xref: /libcore/luni/src/main/java/java/nio/charset/Charset.java

/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

package java.nio.charset;

import com.ibm.icu4jni.charset.NativeConverter;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
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.Collections;
import java.util.Comparator;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Locale;
import java.util.Map;
import java.util.Set;
import java.util.SortedMap;
import java.util.TreeMap;

/**
 * A charset is a named mapping between Unicode characters and byte sequences. Every
 * {@code Charset} can <i>decode</i>, converting a byte sequence into a sequence of characters,
 * and some can also <i>encode</i>, converting a sequence of characters into a byte sequence.
 * Use the method {@link #canEncode} to find out whether a charset supports both.
 *
 * <h4>Characters</h4>
 * <p>In the context of this class, <i>character</i> always refers to a Java character: a Unicode
 * code point in the range U+0000 to U+FFFF. (Java represents supplementary characters using surrogates.)
 * Not all byte sequences will represent a character, and not
 * all characters can necessarily be represented by a given charset. The method {@link #contains}
 * can be used to determine whether every character representable by one charset can also be
 * represented by another (meaning that a lossless transformation is possible from the contained
 * to the container).
 *
 * <h4>Encodings</h4>
 * <p>There are many possible ways to represent Unicode characters as byte sequences.
 * See <a href="http://www.unicode.org/reports/tr17/">UTR#17: Unicode Character Encoding Model</a>
 * for detailed discussion.
 *
 * <p>The most important mappings capable of representing every character are the Unicode
 * Transformation Format (UTF) charsets. Of those, UTF-8 and the UTF-16 family are the most
 * common. UTF-8 (described in <a href="http://www.ietf.org/rfc/rfc3629.txt">RFC 3629</a>)
 * encodes a character using 1 to 4 bytes. UTF-16 uses exactly 2 bytes per character (potentially
 * wasting space, but allowing efficient random access into BMP text), and UTF-32 uses
 * exactly 4 bytes per character (trading off even more space for efficient random access into text
 * that includes supplementary characters).
 *
 * <p>UTF-16 and UTF-32 encode characters directly, using their code point as a two- or four-byte
 * integer. This means that any given UTF-16 or UTF-32 byte sequence is either big- or
 * little-endian. To assist decoders, Unicode includes a special <i>byte order mark</i> (BOM)
 * character U+FEFF used to determine the endianness of a sequence. The corresponding byte-swapped
 * code point U+FFFE is guaranteed never to be assigned. If a UTF-16 decoder sees
 * {@code 0xfe, 0xff}, for example, it knows it's reading a big-endian byte sequence, while
 * {@code 0xff, 0xfe}, would indicate a little-endian byte sequence.
 *
 * <p>UTF-8 can contain a BOM, but since the UTF-8 encoding of a character always uses the same
 * byte sequence, there is no information about endianness to convey. Seeing the bytes
 * corresponding to the UTF-8 encoding of U+FEFF ({@code 0xef, 0xbb, 0xbf}) would only serve to
 * suggest that you're reading UTF-8. Note that BOMs are decoded as the U+FEFF character, and
 * will appear in the output character sequence. This means that a disadvantage to including a BOM
 * in UTF-8 is that most applications that use UTF-8 do not expect to see a BOM. (This is also a
 * reason to prefer UTF-8: it's one less complication to worry about.)
 *
 * <p>Because a BOM indicates how the data that follows should be interpreted, a BOM should occur
 * as the first character in a character sequence.
 *
 * <p>See the <a href="http://unicode.org/faq/utf_bom.html#BOM">Byte Order Mark (BOM) FAQ</a> for
 * more about dealing with BOMs.
 *
 * <h4>Endianness and BOM behavior</h4>
 *
 * <p>The following tables show the endianness and BOM behavior of the UTF-16 variants.
 *
 * <p>This table shows what the encoder writes. "BE" means that the byte sequence is big-endian,
 * "LE" means little-endian. "BE BOM" means a big-endian BOM (that is, {@code 0xfe, 0xff}).
 * <p><table width="100%">
 * <tr> <th>Charset</th>  <th>Encoder writes</th>  </tr>
 * <tr> <td>UTF-16BE</td> <td>BE, no BOM</td>      </tr>
 * <tr> <td>UTF-16LE</td> <td>LE, no BOM</td>      </tr>
 * <tr> <td>UTF-16</td>   <td>BE, with BE BOM</td> </tr>
 * </table>
 *
 * <p>The next table shows how each variant's decoder behaves when reading a byte sequence.
 * The exact meaning of "failure" in the table is dependent on the
 * {@link CodingErrorAction} supplied to {@link CharsetDecoder#malformedInputAction}, so
 * "BE, failure" means "the byte sequence is treated as big-endian, and a little-endian BOM
 * triggers the malformedInputAction".
 *
 * <p>The phrase "includes BOM" means that the output includes the U+FEFF byte order mark character.
 *
 * <p><table width="100%">
 * <tr> <th>Charset</th>  <th>BE BOM</th>           <th>LE BOM</th>           <th>No BOM</th> </tr>
 * <tr> <td>UTF-16BE</td> <td>BE, includes BOM</td> <td>BE, failure</td>      <td>BE</td>     </tr>
 * <tr> <td>UTF-16LE</td> <td>LE, failure</td>      <td>LE, includes BOM</td> <td>LE</td>     </tr>
 * <tr> <td>UTF-16</td>   <td>BE, includes BOM</td> <td>LE, includes BOM</td> <td>BE</td>     </tr>
 * </table>
 *
 * <h4>Charset names</h4>
 * <p>A charset has a canonical name, returned by {@link #name}. Most charsets will
 * also have one or more aliases, returned by {@link #aliases}. A charset can be looked up
 * by canonical name or any of its aliases using {@link #forName}.
 *
 * <h4>Guaranteed-available charsets</h4>
 * <p>The following charsets are available on every Java implementation:
 * <ul>
 * <li>ISO-8859-1
 * <li>US-ASCII
 * <li>UTF-16
 * <li>UTF-16BE
 * <li>UTF-16LE
 * <li>UTF-8
 * </ul>
 * <p>All of these charsets support both decoding and encoding. The charsets whose names begin
 * "UTF" can represent all characters, as mentioned above. The "ISO-8859-1" and "US-ASCII" charsets
 * can only represent small subsets of these characters. Except when required to do otherwise for
 * compatibility, new code should use one of the UTF charsets listed above. The platform's default
 * charset is UTF-8. (This is in contrast to some older implementations, where the default charset
 * depended on the user's locale.)
 *
 * <p>Most implementations will support hundreds of charsets. Use {@link #availableCharsets} or
 * {@link #isSupported} to see what's available. If you intend to use the charset if it's
 * available, just call {@link #forName} and catch the exceptions it throws if the charset isn't
 * available.
 *
 * <p>Additional charsets can be made available by configuring one or more charset
 * providers through provider configuration files. Such files are always named
 * as "java.nio.charset.spi.CharsetProvider" and located in the
 * "META-INF/services" sub folder of one or more classpaths. The files should be
 * encoded in "UTF-8". Each line of their content specifies the class name of a
 * charset provider which extends {@link java.nio.charset.spi.CharsetProvider}.
 * A line should end with '\r', '\n' or '\r\n'. Leading and trailing whitespaces
 * are trimmed. Blank lines, and lines (after trimming) starting with "#" which are
 * regarded as comments, are both ignored. Duplicates of names already found are also
 * ignored. Both the configuration files and the provider classes will be loaded
 * using the thread context class loader.
 *
 * <p>Although class is thread-safe, the {@link CharsetDecoder} and {@link CharsetEncoder} instances
 * it returns are inherently stateful.
 */
public abstract class Charset implements Comparable<Charset> {
    /*
     * The name of configuration files where charset provider class names can be
     * specified.
     */
    private static final String PROVIDER_CONFIGURATION_FILE_NAME = "META-INF/services/java.nio.charset.spi.CharsetProvider";

    /*
     * The encoding of configuration files
     */
    private static final String PROVIDER_CONFIGURATION_FILE_ENCODING = "UTF-8";

    /*
     * The comment string used in configuration files
     */
    private static final String PROVIDER_CONFIGURATION_FILE_COMMENT = "#";

    private static ClassLoader systemClassLoader;

    private static SortedMap<String, Charset> cachedBuiltInCharsets;

    private final String canonicalName;

    // the aliases set
    private final HashSet<String> aliasesSet;

    // cached Charset table
    private final static HashMap<String, Charset> cachedCharsetTable = new HashMap<String, Charset>();

    private static boolean inForNameInternal = false;

    /**
     * Constructs a <code>Charset</code> object. Duplicated aliases are
     * ignored.
     *
     * @param canonicalName
     *            the canonical name of the charset.
     * @param aliases
     *            an array containing all aliases of the charset. May be null.
     * @throws IllegalCharsetNameException
     *             on an illegal value being supplied for either
     *             <code>canonicalName</code> or for any element of
     *             <code>aliases</code>.
     */
    protected Charset(String canonicalName, String[] aliases) {
        if (null == canonicalName) {
            throw new NullPointerException();
        }
        // check whether the given canonical name is legal
        checkCharsetName(canonicalName);
        this.canonicalName = canonicalName;
        // check each alias and put into a set
        this.aliasesSet = new HashSet<String>();
        if (aliases != null) {
            for (int i = 0; i < aliases.length; i++) {
                checkCharsetName(aliases[i]);
                this.aliasesSet.add(aliases[i]);
            }
        }
    }

    /*
     * Checks whether a character is a special character that can be used in
     * charset names, other than letters and digits.
     */
    private static boolean isSpecial(char c) {
        return ('-' == c || '.' == c || ':' == c || '_' == c);
    }

    /*
     * Checks whether a character is a letter (ascii) which are defined in the
     * spec.
     */
    private static boolean isLetter(char c) {
        return ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z');
    }

    /*
     * Checks whether a character is a digit (ascii) which are defined in the
     * spec.
     */
    private static boolean isDigit(char c) {
        return ('0' <= c && c <= '9');
    }

    /*
     * Checks whether a given string is a legal charset name. The argument name
     * should not be null.
     */
    private static void checkCharsetName(String name) {
        // An empty string is illegal charset name
        if (name.length() == 0) {
            throw new IllegalCharsetNameException(name);
        }
        // The first character must be a letter or a digit
        // This is related to HARMONY-68 (won't fix)
        // char first = name.charAt(0);
        // if (!isLetter(first) && !isDigit(first)) {
        // throw new IllegalCharsetNameException(name);
        // }
        // Check the remaining characters
        int length = name.length();
        for (int i = 0; i < length; i++) {
            char c = name.charAt(i);
            if (!isLetter(c) && !isDigit(c) && !isSpecial(c)) {
                throw new IllegalCharsetNameException(name);
            }
        }
    }

    /*
     * Use privileged code to get the context class loader.
     */
    private static ClassLoader getContextClassLoader() {
        final Thread t = Thread.currentThread();
        return AccessController
                .doPrivileged(new PrivilegedAction<ClassLoader>() {
                    public ClassLoader run() {
                        return t.getContextClassLoader();
                    }
                });
    }

    /*
     * Use privileged code to get the system class loader.
     */
    private static void getSystemClassLoader() {
        if (null == systemClassLoader) {
            systemClassLoader = AccessController
                    .doPrivileged(new PrivilegedAction<ClassLoader>() {
                        public ClassLoader run() {
                            return ClassLoader.getSystemClassLoader();
                        }
                    });
        }
    }

    /*
     * Add the charsets supported by the given provider to the map.
     */
    private static void addCharsets(CharsetProvider cp, Map<String, Charset> charsets) {
        Iterator<Charset> it = cp.charsets();
        while (it.hasNext()) {
            Charset cs = it.next();
            // Only new charsets will be added
            if (!charsets.containsKey(cs.name())) {
                charsets.put(cs.name(), cs);
            }
        }
    }

    /*
     * Trim comment string, and then trim white spaces.
     */
    private static String trimClassName(String name) {
        String trimmedName = name;
        int index = name.indexOf(PROVIDER_CONFIGURATION_FILE_COMMENT);
        // Trim comments
        if (index != -1) {
            trimmedName = name.substring(0, index);
        }
        return trimmedName.trim();
    }

    /*
     * Read a configuration file and add the charsets supported by the providers
     * specified by this configuration file to the map.
     */
    private static void loadConfiguredCharsets(URL configFile,
            ClassLoader contextClassLoader, Map<String, Charset> charsets) {
        BufferedReader reader = null;
        try {
            InputStream is = configFile.openStream();
            // Read each line for charset provider class names
            reader = new BufferedReader(new InputStreamReader(is,
                    PROVIDER_CONFIGURATION_FILE_ENCODING));
            String providerClassName = reader.readLine();
            while (null != providerClassName) {
                providerClassName = trimClassName(providerClassName);
                // Skip comments and blank lines
                if (providerClassName.length() > 0) { // Non empty string
                    // Load the charset provider
                    Object cp = null;
                    try {
                        Class<?> c = Class.forName(providerClassName, true,
                                contextClassLoader);
                        cp = c.newInstance();
                    } catch (Exception ex) {
                        // try to use system classloader when context
                        // classloader failed to load config file.
                        try {
                            getSystemClassLoader();
                            Class<?> c = Class.forName(providerClassName, true,
                                    systemClassLoader);
                            cp = c.newInstance();
                        } catch (Exception e) {
                            throw new Error(e.getMessage(), e);
                        }
                    }
                    // Put the charsets supported by this provider into the map
                    addCharsets((CharsetProvider) cp, charsets);
                }
                // Read the next line of the config file
                providerClassName = reader.readLine();
            }
        } catch (IOException ex) {
            // Can't read this configuration file, ignore
        } finally {
            try {
                if (null != reader) {
                    reader.close();
                }
            } catch (IOException ex) {
                // Ignore closing exception
            }
        }
    }

    private static synchronized SortedMap<String, Charset> getCachedBuiltInCharsets() {
        if (cachedBuiltInCharsets == null) {
            cachedBuiltInCharsets = new TreeMap<String, Charset>(String.CASE_INSENSITIVE_ORDER);
            for (String charsetName : NativeConverter.getAvailableCharsetNames()) {
                Charset charset = NativeConverter.charsetForName(charsetName);
                cachedBuiltInCharsets.put(charset.name(), charset);
            }
        }
        return cachedBuiltInCharsets;
    }

    /**
     * Returns an immutable case-insensitive map from canonical names to {@code Charset} instances.
     * If multiple charsets have the same canonical name, it is unspecified which is returned in
     * the map. This method may be slow. If you know which charset you're looking for, use
     * {@link #forName}.
     * @return an immutable case-insensitive map from canonical names to {@code Charset} instances
     */
    @SuppressWarnings("unchecked")
    public static SortedMap<String, Charset> availableCharsets() {
        // Start with a copy of the built-in charsets...
        TreeMap<String, Charset> charsets = new TreeMap<String, Charset>(String.CASE_INSENSITIVE_ORDER);
        charsets.putAll(getCachedBuiltInCharsets());

        // Add all charsets provided by charset providers...
        ClassLoader contextClassLoader = getContextClassLoader();
        Enumeration<URL> e = null;
        try {
            if (contextClassLoader != null) {
                e = contextClassLoader.getResources(PROVIDER_CONFIGURATION_FILE_NAME);
            } else {
                getSystemClassLoader();
                e = systemClassLoader.getResources(PROVIDER_CONFIGURATION_FILE_NAME);
            }
            // Examine each configuration file
            while (e.hasMoreElements()) {
                loadConfiguredCharsets(e.nextElement(), contextClassLoader, charsets);
            }
        } catch (IOException ex) {
            // Unexpected ClassLoader exception, ignore
        }
        return Collections.unmodifiableSortedMap(charsets);
    }

    /*
     * Read a configuration file and try to find the desired charset among those
     * which are supported by the providers specified in this configuration
     * file.
     */
    private static Charset searchConfiguredCharsets(String charsetName,
            ClassLoader contextClassLoader, URL configFile) {
        BufferedReader reader = null;
        try {
            InputStream is = configFile.openStream();
            // Read each line for charset provider class names
            reader = new BufferedReader(new InputStreamReader(is,
                    PROVIDER_CONFIGURATION_FILE_ENCODING));
            String providerClassName = reader.readLine();
            while (null != providerClassName) {
                providerClassName = trimClassName(providerClassName);
                if (providerClassName.length() > 0) { // Non empty string
                    // Load the charset provider
                    Object cp = null;
                    try {
                        Class<?> c = Class.forName(providerClassName, true,
                                contextClassLoader);
                        cp = c.newInstance();
                    } catch (Exception ex) {
                        // try to use system classloader when context
                        // classloader failed to load config file.
                        try {
                            getSystemClassLoader();
                            Class<?> c = Class.forName(providerClassName, true,
                                    systemClassLoader);
                            cp = c.newInstance();
                        } catch (SecurityException e) {
                            // BEGIN android-changed
                            // ignore
                            // END android-changed
                        } catch (Exception e) {
                            throw new Error(e.getMessage(), e);
                        }
                    }
                    // BEGIN android-changed
                    if (cp != null) {
                        // Try to get the desired charset from this provider
                        Charset cs = ((CharsetProvider) cp)
                                .charsetForName(charsetName);
                        if (null != cs) {
                            return cs;
                        }
                    }
                    // END android-changed
                }
                // Read the next line of the config file
                providerClassName = reader.readLine();
            }
            return null;
        } catch (IOException ex) {
            // Can't read this configuration file
            return null;
        } finally {
            try {
                if (null != reader) {
                    reader.close();
                }
            } catch (IOException ex) {
                // Ignore closing exception
            }
        }
    }

    /*
     * Gets a <code>Charset</code> instance for the specified charset name. If
     * the charset is not supported, returns null instead of throwing an
     * exception.
     */
    private synchronized static Charset forNameInternal(String charsetName)
            throws IllegalCharsetNameException {

        Charset cs = lookupCachedOrBuiltInCharset(charsetName);
        if (cs != null || inForNameInternal) {
            return cs;
        }

        // collect all charsets provided by charset providers
        try {
            Enumeration<URL> e = null;
            ClassLoader contextClassLoader = getContextClassLoader();
            if (contextClassLoader != null) {
                e = contextClassLoader.getResources(PROVIDER_CONFIGURATION_FILE_NAME);
            } else {
                getSystemClassLoader();
                if (systemClassLoader == null) {
                    // Non available during class library start-up phase
                    return null;
                } else {
                    e = systemClassLoader.getResources(PROVIDER_CONFIGURATION_FILE_NAME);
                }
            }

            // examine each configuration file
            while (e.hasMoreElements()) {
                inForNameInternal = true;
                cs = searchConfiguredCharsets(charsetName, contextClassLoader, e.nextElement());
                inForNameInternal = false;
                if (cs != null) {
                    cacheCharset(cs);
                    return cs;
                }
            }
        } catch (IOException ex) {
            // Unexpected ClassLoader exception, ignore
        } finally {
            inForNameInternal = false;
        }
        return null;
    }

    private synchronized static Charset lookupCachedOrBuiltInCharset(String charsetName) {
        Charset cs = cachedCharsetTable.get(charsetName);
        if (cs != null) {
            return cs;
        }
        if (charsetName == null) {
            throw new IllegalArgumentException();
        }
        checkCharsetName(charsetName);
        cs = NativeConverter.charsetForName(charsetName);
        if (cs != null) {
            cacheCharset(cs);
        }
        return cs;
    }

    /*
     * save charset into cachedCharsetTable
     */
    private synchronized static void cacheCharset(Charset cs) {
        // Cache the Charset by its canonical name...
        String canonicalName = cs.name();
        if (!cachedCharsetTable.containsKey(canonicalName)) {
            cachedCharsetTable.put(canonicalName, cs);
        }
        // And all its aliases...
        for (String alias : cs.aliasesSet) {
            if (!cachedCharsetTable.containsKey(alias)) {
                cachedCharsetTable.put(alias, cs);
            }
        }
    }

    /**
     * Gets a <code>Charset</code> instance for the specified charset name.
     *
     * @param charsetName
     *            the canonical name of the charset or an alias.
     * @return a <code>Charset</code> instance for the specified charset name.
     * @throws IllegalCharsetNameException
     *             if the specified charset name is illegal.
     * @throws UnsupportedCharsetException
     *             if the desired charset is not supported by this runtime.
     */
    public static Charset forName(String charsetName) {
        Charset c = forNameInternal(charsetName);
        if (c == null) {
            throw new UnsupportedCharsetException(charsetName);
        }
        return c;
    }

    /**
     * Determines whether the specified charset is supported by this runtime.
     *
     * @param charsetName
     *            the name of the charset.
     * @return true if the specified charset is supported, otherwise false.
     * @throws IllegalCharsetNameException
     *             if the specified charset name is illegal.
     */
    public static synchronized boolean isSupported(String charsetName) {
        return forNameInternal(charsetName) != null;
    }

    /**
     * Determines whether this charset is a superset of the given charset. A charset C1 contains
     * charset C2 if every character representable by C2 is also representable by C1. This means
     * that lossless conversion is possible from C2 to C1 (but not necessarily the other way
     * round). It does <i>not</i> imply that the two charsets use the same byte sequences for the
     * characters they share.
     *
     * <p>Note that this method is allowed to be conservative, and some implementations may return
     * false when this charset does contain the other charset. Android's implementation is precise,
     * and will always return true in such cases.
     *
     * @param charset
     *            a given charset.
     * @return true if this charset is a super set of the given charset,
     *         false if it's unknown or this charset is not a superset of
     *         the given charset.
     */
    public abstract boolean contains(Charset charset);

    /**
     * Gets a new instance of an encoder for this charset.
     *
     * @return a new instance of an encoder for this charset.
     */
    public abstract CharsetEncoder newEncoder();

    /**
     * Gets a new instance of a decoder for this charset.
     *
     * @return a new instance of a decoder for this charset.
     */
    public abstract CharsetDecoder newDecoder();

    /**
     * Gets the canonical name of this charset.
     *
     * @return this charset's name in canonical form.
     */
    public final String name() {
        return this.canonicalName;
    }

    /**
     * Gets the set of this charset's aliases.
     *
     * @return an unmodifiable set of this charset's aliases.
     */
    public final Set<String> aliases() {
        return Collections.unmodifiableSet(this.aliasesSet);
    }

    /**
     * Gets the name of this charset for the default locale.
     *
     * <p>The default implementation returns the canonical name of this charset.
     * Subclasses may return a localized display name.
     *
     * @return the name of this charset for the default locale.
     */
    public String displayName() {
        return this.canonicalName;
    }

    /**
     * Gets the name of this charset for the specified locale.
     *
     * <p>The default implementation returns the canonical name of this charset.
     * Subclasses may return a localized display name.
     *
     * @param l
     *            a certain locale
     * @return the name of this charset for the specified locale
     */
    public String displayName(Locale l) {
        return this.canonicalName;
    }

    /**
     * Indicates whether this charset is known to be registered in the IANA
     * Charset Registry.
     *
     * @return true if the charset is known to be registered, otherwise returns
     *         false.
     */
    public final boolean isRegistered() {
        return !canonicalName.startsWith("x-")
                && !canonicalName.startsWith("X-");
    }

    /**
     * Returns true if this charset supports encoding, false otherwise.
     *
     * @return true if this charset supports encoding, false otherwise.
     */
    public boolean canEncode() {
        return true;
    }

    /**
     * Returns a new {@code ByteBuffer} containing the bytes encoding the characters from
     * {@code buffer}.
     * This method uses {@code CodingErrorAction.REPLACE}.
     *
     * <p>Applications should generally create a {@link CharsetEncoder} using {@link #newEncoder}
     * for performance.
     *
     * @param buffer
     *            the character buffer containing the content to be encoded.
     * @return the result of the encoding.
     */
    public final ByteBuffer encode(CharBuffer buffer) {
        try {
            return newEncoder()
                    .onMalformedInput(CodingErrorAction.REPLACE)
                    .onUnmappableCharacter(CodingErrorAction.REPLACE).encode(
                            buffer);
        } catch (CharacterCodingException ex) {
            throw new Error(ex.getMessage(), ex);
        }
    }

    /**
     * Returns a new {@code ByteBuffer} containing the bytes encoding the characters from {@code s}.
     * This method uses {@code CodingErrorAction.REPLACE}.
     *
     * <p>Applications should generally create a {@link CharsetEncoder} using {@link #newEncoder}
     * for performance.
     *
     * @param s the string to be encoded.
     * @return the result of the encoding.
     */
    public final ByteBuffer encode(String s) {
        return encode(CharBuffer.wrap(s));
    }

    /**
     * Returns a new {@code CharBuffer} containing the characters decoded from {@code buffer}.
     * This method uses {@code CodingErrorAction.REPLACE}.
     *
     * <p>Applications should generally create a {@link CharsetDecoder} using {@link #newDecoder}
     * for performance.
     *
     * @param buffer
     *            the byte buffer containing the content to be decoded.
     * @return a character buffer containing the output of the decoding.
     */
    public final CharBuffer decode(ByteBuffer buffer) {
        try {
            return newDecoder()
                    .onMalformedInput(CodingErrorAction.REPLACE)
                    .onUnmappableCharacter(CodingErrorAction.REPLACE).decode(buffer);
        } catch (CharacterCodingException ex) {
            throw new Error(ex.getMessage(), ex);
        }
    }

    /*
     * -------------------------------------------------------------------
     * Methods implementing parent interface Comparable
     * -------------------------------------------------------------------
     */

    /**
     * Compares this charset with the given charset. This comparison is
     * based on the case insensitive canonical names of the charsets.
     *
     * @param charset
     *            the given object to be compared with.
     * @return a negative integer if less than the given object, a positive
     *         integer if larger than it, or 0 if equal to it.
     */
    public final int compareTo(Charset charset) {
        return this.canonicalName.compareToIgnoreCase(charset.canonicalName);
    }

    /*
     * -------------------------------------------------------------------
     * Methods overriding parent class Object
     * -------------------------------------------------------------------
     */

    /**
     * Determines whether this charset equals to the given object. They are
     * considered to be equal if they have the same canonical name.
     *
     * @param obj
     *            the given object to be compared with.
     * @return true if they have the same canonical name, otherwise false.
     */
    @Override
    public final boolean equals(Object obj) {
        if (obj instanceof Charset) {
            Charset that = (Charset) obj;
            return this.canonicalName.equals(that.canonicalName);
        }
        return false;
    }

    /**
     * Gets the hash code of this charset.
     *
     * @return the hash code of this charset.
     */
    @Override
    public final int hashCode() {
        return this.canonicalName.hashCode();
    }

    /**
     * Gets a string representation of this charset. Usually this contains the
     * canonical name of the charset.
     *
     * @return a string representation of this charset.
     */
    @Override
    public final String toString() {
        return "Charset[" + this.canonicalName + "]";
    }

    /**
     * Gets the system default charset from the virtual machine.
     *
     * @return the default charset.
     */
    public static Charset defaultCharset() {
        Charset defaultCharset = null;
        String encoding = AccessController
                .doPrivileged(new PrivilegedAction<String>() {
                    public String run() {
                        return System.getProperty("file.encoding");
                    }
                });
        try {
            defaultCharset = Charset.forName(encoding);
        } catch (UnsupportedCharsetException e) {
            defaultCharset = Charset.forName("UTF-8");
        }
        return defaultCharset;
    }
}