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.util.prefs;
  
  import java.util.*;
  import java.io.*;
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  // These imports needed only as a workaround for a JavaDoc bug
  import java.lang.Integer;
  import java.lang.Long;
  import java.lang.Float;
  import java.lang.Double;
  
  /**
   * This class provides a skeletal implementation of the {@link Preferences}
   * class, greatly easing the task of implementing it.
   *
   * <p><strong>This class is for <tt>Preferences</tt> implementers only.
   * Normal users of the <tt>Preferences</tt> facility should have no need to
   * consult this documentation.  The {@link Preferences} documentation
   * should suffice.</strong>
   *
   * <p>Implementors must override the nine abstract service-provider interface
   * (SPI) methods: {@link #getSpi(String)}, {@link #putSpi(String,String)},
   * {@link #removeSpi(String)}, {@link #childSpi(String)}, {@link
   * #removeNodeSpi()}, {@link #keysSpi()}, {@link #childrenNamesSpi()}, {@link
   * #syncSpi()} and {@link #flushSpi()}.  All of the concrete methods specify
   * precisely how they are implemented atop these SPI methods.  The implementor
   * may, at his discretion, override one or more of the concrete methods if the
   * default implementation is unsatisfactory for any reason, such as
   * performance.
   *
   * <p>The SPI methods fall into three groups concerning exception
   * behavior. The <tt>getSpi</tt> method should never throw exceptions, but it
   * doesn't really matter, as any exception thrown by this method will be
   * intercepted by {@link #get(String,String)}, which will return the specified
   * default value to the caller.  The <tt>removeNodeSpi, keysSpi,
   * childrenNamesSpi, syncSpi</tt> and <tt>flushSpi</tt> methods are specified
   * to throw {@link BackingStoreException}, and the implementation is required
   * to throw this checked exception if it is unable to perform the operation.
   * The exception propagates outward, causing the corresponding API method
   * to fail.
   *
   * <p>The remaining SPI methods {@link #putSpi(String,String)}, {@link
   * #removeSpi(String)} and {@link #childSpi(String)} have more complicated
   * exception behavior.  They are not specified to throw
   * <tt>BackingStoreException</tt>, as they can generally obey their contracts
   * even if the backing store is unavailable.  This is true because they return
   * no information and their effects are not required to become permanent until
   * a subsequent call to {@link Preferences#flush()} or
   * {@link Preferences#sync()}. Generally speaking, these SPI methods should not
   * throw exceptions.  In some implementations, there may be circumstances
   * under which these calls cannot even enqueue the requested operation for
   * later processing.  Even under these circumstances it is generally better to
   * simply ignore the invocation and return, rather than throwing an
   * exception.  Under these circumstances, however, all subsequent invocations
   * of <tt>flush()</tt> and <tt>sync</tt> should return <tt>false</tt>, as
   * returning <tt>true</tt> would imply that all previous operations had
   * successfully been made permanent.
   *
   * <p>There is one circumstance under which <tt>putSpi, removeSpi and
   * childSpi</tt> <i>should</i> throw an exception: if the caller lacks
   * sufficient privileges on the underlying operating system to perform the
   * requested operation.  This will, for instance, occur on most systems
   * if a non-privileged user attempts to modify system preferences.
   * (The required privileges will vary from implementation to
   * implementation.  On some implementations, they are the right to modify the
   * contents of some directory in the file system; on others they are the right
   * to modify contents of some key in a registry.)  Under any of these
   * circumstances, it would generally be undesirable to let the program
   * continue executing as if these operations would become permanent at a later
   * time.  While implementations are not required to throw an exception under
   * these circumstances, they are encouraged to do so.  A {@link
   * SecurityException} would be appropriate.
   *
   * <p>Most of the SPI methods require the implementation to read or write
   * information at a preferences node.  The implementor should beware of the
   * fact that another VM may have concurrently deleted this node from the
   * backing store.  It is the implementation's responsibility to recreate the
   * node if it has been deleted.
   *
   * <p>Implementation note: In Sun's default <tt>Preferences</tt>
   * implementations, the user's identity is inherited from the underlying
   * operating system and does not change for the lifetime of the virtual
   * machine.  It is recognized that server-side <tt>Preferences</tt>
   * implementations may have the user identity change from request to request,
   * implicitly passed to <tt>Preferences</tt> methods via the use of a
   * static {@link ThreadLocal} instance.  Authors of such implementations are
   * <i>strongly</i> encouraged to determine the user at the time preferences
   * are accessed (for example by the {@link #get(String,String)} or {@link
   * #put(String,String)} method) rather than permanently associating a user
   * with each <tt>Preferences</tt> instance.  The latter behavior conflicts
   * with normal <tt>Preferences</tt> usage and would lead to great confusion.
   *
   * @author  Josh Bloch
   * @see     Preferences
   * @since   1.4
   */
  public abstract class AbstractPreferences extends Preferences {
      /**
       * Our name relative to parent.
       */
      private final String name;
  
      /**
       * Our absolute path name.
       */
      private final String absolutePath;
  
      /**
       * Our parent node.
       */
      final AbstractPreferences parent;
  
      /**
       * Our root node.
       */
      private final AbstractPreferences root; // Relative to this node
  
      /**
       * This field should be <tt>true</tt> if this node did not exist in the
       * backing store prior to the creation of this object.  The field
       * is initialized to false, but may be set to true by a subclass
       * constructor (and should not be modified thereafter).  This field
       * indicates whether a node change event should be fired when
       * creation is complete.
       */
      protected boolean newNode = false;
  
      /**
       * All known unremoved children of this node.  (This "cache" is consulted
       * prior to calling childSpi() or getChild().
       */
      private Map<String, AbstractPreferences> kidCache = new HashMap<>();
  
      /**
       * This field is used to keep track of whether or not this node has
       * been removed.  Once it's set to true, it will never be reset to false.
       */
      private boolean removed = false;
  
      /**
       * Registered preference change listeners.
       */
      // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
      // It's not clear if this change provides overall benefit; it might be
      // reverted in future. Discussion: http://b/111195881
      // private PreferenceChangeListener[] prefListeners =
      //     new PreferenceChangeListener[0];
      private final ArrayList<PreferenceChangeListener> prefListeners = new ArrayList<>();
  
      /**
       * Registered node change listeners.
       */
      // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
      // private NodeChangeListener[] nodeListeners = new NodeChangeListener[0];
      private final ArrayList<NodeChangeListener> nodeListeners = new ArrayList<>();
  
      /**
       * An object whose monitor is used to lock this node.  This object
       * is used in preference to the node itself to reduce the likelihood of
       * intentional or unintentional denial of service due to a locked node.
       * To avoid deadlock, a node is <i>never</i> locked by a thread that
       * holds a lock on a descendant of that node.
       */
      protected final Object lock = new Object();
  
      /**
       * Creates a preference node with the specified parent and the specified
       * name relative to its parent.
       *
       * @param parent the parent of this preference node, or null if this
       *               is the root.
       * @param name the name of this preference node, relative to its parent,
       *             or <tt>""</tt> if this is the root.
       * @throws IllegalArgumentException if <tt>name</tt> contains a slash
       *          (<tt>'/'</tt>),  or <tt>parent</tt> is <tt>null</tt> and
       *          name isn't <tt>""</tt>.
       */
      protected AbstractPreferences(AbstractPreferences parent, String name) {
          if (parent==null) {
              if (!name.equals(""))
                  throw new IllegalArgumentException("Root name '"+name+
                                                     "' must be \"\"");
              this.absolutePath = "/";
              root = this;
          } else {
              if (name.indexOf('/') != -1)
                  throw new IllegalArgumentException("Name '" + name +
                                                   "' contains '/'");
              if (name.equals(""))
                throw new IllegalArgumentException("Illegal name: empty string");
  
              root = parent.root;
              absolutePath = (parent==root ? "/" + name
                                           : parent.absolutePath() + "/" + name);
          }
          this.name = name;
          this.parent = parent;
      }
  
      /**
       * Implements the <tt>put</tt> method as per the specification in
       * {@link Preferences#put(String,String)}.
       *
       * <p>This implementation checks that the key and value are legal,
       * obtains this preference node's lock, checks that the node
       * has not been removed, invokes {@link #putSpi(String,String)}, and if
       * there are any preference change listeners, enqueues a notification
       * event for processing by the event dispatch thread.
       *
       * @param key key with which the specified value is to be associated.
       * @param value value to be associated with the specified key.
       * @throws NullPointerException if key or value is <tt>null</tt>.
       * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
       *       <tt>MAX_KEY_LENGTH</tt> or if <tt>value.length</tt> exceeds
       *       <tt>MAX_VALUE_LENGTH</tt>.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public void put(String key, String value) {
          if (key==null || value==null)
              throw new NullPointerException();
          if (key.length() > MAX_KEY_LENGTH)
              throw new IllegalArgumentException("Key too long: "+key);
          if (value.length() > MAX_VALUE_LENGTH)
              throw new IllegalArgumentException("Value too long: "+value);
  
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
  
              putSpi(key, value);
              enqueuePreferenceChangeEvent(key, value);
          }
      }
  
      /**
       * Implements the <tt>get</tt> method as per the specification in
       * {@link Preferences#get(String,String)}.
       *
       * <p>This implementation first checks to see if <tt>key</tt> is
       * <tt>null</tt> throwing a <tt>NullPointerException</tt> if this is
       * the case.  Then it obtains this preference node's lock,
       * checks that the node has not been removed, invokes {@link
       * #getSpi(String)}, and returns the result, unless the <tt>getSpi</tt>
       * invocation returns <tt>null</tt> or throws an exception, in which case
       * this invocation returns <tt>def</tt>.
       *
       * @param key key whose associated value is to be returned.
       * @param def the value to be returned in the event that this
       *        preference node has no value associated with <tt>key</tt>.
       * @return the value associated with <tt>key</tt>, or <tt>def</tt>
       *         if no value is associated with <tt>key</tt>.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @throws NullPointerException if key is <tt>null</tt>.  (A
       *         <tt>null</tt> default <i>is</i> permitted.)
       */
      public String get(String key, String def) {
          if (key==null)
              throw new NullPointerException("Null key");
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
  
              String result = null;
              try {
                  result = getSpi(key);
              } catch (Exception e) {
                  // Ignoring exception causes default to be returned
              }
              return (result==null ? def : result);
          }
      }
  
      /**
       * Implements the <tt>remove(String)</tt> method as per the specification
       * in {@link Preferences#remove(String)}.
       *
       * <p>This implementation obtains this preference node's lock,
       * checks that the node has not been removed, invokes
       * {@link #removeSpi(String)} and if there are any preference
       * change listeners, enqueues a notification event for processing by the
       * event dispatch thread.
       *
       * @param key key whose mapping is to be removed from the preference node.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @throws NullPointerException {@inheritDoc}.
       */
      public void remove(String key) {
          Objects.requireNonNull(key, "Specified key cannot be null");
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
  
              removeSpi(key);
              enqueuePreferenceChangeEvent(key, null);
          }
      }
  
      /**
       * Implements the <tt>clear</tt> method as per the specification in
       * {@link Preferences#clear()}.
       *
       * <p>This implementation obtains this preference node's lock,
       * invokes {@link #keys()} to obtain an array of keys, and
       * iterates over the array invoking {@link #remove(String)} on each key.
       *
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public void clear() throws BackingStoreException {
          synchronized(lock) {
              String[] keys = keys();
              for (int i=0; i<keys.length; i++)
                  remove(keys[i]);
          }
      }
  
      /**
       * Implements the <tt>putInt</tt> method as per the specification in
       * {@link Preferences#putInt(String,int)}.
       *
       * <p>This implementation translates <tt>value</tt> to a string with
       * {@link Integer#toString(int)} and invokes {@link #put(String,String)}
       * on the result.
       *
       * @param key key with which the string form of value is to be associated.
       * @param value value whose string form is to be associated with key.
       * @throws NullPointerException if key is <tt>null</tt>.
       * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
       *         <tt>MAX_KEY_LENGTH</tt>.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public void putInt(String key, int value) {
          put(key, Integer.toString(value));
      }
  
      /**
       * Implements the <tt>getInt</tt> method as per the specification in
       * {@link Preferences#getInt(String,int)}.
       *
       * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
       * null)</tt>}.  If the return value is non-null, the implementation
       * attempts to translate it to an <tt>int</tt> with
       * {@link Integer#parseInt(String)}.  If the attempt succeeds, the return
       * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
       *
       * @param key key whose associated value is to be returned as an int.
       * @param def the value to be returned in the event that this
       *        preference node has no value associated with <tt>key</tt>
       *        or the associated value cannot be interpreted as an int.
       * @return the int value represented by the string associated with
       *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
       *         associated value does not exist or cannot be interpreted as
       *         an int.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
       */
      public int getInt(String key, int def) {
          int result = def;
          try {
              String value = get(key, null);
              if (value != null)
                  result = Integer.parseInt(value);
          } catch (NumberFormatException e) {
              // Ignoring exception causes specified default to be returned
          }
  
          return result;
      }
  
      /**
       * Implements the <tt>putLong</tt> method as per the specification in
       * {@link Preferences#putLong(String,long)}.
       *
       * <p>This implementation translates <tt>value</tt> to a string with
       * {@link Long#toString(long)} and invokes {@link #put(String,String)}
       * on the result.
       *
       * @param key key with which the string form of value is to be associated.
       * @param value value whose string form is to be associated with key.
       * @throws NullPointerException if key is <tt>null</tt>.
       * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
       *         <tt>MAX_KEY_LENGTH</tt>.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public void putLong(String key, long value) {
          put(key, Long.toString(value));
      }
  
      /**
       * Implements the <tt>getLong</tt> method as per the specification in
       * {@link Preferences#getLong(String,long)}.
       *
       * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
       * null)</tt>}.  If the return value is non-null, the implementation
       * attempts to translate it to a <tt>long</tt> with
       * {@link Long#parseLong(String)}.  If the attempt succeeds, the return
       * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
       *
       * @param key key whose associated value is to be returned as a long.
       * @param def the value to be returned in the event that this
       *        preference node has no value associated with <tt>key</tt>
       *        or the associated value cannot be interpreted as a long.
       * @return the long value represented by the string associated with
       *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
       *         associated value does not exist or cannot be interpreted as
       *         a long.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
       */
      public long getLong(String key, long def) {
          long result = def;
          try {
              String value = get(key, null);
              if (value != null)
                  result = Long.parseLong(value);
          } catch (NumberFormatException e) {
              // Ignoring exception causes specified default to be returned
          }
  
          return result;
      }
  
      /**
       * Implements the <tt>putBoolean</tt> method as per the specification in
       * {@link Preferences#putBoolean(String,boolean)}.
       *
       * <p>This implementation translates <tt>value</tt> to a string with
       * {@link String#valueOf(boolean)} and invokes {@link #put(String,String)}
       * on the result.
       *
       * @param key key with which the string form of value is to be associated.
       * @param value value whose string form is to be associated with key.
       * @throws NullPointerException if key is <tt>null</tt>.
       * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
       *         <tt>MAX_KEY_LENGTH</tt>.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public void putBoolean(String key, boolean value) {
          put(key, String.valueOf(value));
      }
  
      /**
       * Implements the <tt>getBoolean</tt> method as per the specification in
       * {@link Preferences#getBoolean(String,boolean)}.
       *
       * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
       * null)</tt>}.  If the return value is non-null, it is compared with
       * <tt>"true"</tt> using {@link String#equalsIgnoreCase(String)}.  If the
       * comparison returns <tt>true</tt>, this invocation returns
       * <tt>true</tt>.  Otherwise, the original return value is compared with
       * <tt>"false"</tt>, again using {@link String#equalsIgnoreCase(String)}.
       * If the comparison returns <tt>true</tt>, this invocation returns
       * <tt>false</tt>.  Otherwise, this invocation returns <tt>def</tt>.
       *
       * @param key key whose associated value is to be returned as a boolean.
       * @param def the value to be returned in the event that this
       *        preference node has no value associated with <tt>key</tt>
       *        or the associated value cannot be interpreted as a boolean.
       * @return the boolean value represented by the string associated with
       *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
       *         associated value does not exist or cannot be interpreted as
       *         a boolean.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
       */
      public boolean getBoolean(String key, boolean def) {
          boolean result = def;
          String value = get(key, null);
          if (value != null) {
              if (value.equalsIgnoreCase("true"))
                  result = true;
              else if (value.equalsIgnoreCase("false"))
                  result = false;
          }
  
          return result;
      }
  
      /**
       * Implements the <tt>putFloat</tt> method as per the specification in
       * {@link Preferences#putFloat(String,float)}.
       *
       * <p>This implementation translates <tt>value</tt> to a string with
       * {@link Float#toString(float)} and invokes {@link #put(String,String)}
       * on the result.
       *
       * @param key key with which the string form of value is to be associated.
       * @param value value whose string form is to be associated with key.
       * @throws NullPointerException if key is <tt>null</tt>.
       * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
       *         <tt>MAX_KEY_LENGTH</tt>.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public void putFloat(String key, float value) {
          put(key, Float.toString(value));
      }
  
      /**
       * Implements the <tt>getFloat</tt> method as per the specification in
       * {@link Preferences#getFloat(String,float)}.
       *
       * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
       * null)</tt>}.  If the return value is non-null, the implementation
       * attempts to translate it to an <tt>float</tt> with
       * {@link Float#parseFloat(String)}.  If the attempt succeeds, the return
       * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
       *
       * @param key key whose associated value is to be returned as a float.
       * @param def the value to be returned in the event that this
       *        preference node has no value associated with <tt>key</tt>
       *        or the associated value cannot be interpreted as a float.
       * @return the float value represented by the string associated with
       *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
       *         associated value does not exist or cannot be interpreted as
       *         a float.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
       */
      public float getFloat(String key, float def) {
          float result = def;
          try {
              String value = get(key, null);
              if (value != null)
                  result = Float.parseFloat(value);
          } catch (NumberFormatException e) {
              // Ignoring exception causes specified default to be returned
          }
  
          return result;
      }
  
      /**
       * Implements the <tt>putDouble</tt> method as per the specification in
       * {@link Preferences#putDouble(String,double)}.
       *
       * <p>This implementation translates <tt>value</tt> to a string with
       * {@link Double#toString(double)} and invokes {@link #put(String,String)}
       * on the result.
       *
       * @param key key with which the string form of value is to be associated.
       * @param value value whose string form is to be associated with key.
       * @throws NullPointerException if key is <tt>null</tt>.
       * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
       *         <tt>MAX_KEY_LENGTH</tt>.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public void putDouble(String key, double value) {
          put(key, Double.toString(value));
      }
  
      /**
       * Implements the <tt>getDouble</tt> method as per the specification in
       * {@link Preferences#getDouble(String,double)}.
       *
       * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
       * null)</tt>}.  If the return value is non-null, the implementation
       * attempts to translate it to an <tt>double</tt> with
       * {@link Double#parseDouble(String)}.  If the attempt succeeds, the return
       * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
       *
       * @param key key whose associated value is to be returned as a double.
       * @param def the value to be returned in the event that this
       *        preference node has no value associated with <tt>key</tt>
       *        or the associated value cannot be interpreted as a double.
       * @return the double value represented by the string associated with
       *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
       *         associated value does not exist or cannot be interpreted as
       *         a double.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
       */
      public double getDouble(String key, double def) {
          double result = def;
          try {
              String value = get(key, null);
              if (value != null)
                  result = Double.parseDouble(value);
          } catch (NumberFormatException e) {
              // Ignoring exception causes specified default to be returned
          }
  
          return result;
      }
  
      /**
       * Implements the <tt>putByteArray</tt> method as per the specification in
       * {@link Preferences#putByteArray(String,byte[])}.
       *
       * @param key key with which the string form of value is to be associated.
       * @param value value whose string form is to be associated with key.
       * @throws NullPointerException if key or value is <tt>null</tt>.
       * @throws IllegalArgumentException if key.length() exceeds MAX_KEY_LENGTH
       *         or if value.length exceeds MAX_VALUE_LENGTH*3/4.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public void putByteArray(String key, byte[] value) {
          put(key, Base64.byteArrayToBase64(value));
      }
  
      /**
       * Implements the <tt>getByteArray</tt> method as per the specification in
       * {@link Preferences#getByteArray(String,byte[])}.
       *
       * @param key key whose associated value is to be returned as a byte array.
       * @param def the value to be returned in the event that this
       *        preference node has no value associated with <tt>key</tt>
       *        or the associated value cannot be interpreted as a byte array.
       * @return the byte array value represented by the string associated with
       *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
       *         associated value does not exist or cannot be interpreted as
       *         a byte array.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.  (A
       *         <tt>null</tt> value for <tt>def</tt> <i>is</i> permitted.)
       */
      public byte[] getByteArray(String key, byte[] def) {
          byte[] result = def;
          String value = get(key, null);
          try {
              if (value != null)
                  result = Base64.base64ToByteArray(value);
          }
          catch (RuntimeException e) {
              // Ignoring exception causes specified default to be returned
          }
  
          return result;
      }
  
      /**
       * Implements the <tt>keys</tt> method as per the specification in
       * {@link Preferences#keys()}.
       *
       * <p>This implementation obtains this preference node's lock, checks that
       * the node has not been removed and invokes {@link #keysSpi()}.
       *
       * @return an array of the keys that have an associated value in this
       *         preference node.
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public String[] keys() throws BackingStoreException {
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
  
              return keysSpi();
          }
      }
  
      /**
       * Implements the <tt>children</tt> method as per the specification in
       * {@link Preferences#childrenNames()}.
       *
       * <p>This implementation obtains this preference node's lock, checks that
       * the node has not been removed, constructs a <tt>TreeSet</tt> initialized
       * to the names of children already cached (the children in this node's
       * "child-cache"), invokes {@link #childrenNamesSpi()}, and adds all of the
       * returned child-names into the set.  The elements of the tree set are
       * dumped into a <tt>String</tt> array using the <tt>toArray</tt> method,
       * and this array is returned.
       *
       * @return the names of the children of this preference node.
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @see #cachedChildren()
       */
      public String[] childrenNames() throws BackingStoreException {
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
  
              Set<String> s = new TreeSet<>(kidCache.keySet());
              for (String kid : childrenNamesSpi())
                  s.add(kid);
              return s.toArray(EMPTY_STRING_ARRAY);
          }
      }
  
      private static final String[] EMPTY_STRING_ARRAY = new String[0];
  
      /**
       * Returns all known unremoved children of this node.
       *
       * @return all known unremoved children of this node.
       */
      protected final AbstractPreferences[] cachedChildren() {
          return kidCache.values().toArray(EMPTY_ABSTRACT_PREFS_ARRAY);
      }
  
      private static final AbstractPreferences[] EMPTY_ABSTRACT_PREFS_ARRAY
          = new AbstractPreferences[0];
  
      /**
       * Implements the <tt>parent</tt> method as per the specification in
       * {@link Preferences#parent()}.
       *
       * <p>This implementation obtains this preference node's lock, checks that
       * the node has not been removed and returns the parent value that was
       * passed to this node's constructor.
       *
       * @return the parent of this preference node.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public Preferences parent() {
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
  
              return parent;
          }
      }
  
      /**
       * Implements the <tt>node</tt> method as per the specification in
       * {@link Preferences#node(String)}.
       *
       * <p>This implementation obtains this preference node's lock and checks
       * that the node has not been removed.  If <tt>path</tt> is <tt>""</tt>,
       * this node is returned; if <tt>path</tt> is <tt>"/"</tt>, this node's
       * root is returned.  If the first character in <tt>path</tt> is
       * not <tt>'/'</tt>, the implementation breaks <tt>path</tt> into
       * tokens and recursively traverses the path from this node to the
       * named node, "consuming" a name and a slash from <tt>path</tt> at
       * each step of the traversal.  At each step, the current node is locked
       * and the node's child-cache is checked for the named node.  If it is
       * not found, the name is checked to make sure its length does not
       * exceed <tt>MAX_NAME_LENGTH</tt>.  Then the {@link #childSpi(String)}
       * method is invoked, and the result stored in this node's child-cache.
       * If the newly created <tt>Preferences</tt> object's {@link #newNode}
       * field is <tt>true</tt> and there are any node change listeners,
       * a notification event is enqueued for processing by the event dispatch
       * thread.
       *
       * <p>When there are no more tokens, the last value found in the
       * child-cache or returned by <tt>childSpi</tt> is returned by this
       * method.  If during the traversal, two <tt>"/"</tt> tokens occur
       * consecutively, or the final token is <tt>"/"</tt> (rather than a name),
       * an appropriate <tt>IllegalArgumentException</tt> is thrown.
       *
       * <p> If the first character of <tt>path</tt> is <tt>'/'</tt>
       * (indicating an absolute path name) this preference node's
       * lock is dropped prior to breaking <tt>path</tt> into tokens, and
       * this method recursively traverses the path starting from the root
       * (rather than starting from this node).  The traversal is otherwise
       * identical to the one described for relative path names.  Dropping
       * the lock on this node prior to commencing the traversal at the root
       * node is essential to avoid the possibility of deadlock, as per the
       * {@link #lock locking invariant}.
       *
       * @param path the path name of the preference node to return.
       * @return the specified preference node.
       * @throws IllegalArgumentException if the path name is invalid (i.e.,
       *         it contains multiple consecutive slash characters, or ends
       *         with a slash character and is more than one character long).
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       */
      public Preferences node(String path) {
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
              if (path.equals(""))
                  return this;
              if (path.equals("/"))
                  return root;
              if (path.charAt(0) != '/')
                  return node(new StringTokenizer(path, "/", true));
          }
  
          // Absolute path.  Note that we've dropped our lock to avoid deadlock
          return root.node(new StringTokenizer(path.substring(1), "/", true));
      }
  
      /**
       * tokenizer contains <name> {'/' <name>}*
       */
      private Preferences node(StringTokenizer path) {
          String token = path.nextToken();
          if (token.equals("/"))  // Check for consecutive slashes
              throw new IllegalArgumentException("Consecutive slashes in path");
          synchronized(lock) {
              AbstractPreferences child = kidCache.get(token);
              if (child == null) {
                  if (token.length() > MAX_NAME_LENGTH)
                      throw new IllegalArgumentException(
                          "Node name " + token + " too long");
                  child = childSpi(token);
                  if (child.newNode)
                      enqueueNodeAddedEvent(child);
                  kidCache.put(token, child);
              }
              if (!path.hasMoreTokens())
                  return child;
              path.nextToken();  // Consume slash
              if (!path.hasMoreTokens())
                  throw new IllegalArgumentException("Path ends with slash");
              return child.node(path);
          }
      }
  
      /**
       * Implements the <tt>nodeExists</tt> method as per the specification in
       * {@link Preferences#nodeExists(String)}.
       *
       * <p>This implementation is very similar to {@link #node(String)},
       * except that {@link #getChild(String)} is used instead of {@link
       * #childSpi(String)}.
       *
       * @param path the path name of the node whose existence is to be checked.
       * @return true if the specified node exists.
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       * @throws IllegalArgumentException if the path name is invalid (i.e.,
       *         it contains multiple consecutive slash characters, or ends
       *         with a slash character and is more than one character long).
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method and
       *         <tt>pathname</tt> is not the empty string (<tt>""</tt>).
       */
      public boolean nodeExists(String path)
          throws BackingStoreException
      {
          synchronized(lock) {
              if (path.equals(""))
                  return !removed;
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
              if (path.equals("/"))
                  return true;
              if (path.charAt(0) != '/')
                  return nodeExists(new StringTokenizer(path, "/", true));
          }
  
          // Absolute path.  Note that we've dropped our lock to avoid deadlock
          return root.nodeExists(new StringTokenizer(path.substring(1), "/",
                                                     true));
      }
  
      /**
       * tokenizer contains <name> {'/' <name>}*
       */
      private boolean nodeExists(StringTokenizer path)
          throws BackingStoreException
      {
          String token = path.nextToken();
          if (token.equals("/"))  // Check for consecutive slashes
              throw new IllegalArgumentException("Consecutive slashes in path");
          synchronized(lock) {
              AbstractPreferences child = kidCache.get(token);
              if (child == null)
                  child = getChild(token);
              if (child==null)
                  return false;
              if (!path.hasMoreTokens())
                  return true;
              path.nextToken();  // Consume slash
              if (!path.hasMoreTokens())
                  throw new IllegalArgumentException("Path ends with slash");
              return child.nodeExists(path);
          }
      }
  
      /**
  
       * Implements the <tt>removeNode()</tt> method as per the specification in
       * {@link Preferences#removeNode()}.
       *
       * <p>This implementation checks to see that this node is the root; if so,
       * it throws an appropriate exception.  Then, it locks this node's parent,
       * and calls a recursive helper method that traverses the subtree rooted at
       * this node.  The recursive method locks the node on which it was called,
       * checks that it has not already been removed, and then ensures that all
       * of its children are cached: The {@link #childrenNamesSpi()} method is
       * invoked and each returned child name is checked for containment in the
       * child-cache.  If a child is not already cached, the {@link
       * #childSpi(String)} method is invoked to create a <tt>Preferences</tt>
       * instance for it, and this instance is put into the child-cache.  Then
       * the helper method calls itself recursively on each node contained in its
       * child-cache.  Next, it invokes {@link #removeNodeSpi()}, marks itself
       * as removed, and removes itself from its parent's child-cache.  Finally,
       * if there are any node change listeners, it enqueues a notification
       * event for processing by the event dispatch thread.
       *
       * <p>Note that the helper method is always invoked with all ancestors up
       * to the "closest non-removed ancestor" locked.
       *
       * @throws IllegalStateException if this node (or an ancestor) has already
       *         been removed with the {@link #removeNode()} method.
       * @throws UnsupportedOperationException if this method is invoked on
       *         the root node.
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       */
      public void removeNode() throws BackingStoreException {
          if (this==root)
              throw new UnsupportedOperationException("Can't remove the root!");
          synchronized(parent.lock) {
              removeNode2();
              parent.kidCache.remove(name);
          }
      }
  
      /*
       * Called with locks on all nodes on path from parent of "removal root"
       * to this (including the former but excluding the latter).
       */
      private void removeNode2() throws BackingStoreException {
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node already removed.");
  
              // Ensure that all children are cached
              String[] kidNames = childrenNamesSpi();
              for (int i=0; i<kidNames.length; i++)
                  if (!kidCache.containsKey(kidNames[i]))
                      kidCache.put(kidNames[i], childSpi(kidNames[i]));
  
              // Recursively remove all cached children
              for (Iterator<AbstractPreferences> i = kidCache.values().iterator();
                   i.hasNext();) {
                  try {
                      i.next().removeNode2();
                      i.remove();
                  } catch (BackingStoreException x) { }
              }
  
              // Now we have no descendants - it's time to die!
              removeNodeSpi();
              removed = true;
              parent.enqueueNodeRemovedEvent(this);
          }
      }
  
      /**
       * Implements the <tt>name</tt> method as per the specification in
       * {@link Preferences#name()}.
       *
       * <p>This implementation merely returns the name that was
       * passed to this node's constructor.
       *
       * @return this preference node's name, relative to its parent.
       */
      public String name() {
          return name;
      }
  
      /**
       * Implements the <tt>absolutePath</tt> method as per the specification in
       * {@link Preferences#absolutePath()}.
       *
       * <p>This implementation merely returns the absolute path name that
       * was computed at the time that this node was constructed (based on
       * the name that was passed to this node's constructor, and the names
       * that were passed to this node's ancestors' constructors).
       *
       * @return this preference node's absolute path name.
       */
      public String absolutePath() {
          return absolutePath;
      }
  
      /**
       * Implements the <tt>isUserNode</tt> method as per the specification in
       * {@link Preferences#isUserNode()}.
       *
       * <p>This implementation compares this node's root node (which is stored
       * in a private field) with the value returned by
       * {@link Preferences#userRoot()}.  If the two object references are
       * identical, this method returns true.
       *
       * @return <tt>true</tt> if this preference node is in the user
       *         preference tree, <tt>false</tt> if it's in the system
       *         preference tree.
       */
      public boolean isUserNode() {
          return AccessController.doPrivileged(
              new PrivilegedAction<Boolean>() {
                  public Boolean run() {
                      return root == Preferences.userRoot();
              }
              }).booleanValue();
      }
  
      public void addPreferenceChangeListener(PreferenceChangeListener pcl) {
          if (pcl==null)
              throw new NullPointerException("Change listener is null.");
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
  
              // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
              /*
              // Copy-on-write
              PreferenceChangeListener[] old = prefListeners;
              prefListeners = new PreferenceChangeListener[old.length + 1];
              System.arraycopy(old, 0, prefListeners, 0, old.length);
              prefListeners[old.length] = pcl;
              */
              prefListeners.add(pcl);
          }
          startEventDispatchThreadIfNecessary();
      }
  
      public void removePreferenceChangeListener(PreferenceChangeListener pcl) {
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
              // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
              // if ((prefListeners == null) || (prefListeners.length == 0))
              if (!prefListeners.contains(pcl)) {
                  throw new IllegalArgumentException("Listener not registered.");
              }
  
              // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
              /*
              // Copy-on-write
              PreferenceChangeListener[] newPl =
                  new PreferenceChangeListener[prefListeners.length - 1];
              int i = 0;
              while (i < newPl.length && prefListeners[i] != pcl)
                  newPl[i] = prefListeners[i++];
  
              if (i == newPl.length &&  prefListeners[i] != pcl)
                  throw new IllegalArgumentException("Listener not registered.");
              while (i < newPl.length)
                  newPl[i] = prefListeners[++i];
              prefListeners = newPl;
              */
              prefListeners.remove(pcl);
          }
      }
  
      public void addNodeChangeListener(NodeChangeListener ncl) {
          if (ncl==null)
              throw new NullPointerException("Change listener is null.");
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
  
              // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
              /*
              // Copy-on-write
              if (nodeListeners == null) {
                  nodeListeners = new NodeChangeListener[1];
                  nodeListeners[0] = ncl;
              } else {
                  NodeChangeListener[] old = nodeListeners;
                  nodeListeners = new NodeChangeListener[old.length + 1];
                  System.arraycopy(old, 0, nodeListeners, 0, old.length);
                  nodeListeners[old.length] = ncl;
              }
              */
              nodeListeners.add(ncl);
          }
          startEventDispatchThreadIfNecessary();
      }
  
      public void removeNodeChangeListener(NodeChangeListener ncl) {
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed.");
              // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
              // if ((nodeListeners == null) || (nodeListeners.length == 0))
              if (!nodeListeners.contains(ncl)) {
                  throw new IllegalArgumentException("Listener not registered.");
              }
  
              // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
              /*
              // Copy-on-write
              int i = 0;
              while (i < nodeListeners.length && nodeListeners[i] != ncl)
                  i++;
              if (i == nodeListeners.length)
                  throw new IllegalArgumentException("Listener not registered.");
              NodeChangeListener[] newNl =
                  new NodeChangeListener[nodeListeners.length - 1];
              if (i != 0)
                  System.arraycopy(nodeListeners, 0, newNl, 0, i);
              if (i != newNl.length)
                  System.arraycopy(nodeListeners, i + 1,
                                   newNl, i, newNl.length - i);
              nodeListeners = newNl;
              */
              nodeListeners.remove(ncl);
          }
      }
  
      // "SPI" METHODS
  
      /**
       * Put the given key-value association into this preference node.  It is
       * guaranteed that <tt>key</tt> and <tt>value</tt> are non-null and of
       * legal length.  Also, it is guaranteed that this node has not been
       * removed.  (The implementor needn't check for any of these things.)
       *
       * <p>This method is invoked with the lock on this node held.
       * @param key the key
       * @param value the value
       */
      protected abstract void putSpi(String key, String value);
  
      /**
       * Return the value associated with the specified key at this preference
       * node, or <tt>null</tt> if there is no association for this key, or the
       * association cannot be determined at this time.  It is guaranteed that
       * <tt>key</tt> is non-null.  Also, it is guaranteed that this node has
       * not been removed.  (The implementor needn't check for either of these
       * things.)
       *
       * <p> Generally speaking, this method should not throw an exception
       * under any circumstances.  If, however, if it does throw an exception,
       * the exception will be intercepted and treated as a <tt>null</tt>
       * return value.
       *
       * <p>This method is invoked with the lock on this node held.
       *
       * @param key the key
       * @return the value associated with the specified key at this preference
       *          node, or <tt>null</tt> if there is no association for this
       *          key, or the association cannot be determined at this time.
       */
      protected abstract String getSpi(String key);
  
      /**
       * Remove the association (if any) for the specified key at this
       * preference node.  It is guaranteed that <tt>key</tt> is non-null.
       * Also, it is guaranteed that this node has not been removed.
       * (The implementor needn't check for either of these things.)
       *
       * <p>This method is invoked with the lock on this node held.
       * @param key the key
       */
      protected abstract void removeSpi(String key);
  
      /**
       * Removes this preference node, invalidating it and any preferences that
       * it contains.  The named child will have no descendants at the time this
       * invocation is made (i.e., the {@link Preferences#removeNode()} method
       * invokes this method repeatedly in a bottom-up fashion, removing each of
       * a node's descendants before removing the node itself).
       *
       * <p>This method is invoked with the lock held on this node and its
       * parent (and all ancestors that are being removed as a
       * result of a single invocation to {@link Preferences#removeNode()}).
       *
       * <p>The removal of a node needn't become persistent until the
       * <tt>flush</tt> method is invoked on this node (or an ancestor).
       *
       * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
       * will propagate out beyond the enclosing {@link #removeNode()}
       * invocation.
       *
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       */
      protected abstract void removeNodeSpi() throws BackingStoreException;
  
      /**
       * Returns all of the keys that have an associated value in this
       * preference node.  (The returned array will be of size zero if
       * this node has no preferences.)  It is guaranteed that this node has not
       * been removed.
       *
       * <p>This method is invoked with the lock on this node held.
       *
       * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
       * will propagate out beyond the enclosing {@link #keys()} invocation.
       *
       * @return an array of the keys that have an associated value in this
       *         preference node.
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       */
      protected abstract String[] keysSpi() throws BackingStoreException;
  
      /**
       * Returns the names of the children of this preference node.  (The
       * returned array will be of size zero if this node has no children.)
       * This method need not return the names of any nodes already cached,
       * but may do so without harm.
       *
       * <p>This method is invoked with the lock on this node held.
       *
       * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
       * will propagate out beyond the enclosing {@link #childrenNames()}
       * invocation.
       *
       * @return an array containing the names of the children of this
       *         preference node.
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       */
      protected abstract String[] childrenNamesSpi()
          throws BackingStoreException;
  
      /**
       * Returns the named child if it exists, or <tt>null</tt> if it does not.
       * It is guaranteed that <tt>nodeName</tt> is non-null, non-empty,
       * does not contain the slash character ('/'), and is no longer than
       * {@link #MAX_NAME_LENGTH} characters.  Also, it is guaranteed
       * that this node has not been removed.  (The implementor needn't check
       * for any of these things if he chooses to override this method.)
       *
       * <p>Finally, it is guaranteed that the named node has not been returned
       * by a previous invocation of this method or {@link #childSpi} after the
       * last time that it was removed.  In other words, a cached value will
       * always be used in preference to invoking this method.  (The implementor
       * needn't maintain his own cache of previously returned children if he
       * chooses to override this method.)
       *
       * <p>This implementation obtains this preference node's lock, invokes
       * {@link #childrenNames()} to get an array of the names of this node's
       * children, and iterates over the array comparing the name of each child
       * with the specified node name.  If a child node has the correct name,
       * the {@link #childSpi(String)} method is invoked and the resulting
       * node is returned.  If the iteration completes without finding the
       * specified name, <tt>null</tt> is returned.
       *
       * @param nodeName name of the child to be searched for.
       * @return the named child if it exists, or null if it does not.
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       */
      protected AbstractPreferences getChild(String nodeName)
              throws BackingStoreException {
          synchronized(lock) {
              // assert kidCache.get(nodeName)==null;
              String[] kidNames = childrenNames();
              for (int i=0; i<kidNames.length; i++)
                  if (kidNames[i].equals(nodeName))
                      return childSpi(kidNames[i]);
          }
          return null;
      }
  
      /**
       * Returns the named child of this preference node, creating it if it does
       * not already exist.  It is guaranteed that <tt>name</tt> is non-null,
       * non-empty, does not contain the slash character ('/'), and is no longer
       * than {@link #MAX_NAME_LENGTH} characters.  Also, it is guaranteed that
       * this node has not been removed.  (The implementor needn't check for any
       * of these things.)
       *
       * <p>Finally, it is guaranteed that the named node has not been returned
       * by a previous invocation of this method or {@link #getChild(String)}
       * after the last time that it was removed.  In other words, a cached
       * value will always be used in preference to invoking this method.
       * Subclasses need not maintain their own cache of previously returned
       * children.
       *
       * <p>The implementer must ensure that the returned node has not been
       * removed.  If a like-named child of this node was previously removed, the
       * implementer must return a newly constructed <tt>AbstractPreferences</tt>
       * node; once removed, an <tt>AbstractPreferences</tt> node
       * cannot be "resuscitated."
       *
       * <p>If this method causes a node to be created, this node is not
       * guaranteed to be persistent until the <tt>flush</tt> method is
       * invoked on this node or one of its ancestors (or descendants).
       *
       * <p>This method is invoked with the lock on this node held.
       *
       * @param name The name of the child node to return, relative to
       *        this preference node.
       * @return The named child node.
       */
      protected abstract AbstractPreferences childSpi(String name);
  
      /**
       * Returns the absolute path name of this preferences node.
       */
      public String toString() {
          return (this.isUserNode() ? "User" : "System") +
                 " Preference Node: " + this.absolutePath();
      }
  
      /**
       * Implements the <tt>sync</tt> method as per the specification in
       * {@link Preferences#sync()}.
       *
       * <p>This implementation calls a recursive helper method that locks this
       * node, invokes syncSpi() on it, unlocks this node, and recursively
       * invokes this method on each "cached child."  A cached child is a child
       * of this node that has been created in this VM and not subsequently
       * removed.  In effect, this method does a depth first traversal of the
       * "cached subtree" rooted at this node, calling syncSpi() on each node in
       * the subTree while only that node is locked. Note that syncSpi() is
       * invoked top-down.
       *
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       * @throws IllegalStateException if this node (or an ancestor) has been
       *         removed with the {@link #removeNode()} method.
       * @see #flush()
       */
      public void sync() throws BackingStoreException {
          sync2();
      }
  
      private void sync2() throws BackingStoreException {
          AbstractPreferences[] cachedKids;
  
          synchronized(lock) {
              if (removed)
                  throw new IllegalStateException("Node has been removed");
              syncSpi();
              cachedKids = cachedChildren();
          }
  
          for (int i=0; i<cachedKids.length; i++)
              cachedKids[i].sync2();
      }
  
      /**
       * This method is invoked with this node locked.  The contract of this
       * method is to synchronize any cached preferences stored at this node
       * with any stored in the backing store.  (It is perfectly possible that
       * this node does not exist on the backing store, either because it has
       * been deleted by another VM, or because it has not yet been created.)
       * Note that this method should <i>not</i> synchronize the preferences in
       * any subnodes of this node.  If the backing store naturally syncs an
       * entire subtree at once, the implementer is encouraged to override
       * sync(), rather than merely overriding this method.
       *
       * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
       * will propagate out beyond the enclosing {@link #sync()} invocation.
       *
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       */
      protected abstract void syncSpi() throws BackingStoreException;
  
      /**
       * Implements the <tt>flush</tt> method as per the specification in
       * {@link Preferences#flush()}.
       *
       * <p>This implementation calls a recursive helper method that locks this
       * node, invokes flushSpi() on it, unlocks this node, and recursively
       * invokes this method on each "cached child."  A cached child is a child
       * of this node that has been created in this VM and not subsequently
       * removed.  In effect, this method does a depth first traversal of the
       * "cached subtree" rooted at this node, calling flushSpi() on each node in
       * the subTree while only that node is locked. Note that flushSpi() is
       * invoked top-down.
       *
       * <p> If this method is invoked on a node that has been removed with
       * the {@link #removeNode()} method, flushSpi() is invoked on this node,
       * but not on others.
       *
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       * @see #flush()
       */
      public void flush() throws BackingStoreException {
          flush2();
      }
  
      private void flush2() throws BackingStoreException {
          AbstractPreferences[] cachedKids;
  
          synchronized(lock) {
              flushSpi();
              if(removed)
                  return;
              cachedKids = cachedChildren();
          }
  
          for (int i = 0; i < cachedKids.length; i++)
              cachedKids[i].flush2();
      }
  
      /**
       * This method is invoked with this node locked.  The contract of this
       * method is to force any cached changes in the contents of this
       * preference node to the backing store, guaranteeing their persistence.
       * (It is perfectly possible that this node does not exist on the backing
       * store, either because it has been deleted by another VM, or because it
       * has not yet been created.)  Note that this method should <i>not</i>
       * flush the preferences in any subnodes of this node.  If the backing
       * store naturally flushes an entire subtree at once, the implementer is
       * encouraged to override flush(), rather than merely overriding this
       * method.
       *
       * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
       * will propagate out beyond the enclosing {@link #flush()} invocation.
       *
       * @throws BackingStoreException if this operation cannot be completed
       *         due to a failure in the backing store, or inability to
       *         communicate with it.
       */
      protected abstract void flushSpi() throws BackingStoreException;
  
      /**
       * Returns <tt>true</tt> iff this node (or an ancestor) has been
       * removed with the {@link #removeNode()} method.  This method
       * locks this node prior to returning the contents of the private
       * field used to track this state.
       *
       * @return <tt>true</tt> iff this node (or an ancestor) has been
       *       removed with the {@link #removeNode()} method.
       */
      protected boolean isRemoved() {
          synchronized(lock) {
              return removed;
          }
      }
  
      /**
       * Queue of pending notification events.  When a preference or node
       * change event for which there are one or more listeners occurs,
       * it is placed on this queue and the queue is notified.  A background
       * thread waits on this queue and delivers the events.  This decouples
       * event delivery from preference activity, greatly simplifying
       * locking and reducing opportunity for deadlock.
       */
      private static final List<EventObject> eventQueue = new LinkedList<>();
  
      /**
       * These two classes are used to distinguish NodeChangeEvents on
       * eventQueue so the event dispatch thread knows whether to call
       * childAdded or childRemoved.
       */
      private class NodeAddedEvent extends NodeChangeEvent {
          private static final long serialVersionUID = -6743557530157328528L;
          NodeAddedEvent(Preferences parent, Preferences child) {
              super(parent, child);
          }
      }
      private class NodeRemovedEvent extends NodeChangeEvent {
          private static final long serialVersionUID = 8735497392918824837L;
          NodeRemovedEvent(Preferences parent, Preferences child) {
              super(parent, child);
          }
      }
  
      /**
       * A single background thread ("the event notification thread") monitors
       * the event queue and delivers events that are placed on the queue.
       */
      private static class EventDispatchThread extends Thread {
          public void run() {
              while(true) {
                  // Wait on eventQueue till an event is present
                  EventObject event = null;
                  synchronized(eventQueue) {
                      try {
                          while (eventQueue.isEmpty())
                              eventQueue.wait();
                          event = eventQueue.remove(0);
                      } catch (InterruptedException e) {
                          // XXX Log "Event dispatch thread interrupted. Exiting"
                          return;
                      }
                  }
  
                  // Now we have event & hold no locks; deliver evt to listeners
                  AbstractPreferences src=(AbstractPreferences)event.getSource();
                  if (event instanceof PreferenceChangeEvent) {
                      PreferenceChangeEvent pce = (PreferenceChangeEvent)event;
                      PreferenceChangeListener[] listeners = src.prefListeners();
                      for (int i=0; i<listeners.length; i++)
                          listeners[i].preferenceChange(pce);
                  } else {
                      NodeChangeEvent nce = (NodeChangeEvent)event;
                      NodeChangeListener[] listeners = src.nodeListeners();
                      if (nce instanceof NodeAddedEvent) {
                          for (int i=0; i<listeners.length; i++)
                              listeners[i].childAdded(nce);
                      } else {
                          // assert nce instanceof NodeRemovedEvent;
                          for (int i=0; i<listeners.length; i++)
                              listeners[i].childRemoved(nce);
                      }
                  }
              }
          }
      }
  
      private static Thread eventDispatchThread = null;
  
      /**
       * This method starts the event dispatch thread the first time it
       * is called.  The event dispatch thread will be started only
       * if someone registers a listener.
       */
      private static synchronized void startEventDispatchThreadIfNecessary() {
          if (eventDispatchThread == null) {
              // XXX Log "Starting event dispatch thread"
              eventDispatchThread = new EventDispatchThread();
              eventDispatchThread.setDaemon(true);
              eventDispatchThread.start();
          }
      }
  
      // BEGIN Android-changed: Copy-on-read List of listeners, not copy-on-write array.
      // Changed documentation.
      /*
      /**
       * Return this node's preference/node change listeners.  Even though
       * we're using a copy-on-write lists, we use synchronized accessors to
       * ensure information transmission from the writing thread to the
       * reading thread.
       *
      */
      /**
       * Return this node's preference/node change listeners. All accesses to prefListeners
       * and nodeListeners are guarded by |lock|. We return a copy of the list so that the
       * EventQueue thread will iterate over a fixed snapshot of the listeners at the time of
       * this call.
       */
      // END Android-changed: Copy-on-read List of listeners, not copy-on-write array.
      PreferenceChangeListener[] prefListeners() {
          synchronized(lock) {
              // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
              // return prefListeners;
              return prefListeners.toArray(new PreferenceChangeListener[prefListeners.size()]);
          }
      }
      NodeChangeListener[] nodeListeners() {
          synchronized(lock) {
              // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
              // return nodeListeners;
              return nodeListeners.toArray(new NodeChangeListener[nodeListeners.size()]);
          }
      }
  
      /**
       * Enqueue a preference change event for delivery to registered
       * preference change listeners unless there are no registered
       * listeners.  Invoked with this.lock held.
       */
      private void enqueuePreferenceChangeEvent(String key, String newValue) {
          // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
          // if (prefListeners.length != 0) {
          if (!prefListeners.isEmpty()) {
              synchronized(eventQueue) {
                  eventQueue.add(new PreferenceChangeEvent(this, key, newValue));
                  eventQueue.notify();
              }
          }
      }
  
      /**
       * Enqueue a "node added" event for delivery to registered node change
       * listeners unless there are no registered listeners.  Invoked with
       * this.lock held.
       */
      private void enqueueNodeAddedEvent(Preferences child) {
          // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
          // if (nodeListeners.length != 0) {
          if (!nodeListeners.isEmpty()) {
              synchronized(eventQueue) {
                  eventQueue.add(new NodeAddedEvent(this, child));
                  eventQueue.notify();
              }
          }
      }
  
      /**
       * Enqueue a "node removed" event for delivery to registered node change
       * listeners unless there are no registered listeners.  Invoked with
       * this.lock held.
       */
      private void enqueueNodeRemovedEvent(Preferences child) {
          // Android-changed: Copy-on-read List of listeners, not copy-on-write array.
          // if (nodeListeners.length != 0) {
          if (!nodeListeners.isEmpty()) {
              synchronized(eventQueue) {
                  eventQueue.add(new NodeRemovedEvent(this, child));
                  eventQueue.notify();
              }
          }
      }
  
      /**
       * Implements the <tt>exportNode</tt> method as per the specification in
       * {@link Preferences#exportNode(OutputStream)}.
       *
       * @param os the output stream on which to emit the XML document.
       * @throws IOException if writing to the specified output stream
       *         results in an <tt>IOException</tt>.
       * @throws BackingStoreException if preference data cannot be read from
       *         backing store.
       */
      public void exportNode(OutputStream os)
          throws IOException, BackingStoreException
      {
          XmlSupport.export(os, this, false);
      }
  
      /**
       * Implements the <tt>exportSubtree</tt> method as per the specification in
       * {@link Preferences#exportSubtree(OutputStream)}.
       *
       * @param os the output stream on which to emit the XML document.
       * @throws IOException if writing to the specified output stream
       *         results in an <tt>IOException</tt>.
       * @throws BackingStoreException if preference data cannot be read from
       *         backing store.
       */
      public void exportSubtree(OutputStream os)
          throws IOException, BackingStoreException
      {
          XmlSupport.export(os, this, true);
      }
  }
  