content: /*
   * Copyright (C) 2014 The Android Open Source Project
   * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * 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
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * 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).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  package java.nio.charset;
  
  import com.android.icu.charset.CharsetICU;
  import java.io.UnsupportedEncodingException;
  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.AbstractMap;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.Locale;
  import java.util.Map;
  import java.util.NoSuchElementException;
  import java.util.Set;
  import java.util.ServiceLoader;
  import java.util.ServiceConfigurationError;
  import java.util.SortedMap;
  import java.util.TreeMap;
  import sun.misc.ASCIICaseInsensitiveComparator;
  import sun.nio.cs.ThreadLocalCoders;
  import sun.security.action.GetPropertyAction;
  
  
  // Android-changed: Docs to say UTF-8 is always the platform default charset.
  
  /**
   * 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
   * CharsetProvider} class.
   *
   * <p> All of the methods defined in this class are safe for use by multiple
   * concurrent threads.
   *
   *
   * <a name="names"></a><a name="charenc"></a>
   * <h2>Charset names</h2>
   *
   * <p> Charsets are named by strings composed of the following characters:
   *
   * <ul>
   *
   *   <li> The uppercase letters <tt>'A'</tt> through <tt>'Z'</tt>
   *        (<tt>'&#92;u0041'</tt>&nbsp;through&nbsp;<tt>'&#92;u005a'</tt>),
   *
   *   <li> The lowercase letters <tt>'a'</tt> through <tt>'z'</tt>
   *        (<tt>'&#92;u0061'</tt>&nbsp;through&nbsp;<tt>'&#92;u007a'</tt>),
   *
   *   <li> The digits <tt>'0'</tt> through <tt>'9'</tt>
   *        (<tt>'&#92;u0030'</tt>&nbsp;through&nbsp;<tt>'&#92;u0039'</tt>),
   *
   *   <li> The dash character <tt>'-'</tt>
   *        (<tt>'&#92;u002d'</tt>,&nbsp;<small>HYPHEN-MINUS</small>),
   *
   *   <li> The plus character <tt>'+'</tt>
   *        (<tt>'&#92;u002b'</tt>,&nbsp;<small>PLUS SIGN</small>),
   *
   *   <li> The period character <tt>'.'</tt>
   *        (<tt>'&#92;u002e'</tt>,&nbsp;<small>FULL STOP</small>),
   *
   *   <li> The colon character <tt>':'</tt>
   *        (<tt>'&#92;u003a'</tt>,&nbsp;<small>COLON</small>), and
   *
   *   <li> The underscore character <tt>'_'</tt>
   *        (<tt>'&#92;u005f'</tt>,&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 name="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 <tt>getEncoding()</tt> methods of the
   * {@link java.io.InputStreamReader#getEncoding InputStreamReader} and {@link
   * java.io.OutputStreamWriter#getEncoding OutputStreamWriter} classes.
   *
   * <p><a name="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 <tt>"X-"</tt> or <tt>"x-"</tt>.
   *
   * <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 name="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 width="80%" summary="Description of standard charsets">
   * <tr><th align="left">Charset</th><th align="left">Description</th></tr>
   * <tr><td valign=top><tt>US-ASCII</tt></td>
   *     <td>Seven-bit ASCII, a.k.a. <tt>ISO646-US</tt>,
   *         a.k.a. the Basic Latin block of the Unicode character set</td></tr>
   * <tr><td valign=top><tt>ISO-8859-1&nbsp;&nbsp;</tt></td>
   *     <td>ISO Latin Alphabet No. 1, a.k.a. <tt>ISO-LATIN-1</tt></td></tr>
   * <tr><td valign=top><tt>UTF-8</tt></td>
   *     <td>Eight-bit UCS Transformation Format</td></tr>
   * <tr><td valign=top><tt>UTF-16BE</tt></td>
   *     <td>Sixteen-bit UCS Transformation Format,
   *         big-endian byte&nbsp;order</td></tr>
   * <tr><td valign=top><tt>UTF-16LE</tt></td>
   *     <td>Sixteen-bit UCS Transformation Format,
   *         little-endian byte&nbsp;order</td></tr>
   * <tr><td valign=top><tt>UTF-16</tt></td>
   *     <td>Sixteen-bit UCS Transformation Format,
   *         byte&nbsp;order identified by an optional byte-order mark</td></tr>
   * </table></blockquote>
   *
   * <p> The <tt>UTF-8</tt> 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 <tt>UTF-16</tt> 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 <tt>UTF-16</tt> 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 <tt>'&#92;uFEFF'</tt>.  Byte-order marks are handled
   * as follows:
   *
   * <ul>
   *
   *   <li><p> When decoding, the <tt>UTF-16BE</tt> and <tt>UTF-16LE</tt>
   *   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 <tt>UTF-16</tt> 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>Android note: The Android platform default is always UTF-8.
   *
   * <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 <tt>US-ASCII</tt> is both the
   * name of a coded character set and of the charset that encodes it, while
   * <tt>EUC-JP</tt> 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 CharsetProvider
   * @see Character
   */
  
  public abstract class Charset
      implements Comparable<Charset>
  {
  
      /* -- Static methods -- */
  
      private static volatile String bugLevel = null;
  
      static boolean atBugLevel(String bl) {              // package-private
          String level = bugLevel;
          if (level == null) {
              if (!sun.misc.VM.isBooted())
                  return false;
              bugLevel = level = AccessController.doPrivileged(
                  new GetPropertyAction("sun.nio.cs.bugLevel", ""));
          }
          return level.equals(bl);
      }
  
      /**
       * 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 (!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;
              if (c == '.' && i != 0) continue;
              throw new IllegalCharsetNameException(s);
          }
      }
  
      // Android-removed: We use ICU's list of standard charsets.
      /* The standard set of charsets */
      // private static CharsetProvider standardProvider = new StandardCharsets();
  
      // Cache of the most-recently-returned charsets,
      // along with the names that were used to find them
      //
      // cache1/2 usage is explained in the lookup method
      //
      private static volatile Map.Entry<String, Charset> cache1 = null; // "Level 1" cache
      private static final HashMap<String, Charset> cache2 = new HashMap<>(); // "Level 2" cache
  
      private static void cache(String charsetName, Charset cs) {
          synchronized(cache2) {
              String canonicalName = cs.name();
              Charset canonicalCharset = cache2.get(canonicalName);
  
              if (canonicalCharset != null) {
                  cs = canonicalCharset;
              } else {
                  cache2.put(canonicalName, cs);
  
                  for (String alias : cs.aliases()) {
                      cache2.put(alias, cs);
                  }
              }
  
              cache2.put(charsetName, cs);
          }
  
          cache1 = new AbstractMap.SimpleImmutableEntry<>(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<CharsetProvider>() {
  
                  ServiceLoader<CharsetProvider> sl =
                      ServiceLoader.load(CharsetProvider.class);
                  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 (!sun.misc.VM.isBooted())
              return null;
  
          if (gate.get() != null)
              // Avoid recursive provider lookups
              return null;
          try {
              gate.set(gate);
  
              return AccessController.doPrivileged(
                  new PrivilegedAction<Charset>() {
                      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);
          }
      }
  
      // Android-removed: Remove support for the extended charset provider.
      //
      /* The extended set of charsets */
      // private static Object extendedProviderLock = new Object();
      // private static boolean extendedProviderProbed = false;
      // private static CharsetProvider extendedProvider = null;
      //
      // private static void probeExtendedProvider() {
      //     AccessController.doPrivileged(new PrivilegedAction<Object>() {
      //            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 Charset lookupExtendedCharset(String charsetName) {
      //     CharsetProvider ecp = null;
      //     synchronized (extendedProviderLock) {
      //         if (!extendedProviderProbed) {
      //             probeExtendedProvider();
      //             extendedProviderProbed = true;
      //         }
      //         ecp = extendedProvider;
      //     }
      //     return (ecp != null) ? ecp.charsetForName(charsetName) : null;
      // }
  
      // We expect most programs to use one Charset repeatedly, so the most recently used Charset
      // instance is stored in the level 1 cache. We convey a hint to this effect to the VM by putting
      // the level 1 cache miss code in a separate method. Since charsetName is not necessarily in
      // canonical form, we store the mapping from both the canonical name and the aliases to the
      // instance in a map for level 2 cache.
      private static Charset lookup(String charsetName) {
          if (charsetName == null)
              throw new IllegalArgumentException("Null charset name");
  
  
          final Map.Entry<String, Charset> cached = cache1;
          if (cached != null && charsetName.equals(cached.getKey()))
              return cached.getValue();
          return lookup2(charsetName);
      }
  
      private static Charset lookup2(String charsetName) {
          Charset cs;
          synchronized (cache2) {
              if ((cs = cache2.get(charsetName)) != null) {
                  cache1 = new AbstractMap.SimpleImmutableEntry<>(charsetName, cs);
                  return cs;
              }
          }
  
          // Android-changed: Drop support for "standard" and "extended"
          // providers.
          if ((cs = CharsetICU.charsetForName(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.
       *
       * @param  charsetName
       *         The name of the requested charset; may be either
       *         a canonical name or an alias
       *
       * @return  <tt>true</tt> 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 <tt>charsetName</tt> is null
       */
      public static boolean isSupported(String charsetName) {
          return (lookup(charsetName) != null);
      }
  
      /**
       * 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 <tt>charsetName</tt> 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);
      }
  
      // BEGIN Android-added: forNameUEE(String) method.
      /**
       * Equivalent to {@code forName} but only throws {@code UnsupportedEncodingException},
       * which is all pre-nio code claims to throw.
       *
       * @hide internal use only
       */
      public static Charset forNameUEE(String charsetName) throws UnsupportedEncodingException {
          try {
              return Charset.forName(charsetName);
          } catch (Exception cause) {
              UnsupportedEncodingException ex = new UnsupportedEncodingException(charsetName);
              ex.initCause(cause);
              throw ex;
          }
      }
      // END Android-added: forNameUEE(String) method.
  
      // 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.
       *
       * <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<SortedMap<String,Charset>>() {
                  public SortedMap<String,Charset> run() {
                      TreeMap<String,Charset> m =
                          new TreeMap<String,Charset>(
                              ASCIICaseInsensitiveComparator.CASE_INSENSITIVE_ORDER);
                      for (String charsetName : CharsetICU.getAvailableCharsetNames()) {
                          Charset charset = CharsetICU.charsetForName(charsetName);
                          m.put(charset.name(), charset);
                      }
                      // Android-changed: No more "standard" provider.
                      // 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 Charset defaultCharset;
  
      /**
       * Returns the default charset of this Java virtual machine.
       *
       * <p>Android note: The Android platform default is always UTF-8.
       *
       * @return  A charset object for the default charset
       *
       * @since 1.5
       */
      public static Charset defaultCharset() {
          // Android-changed: Use UTF_8 unconditionally.
          synchronized (Charset.class) {
              if (defaultCharset == null) {
                  defaultCharset = StandardCharsets.UTF_8;
              }
  
              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.
       *
       * @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
       */
      @libcore.api.IntraCoreApi
      protected Charset(String canonicalName, String[] aliases) {
          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;
      }
  
      /**
       * 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.
       *
       * @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<String>(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.
       *
       * <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 <a
       * href="http://www.iana.org/assignments/character-sets">IANA Charset
       * Registry</a>.
       *
       * @return  <tt>true</tt> 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.
       *
       * <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.
       *
       * <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 <tt>true</tt> then the given charset is known to be
       * contained by this charset; if it returns <tt>false</tt>, however, then
       * it is not necessarily the case that the given charset is not contained
       * in this charset.
       *
       * @param   cs
       *          The given charset
       *
       * @return  <tt>true</tt> if the given charset is contained in this charset
       */
      public abstract boolean contains(Charset cs);
  
      /**
       * 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.
       *
       * @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.
       *
       * <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
       * <tt>false</tt>. </p>
       *
       * @return  <tt>true</tt> if, and only if, this charset supports encoding
       */
      public boolean canEncode() {
          return true;
      }
  
      /**
       * Convenience method that decodes bytes in this charset into Unicode
       * characters.
       *
       * <p> An invocation of this method upon a charset <tt>cs</tt> 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(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.
       *
       * <p> An invocation of this method upon a charset <tt>cs</tt> 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(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.
       *
       * <p> An invocation of this method upon a charset <tt>cs</tt> 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.
       *
       * <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.
       *
       * @return  An integer hashcode
       */
      public final int hashCode() {
          return name().hashCode();
      }
  
      /**
       * 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  <tt>true</tt> 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.
       *
       * @return  A string describing this charset
       */
      public final String toString() {
          return name();
      }
  
  }
  