char buffer containing the decoded characters
/**
     * Convenience method that decodes bytes in this charset into Unicode
     * characters.
     *
     * <p> An invocation of this method upon a charset {@code cs} returns the
     * same result as the expression
     *
     * <pre>
     *     cs.newDecoder()
     *       .onMalformedInput(CodingErrorAction.REPLACE)
     *       .onUnmappableCharacter(CodingErrorAction.REPLACE)
     *       .decode(bb); </pre>
     *
     * except that it is potentially more efficient because it can cache
     * decoders between successive invocations.
     *
     * <p> 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.  </p>
     *
     * @param  bb  The byte buffer to be decoded
     *
     * @return  A char buffer containing the decoded characters
     */
    public final CharBuffer decode(ByteBuffer bb) {
        try {
            return ThreadLocalCoders.decoderFor(this)
                .onMalformedInput(CodingErrorAction.REPLACE)
                .onUnmappableCharacter(CodingErrorAction.REPLACE)
                .decode(bb);
        } catch (CharacterCodingException x) {
            throw new Error(x);         // Can't happen
        }
    }

    
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 CharsetEncoder.encode(CharBuffer) method directly. 


Params:
  • cb –  The char buffer to be encoded
Returns: A byte buffer containing the encoded characters
/**
     * Convenience method that encodes Unicode characters into bytes in this
     * charset.
     *
     * <p> An invocation of this method upon a charset {@code cs} returns the
     * same result as the expression
     *
     * <pre>
     *     cs.newEncoder()
     *       .onMalformedInput(CodingErrorAction.REPLACE)
     *       .onUnmappableCharacter(CodingErrorAction.REPLACE)
     *       .encode(bb); </pre>
     *
     * except that it is potentially more efficient because it can cache
     * encoders between successive invocations.
     *
     * <p> 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.  </p>
     *
     * @param  cb  The char buffer to be encoded
     *
     * @return  A byte buffer containing the encoded characters
     */
    public final ByteBuffer encode(CharBuffer cb) {
        try {
            return ThreadLocalCoders.encoderFor(this)
                .onMalformedInput(CodingErrorAction.REPLACE)
                .onUnmappableCharacter(CodingErrorAction.REPLACE)
                .encode(cb);
        } catch (CharacterCodingException x) {
            throw new Error(x);         // Can't happen
        }
    }

    
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)); 


Params:
  • str –  The string to be encoded
Returns: A byte buffer containing the encoded characters
/**
     * Convenience method that encodes a string into bytes in this charset.
     *
     * <p> An invocation of this method upon a charset {@code cs} returns the
     * same result as the expression
     *
     * <pre>
     *     cs.encode(CharBuffer.wrap(s)); </pre>
     *
     * @param  str  The string to be encoded
     *
     * @return  A byte buffer containing the encoded characters
     */
    public final ByteBuffer encode(String str) {
        return encode(CharBuffer.wrap(str));
    }

    
Compares this charset to another.

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


Params:
  • that – 
        The charset to which this charset is to be compared
Returns:A negative integer, zero, or a positive integer as this charset
        is less than, equal to, or greater than the specified charset
/**
     * Compares this charset to another.
     *
     * <p> Charsets are ordered by their canonical names, without regard to
     * case. </p>
     *
     * @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
     */
    public final int compareTo(Charset that) {
        return (name().compareToIgnoreCase(that.name()));
    }

    
Computes a hashcode for this charset.

Returns: An integer hashcode
/**
     * Computes a hashcode for this charset.
     *
     * @return  An integer hashcode
     */
    public final int hashCode() {
        return name().hashCode();
    }

    
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.  


Returns: true if, and only if, this charset is equal to the given object
/**
     * Tells whether or not this object is equal to another.
     *
     * <p> 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.  </p>
     *
     * @return  {@code true} if, and only if, this charset is equal to the
     *          given object
     */
    public final boolean equals(Object ob) {
        if (!(ob instanceof Charset))
            return false;
        if (this == ob)
            return true;
        return name.equals(((Charset)ob).name());
    }

    
Returns a string describing this charset.

Returns: A string describing this charset
/**
     * Returns a string describing this charset.
     *
     * @return  A string describing this charset
     */
    public final String toString() {
        return name();
    }

}