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    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
    *
    * This code is free software; you can redistribute it and/or modify it
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    * published by the Free Software Foundation.  Oracle designates this
    * particular file as subject to the "Classpath" exception as provided
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   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
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  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.
      */
     private PreferenceChangeListener[] prefListeners =
         new PreferenceChangeListener[0];
 
     /**
      * Registered node change listeners.
      */
     private NodeChangeListener[] nodeListeners = new NodeChangeListener[0];
 
     /**
      * 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.
      */
     public void remove(String key) {
         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.");
 
             // Copy-on-write
             PreferenceChangeListener[] old = prefListeners;
             prefListeners = new PreferenceChangeListener[old.length + 1];
             System.arraycopy(old, 0, prefListeners, 0, old.length);
             prefListeners[old.length] = pcl;
         }
         startEventDispatchThreadIfNecessary();
     }
 
     public void removePreferenceChangeListener(PreferenceChangeListener pcl) {
         synchronized(lock) {
             if (removed)
                 throw new IllegalStateException("Node has been removed.");
             if ((prefListeners == null) || (prefListeners.length == 0))
                 throw new IllegalArgumentException("Listener not registered.");
 
             // 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;
         }
     }
 
     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.");
 
             // 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;
             }
         }
         startEventDispatchThreadIfNecessary();
     }
 
     public void removeNodeChangeListener(NodeChangeListener ncl) {
         synchronized(lock) {
             if (removed)
                 throw new IllegalStateException("Node has been removed.");
             if ((nodeListeners == null) || (nodeListeners.length == 0))
                 throw new IllegalArgumentException("Listener not registered.");
 
             // 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;
         }
     }
 
     // "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.
      */
     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.
      *
      * @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.
      */
     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();
         }
     }
 
     /**
      * 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.
      */
     PreferenceChangeListener[] prefListeners() {
         synchronized(lock) {
             return prefListeners;
         }
     }
     NodeChangeListener[] nodeListeners() {
         synchronized(lock) {
             return nodeListeners;
         }
     }
 
     /**
      * 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) {
         if (prefListeners.length != 0) {
             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) {
         if (nodeListeners.length != 0) {
             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) {
         if (nodeListeners.length != 0) {
             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);
     }
 }