rev 5431 : 7126277: Alternative hashing implementation

   1 /*
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  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
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  25 
  26 package java.util;
  27 import java.io.*;
  28 
  29 /**
  30  * This class implements a hash table, which maps keys to values. Any
  31  * non-<code>null</code> object can be used as a key or as a value. <p>
  32  *
  33  * To successfully store and retrieve objects from a hashtable, the
  34  * objects used as keys must implement the <code>hashCode</code>
  35  * method and the <code>equals</code> method. <p>
  36  *
  37  * An instance of <code>Hashtable</code> has two parameters that affect its
  38  * performance: <i>initial capacity</i> and <i>load factor</i>.  The
  39  * <i>capacity</i> is the number of <i>buckets</i> in the hash table, and the
  40  * <i>initial capacity</i> is simply the capacity at the time the hash table
  41  * is created.  Note that the hash table is <i>open</i>: in the case of a "hash
  42  * collision", a single bucket stores multiple entries, which must be searched
  43  * sequentially.  The <i>load factor</i> is a measure of how full the hash
  44  * table is allowed to get before its capacity is automatically increased.
  45  * The initial capacity and load factor parameters are merely hints to
  46  * the implementation.  The exact details as to when and whether the rehash
  47  * method is invoked are implementation-dependent.<p>
  48  *
  49  * Generally, the default load factor (.75) offers a good tradeoff between
  50  * time and space costs.  Higher values decrease the space overhead but
  51  * increase the time cost to look up an entry (which is reflected in most
  52  * <tt>Hashtable</tt> operations, including <tt>get</tt> and <tt>put</tt>).<p>
  53  *
  54  * The initial capacity controls a tradeoff between wasted space and the
  55  * need for <code>rehash</code> operations, which are time-consuming.
  56  * No <code>rehash</code> operations will <i>ever</i> occur if the initial
  57  * capacity is greater than the maximum number of entries the
  58  * <tt>Hashtable</tt> will contain divided by its load factor.  However,
  59  * setting the initial capacity too high can waste space.<p>
  60  *
  61  * If many entries are to be made into a <code>Hashtable</code>,
  62  * creating it with a sufficiently large capacity may allow the
  63  * entries to be inserted more efficiently than letting it perform
  64  * automatic rehashing as needed to grow the table. <p>
  65  *
  66  * This example creates a hashtable of numbers. It uses the names of
  67  * the numbers as keys:
  68  * <pre>   {@code
  69  *   Hashtable<String, Integer> numbers
  70  *     = new Hashtable<String, Integer>();
  71  *   numbers.put("one", 1);
  72  *   numbers.put("two", 2);
  73  *   numbers.put("three", 3);}</pre>
  74  *
  75  * <p>To retrieve a number, use the following code:
  76  * <pre>   {@code
  77  *   Integer n = numbers.get("two");
  78  *   if (n != null) {
  79  *     System.out.println("two = " + n);
  80  *   }}</pre>
  81  *
  82  * <p>The iterators returned by the <tt>iterator</tt> method of the collections
  83  * returned by all of this class's "collection view methods" are
  84  * <em>fail-fast</em>: if the Hashtable is structurally modified at any time
  85  * after the iterator is created, in any way except through the iterator's own
  86  * <tt>remove</tt> method, the iterator will throw a {@link
  87  * ConcurrentModificationException}.  Thus, in the face of concurrent
  88  * modification, the iterator fails quickly and cleanly, rather than risking
  89  * arbitrary, non-deterministic behavior at an undetermined time in the future.
  90  * The Enumerations returned by Hashtable's keys and elements methods are
  91  * <em>not</em> fail-fast.
  92  *
  93  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
  94  * as it is, generally speaking, impossible to make any hard guarantees in the
  95  * presence of unsynchronized concurrent modification.  Fail-fast iterators
  96  * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
  97  * Therefore, it would be wrong to write a program that depended on this
  98  * exception for its correctness: <i>the fail-fast behavior of iterators
  99  * should be used only to detect bugs.</i>
 100  *
 101  * <p>As of the Java 2 platform v1.2, this class was retrofitted to
 102  * implement the {@link Map} interface, making it a member of the
 103  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 104  *
 105  * Java Collections Framework</a>.  Unlike the new collection
 106  * implementations, {@code Hashtable} is synchronized.  If a
 107  * thread-safe implementation is not needed, it is recommended to use
 108  * {@link HashMap} in place of {@code Hashtable}.  If a thread-safe
 109  * highly-concurrent implementation is desired, then it is recommended
 110  * to use {@link java.util.concurrent.ConcurrentHashMap} in place of
 111  * {@code Hashtable}.
 112  *
 113  * @author  Arthur van Hoff
 114  * @author  Josh Bloch
 115  * @author  Neal Gafter
 116  * @see     Object#equals(java.lang.Object)
 117  * @see     Object#hashCode()
 118  * @see     Hashtable#rehash()
 119  * @see     Collection
 120  * @see     Map
 121  * @see     HashMap
 122  * @see     TreeMap
 123  * @since JDK1.0
 124  */
 125 public class Hashtable<K,V>
 126     extends Dictionary<K,V>
 127     implements Map<K,V>, Cloneable, java.io.Serializable {
 128 
 129     /**
 130      * The hash table data.
 131      */
 132     private transient Entry<?,?>[] table;
 133 
 134     /**
 135      * The total number of entries in the hash table.
 136      */
 137     private transient int count;
 138 
 139     /**
 140      * The table is rehashed when its size exceeds this threshold.  (The
 141      * value of this field is (int)(capacity * loadFactor).)
 142      *
 143      * @serial
 144      */
 145     private int threshold;
 146 
 147     /**
 148      * The load factor for the hashtable.
 149      *
 150      * @serial
 151      */
 152     private float loadFactor;
 153 
 154     /**
 155      * The number of times this Hashtable has been structurally modified
 156      * Structural modifications are those that change the number of entries in
 157      * the Hashtable or otherwise modify its internal structure (e.g.,
 158      * rehash).  This field is used to make iterators on Collection-views of
 159      * the Hashtable fail-fast.  (See ConcurrentModificationException).
 160      */
 161     private transient int modCount = 0;
 162 
 163     /** use serialVersionUID from JDK 1.0.2 for interoperability */
 164     private static final long serialVersionUID = 1421746759512286392L;
 165 














































 166     /**
 167      * Constructs a new, empty hashtable with the specified initial
 168      * capacity and the specified load factor.
 169      *
 170      * @param      initialCapacity   the initial capacity of the hashtable.
 171      * @param      loadFactor        the load factor of the hashtable.
 172      * @exception  IllegalArgumentException  if the initial capacity is less
 173      *             than zero, or if the load factor is nonpositive.
 174      */
 175     public Hashtable(int initialCapacity, float loadFactor) {
 176         if (initialCapacity < 0)
 177             throw new IllegalArgumentException("Illegal Capacity: "+
 178                                                initialCapacity);
 179         if (loadFactor <= 0 || Float.isNaN(loadFactor))
 180             throw new IllegalArgumentException("Illegal Load: "+loadFactor);
 181 
 182         if (initialCapacity==0)
 183             initialCapacity = 1;
 184         this.loadFactor = loadFactor;
 185         table = new Entry<?,?>[initialCapacity];
 186         threshold = (int)(initialCapacity * loadFactor);
 187     }
 188 
 189     /**
 190      * Constructs a new, empty hashtable with the specified initial capacity
 191      * and default load factor (0.75).
 192      *
 193      * @param     initialCapacity   the initial capacity of the hashtable.
 194      * @exception IllegalArgumentException if the initial capacity is less
 195      *              than zero.
 196      */
 197     public Hashtable(int initialCapacity) {
 198         this(initialCapacity, 0.75f);
 199     }
 200 
 201     /**
 202      * Constructs a new, empty hashtable with a default initial capacity (11)
 203      * and load factor (0.75).
 204      */
 205     public Hashtable() {
 206         this(11, 0.75f);
 207     }
 208 
 209     /**
 210      * Constructs a new hashtable with the same mappings as the given
 211      * Map.  The hashtable is created with an initial capacity sufficient to
 212      * hold the mappings in the given Map and a default load factor (0.75).
 213      *
 214      * @param t the map whose mappings are to be placed in this map.
 215      * @throws NullPointerException if the specified map is null.
 216      * @since   1.2
 217      */
 218     public Hashtable(Map<? extends K, ? extends V> t) {
 219         this(Math.max(2*t.size(), 11), 0.75f);
 220         putAll(t);
 221     }
 222 
 223     /**
 224      * Returns the number of keys in this hashtable.
 225      *
 226      * @return  the number of keys in this hashtable.
 227      */
 228     public synchronized int size() {
 229         return count;
 230     }
 231 
 232     /**
 233      * Tests if this hashtable maps no keys to values.
 234      *
 235      * @return  <code>true</code> if this hashtable maps no keys to values;
 236      *          <code>false</code> otherwise.
 237      */
 238     public synchronized boolean isEmpty() {
 239         return count == 0;
 240     }
 241 
 242     /**
 243      * Returns an enumeration of the keys in this hashtable.
 244      *
 245      * @return  an enumeration of the keys in this hashtable.
 246      * @see     Enumeration
 247      * @see     #elements()
 248      * @see     #keySet()
 249      * @see     Map
 250      */
 251     public synchronized Enumeration<K> keys() {
 252         return this.<K>getEnumeration(KEYS);
 253     }
 254 
 255     /**
 256      * Returns an enumeration of the values in this hashtable.
 257      * Use the Enumeration methods on the returned object to fetch the elements
 258      * sequentially.
 259      *
 260      * @return  an enumeration of the values in this hashtable.
 261      * @see     java.util.Enumeration
 262      * @see     #keys()
 263      * @see     #values()
 264      * @see     Map
 265      */
 266     public synchronized Enumeration<V> elements() {
 267         return this.<V>getEnumeration(VALUES);
 268     }
 269 
 270     /**
 271      * Tests if some key maps into the specified value in this hashtable.
 272      * This operation is more expensive than the {@link #containsKey
 273      * containsKey} method.
 274      *
 275      * <p>Note that this method is identical in functionality to
 276      * {@link #containsValue containsValue}, (which is part of the
 277      * {@link Map} interface in the collections framework).
 278      *
 279      * @param      value   a value to search for
 280      * @return     <code>true</code> if and only if some key maps to the
 281      *             <code>value</code> argument in this hashtable as
 282      *             determined by the <tt>equals</tt> method;
 283      *             <code>false</code> otherwise.
 284      * @exception  NullPointerException  if the value is <code>null</code>
 285      */
 286     public synchronized boolean contains(Object value) {
 287         if (value == null) {
 288             throw new NullPointerException();
 289         }
 290 
 291         Entry<?,?> tab[] = table;
 292         for (int i = tab.length ; i-- > 0 ;) {
 293             for (Entry<?,?> e = tab[i] ; e != null ; e = e.next) {
 294                 if (e.value.equals(value)) {
 295                     return true;
 296                 }
 297             }
 298         }
 299         return false;
 300     }
 301 
 302     /**
 303      * Returns true if this hashtable maps one or more keys to this value.
 304      *
 305      * <p>Note that this method is identical in functionality to {@link
 306      * #contains contains} (which predates the {@link Map} interface).
 307      *
 308      * @param value value whose presence in this hashtable is to be tested
 309      * @return <tt>true</tt> if this map maps one or more keys to the
 310      *         specified value
 311      * @throws NullPointerException  if the value is <code>null</code>
 312      * @since 1.2
 313      */
 314     public boolean containsValue(Object value) {
 315         return contains(value);
 316     }
 317 
 318     /**
 319      * Tests if the specified object is a key in this hashtable.
 320      *
 321      * @param   key   possible key
 322      * @return  <code>true</code> if and only if the specified object
 323      *          is a key in this hashtable, as determined by the
 324      *          <tt>equals</tt> method; <code>false</code> otherwise.
 325      * @throws  NullPointerException  if the key is <code>null</code>
 326      * @see     #contains(Object)
 327      */
 328     public synchronized boolean containsKey(Object key) {
 329         Entry<?,?> tab[] = table;
 330         int hash = key.hashCode();
 331         int index = (hash & 0x7FFFFFFF) % tab.length;
 332         for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
 333             if ((e.hash == hash) && e.key.equals(key)) {
 334                 return true;
 335             }
 336         }
 337         return false;
 338     }
 339 
 340     /**
 341      * Returns the value to which the specified key is mapped,
 342      * or {@code null} if this map contains no mapping for the key.
 343      *
 344      * <p>More formally, if this map contains a mapping from a key
 345      * {@code k} to a value {@code v} such that {@code (key.equals(k))},
 346      * then this method returns {@code v}; otherwise it returns
 347      * {@code null}.  (There can be at most one such mapping.)
 348      *
 349      * @param key the key whose associated value is to be returned
 350      * @return the value to which the specified key is mapped, or
 351      *         {@code null} if this map contains no mapping for the key
 352      * @throws NullPointerException if the specified key is null
 353      * @see     #put(Object, Object)
 354      */
 355     @SuppressWarnings("unchecked")
 356     public synchronized V get(Object key) {
 357         Entry<?,?> tab[] = table;
 358         int hash = key.hashCode();
 359         int index = (hash & 0x7FFFFFFF) % tab.length;
 360         for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
 361             if ((e.hash == hash) && e.key.equals(key)) {
 362                 return (V)e.value;
 363             }
 364         }
 365         return null;
 366     }
 367 
 368     /**
 369      * The maximum size of array to allocate.
 370      * Some VMs reserve some header words in an array.
 371      * Attempts to allocate larger arrays may result in
 372      * OutOfMemoryError: Requested array size exceeds VM limit
 373      */
 374     private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
 375 
 376     /**
 377      * Increases the capacity of and internally reorganizes this
 378      * hashtable, in order to accommodate and access its entries more
 379      * efficiently.  This method is called automatically when the
 380      * number of keys in the hashtable exceeds this hashtable's capacity
 381      * and load factor.
 382      */
 383     @SuppressWarnings("unchecked")
 384     protected void rehash() {
 385         int oldCapacity = table.length;
 386         Entry<?,?>[] oldMap = table;
 387 
 388         // overflow-conscious code
 389         int newCapacity = (oldCapacity << 1) + 1;
 390         if (newCapacity - MAX_ARRAY_SIZE > 0) {
 391             if (oldCapacity == MAX_ARRAY_SIZE)
 392                 // Keep running with MAX_ARRAY_SIZE buckets
 393                 return;
 394             newCapacity = MAX_ARRAY_SIZE;
 395         }
 396         Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];
 397 
 398         modCount++;
 399         threshold = (int)(newCapacity * loadFactor);
 400         table = newMap;
 401 
 402         for (int i = oldCapacity ; i-- > 0 ;) {
 403             for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
 404                 Entry<K,V> e = old;
 405                 old = old.next;
 406 
 407                 int index = (e.hash & 0x7FFFFFFF) % newCapacity;
 408                 e.next = (Entry<K,V>)newMap[index];
 409                 newMap[index] = e;
 410             }
 411         }
 412     }
 413 
 414     /**
 415      * Maps the specified <code>key</code> to the specified
 416      * <code>value</code> in this hashtable. Neither the key nor the
 417      * value can be <code>null</code>. <p>
 418      *
 419      * The value can be retrieved by calling the <code>get</code> method
 420      * with a key that is equal to the original key.
 421      *
 422      * @param      key     the hashtable key
 423      * @param      value   the value
 424      * @return     the previous value of the specified key in this hashtable,
 425      *             or <code>null</code> if it did not have one
 426      * @exception  NullPointerException  if the key or value is
 427      *               <code>null</code>
 428      * @see     Object#equals(Object)
 429      * @see     #get(Object)
 430      */
 431     public synchronized V put(K key, V value) {
 432         // Make sure the value is not null
 433         if (value == null) {
 434             throw new NullPointerException();
 435         }
 436 
 437         // Makes sure the key is not already in the hashtable.
 438         Entry<?,?> tab[] = table;
 439         int hash = key.hashCode();
 440         int index = (hash & 0x7FFFFFFF) % tab.length;
 441         @SuppressWarnings("unchecked")
 442         Entry<K,V> entry = (Entry<K,V>)tab[index];
 443         for(; entry != null ; entry = entry.next) {
 444             if ((entry.hash == hash) && entry.key.equals(key)) {
 445                 V old = entry.value;
 446                 entry.value = value;
 447                 return old;
 448             }
 449         }
 450 
 451         modCount++;
 452         if (count >= threshold) {
 453             // Rehash the table if the threshold is exceeded
 454             rehash();
 455 
 456             tab = table;

 457             index = (hash & 0x7FFFFFFF) % tab.length;
 458         }
 459 
 460         // Creates the new entry.
 461         @SuppressWarnings("unchecked")
 462         Entry<K,V> e = (Entry<K,V>)tab[index];
 463         tab[index] = new Entry<>(hash, key, value, e);
 464         count++;
 465         return null;
 466     }
 467 
 468     /**
 469      * Removes the key (and its corresponding value) from this
 470      * hashtable. This method does nothing if the key is not in the hashtable.
 471      *
 472      * @param   key   the key that needs to be removed
 473      * @return  the value to which the key had been mapped in this hashtable,
 474      *          or <code>null</code> if the key did not have a mapping
 475      * @throws  NullPointerException  if the key is <code>null</code>
 476      */
 477     public synchronized V remove(Object key) {
 478         Entry<?,?> tab[] = table;
 479         int hash = key.hashCode();
 480         int index = (hash & 0x7FFFFFFF) % tab.length;
 481         @SuppressWarnings("unchecked")
 482         Entry<K,V> e = (Entry<K,V>)tab[index];
 483         for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) {
 484             if ((e.hash == hash) && e.key.equals(key)) {
 485                 modCount++;
 486                 if (prev != null) {
 487                     prev.next = e.next;
 488                 } else {
 489                     tab[index] = e.next;
 490                 }
 491                 count--;
 492                 V oldValue = e.value;
 493                 e.value = null;
 494                 return oldValue;
 495             }
 496         }
 497         return null;
 498     }
 499 
 500     /**
 501      * Copies all of the mappings from the specified map to this hashtable.
 502      * These mappings will replace any mappings that this hashtable had for any
 503      * of the keys currently in the specified map.
 504      *
 505      * @param t mappings to be stored in this map
 506      * @throws NullPointerException if the specified map is null
 507      * @since 1.2
 508      */
 509     public synchronized void putAll(Map<? extends K, ? extends V> t) {
 510         for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
 511             put(e.getKey(), e.getValue());
 512     }
 513 
 514     /**
 515      * Clears this hashtable so that it contains no keys.
 516      */
 517     public synchronized void clear() {
 518         Entry<?,?> tab[] = table;
 519         modCount++;
 520         for (int index = tab.length; --index >= 0; )
 521             tab[index] = null;
 522         count = 0;
 523     }
 524 
 525     /**
 526      * Creates a shallow copy of this hashtable. All the structure of the
 527      * hashtable itself is copied, but the keys and values are not cloned.
 528      * This is a relatively expensive operation.
 529      *
 530      * @return  a clone of the hashtable
 531      */
 532     public synchronized Object clone() {
 533         try {
 534             Hashtable<?,?> t = (Hashtable<?,?>)super.clone();
 535             t.table = new Entry<?,?>[table.length];
 536             for (int i = table.length ; i-- > 0 ; ) {
 537                 t.table[i] = (table[i] != null)
 538                     ? (Entry<?,?>) table[i].clone() : null;
 539             }
 540             t.keySet = null;
 541             t.entrySet = null;
 542             t.values = null;
 543             t.modCount = 0;
 544             return t;
 545         } catch (CloneNotSupportedException e) {
 546             // this shouldn't happen, since we are Cloneable
 547             throw new InternalError(e);
 548         }
 549     }
 550 
 551     /**
 552      * Returns a string representation of this <tt>Hashtable</tt> object
 553      * in the form of a set of entries, enclosed in braces and separated
 554      * by the ASCII characters "<tt>,&nbsp;</tt>" (comma and space). Each
 555      * entry is rendered as the key, an equals sign <tt>=</tt>, and the
 556      * associated element, where the <tt>toString</tt> method is used to
 557      * convert the key and element to strings.
 558      *
 559      * @return  a string representation of this hashtable
 560      */
 561     public synchronized String toString() {
 562         int max = size() - 1;
 563         if (max == -1)
 564             return "{}";
 565 
 566         StringBuilder sb = new StringBuilder();
 567         Iterator<Map.Entry<K,V>> it = entrySet().iterator();
 568 
 569         sb.append('{');
 570         for (int i = 0; ; i++) {
 571             Map.Entry<K,V> e = it.next();
 572             K key = e.getKey();
 573             V value = e.getValue();
 574             sb.append(key   == this ? "(this Map)" : key.toString());
 575             sb.append('=');
 576             sb.append(value == this ? "(this Map)" : value.toString());
 577 
 578             if (i == max)
 579                 return sb.append('}').toString();
 580             sb.append(", ");
 581         }
 582     }
 583 
 584 
 585     private <T> Enumeration<T> getEnumeration(int type) {
 586         if (count == 0) {
 587             return Collections.emptyEnumeration();
 588         } else {
 589             return new Enumerator<>(type, false);
 590         }
 591     }
 592 
 593     private <T> Iterator<T> getIterator(int type) {
 594         if (count == 0) {
 595             return Collections.emptyIterator();
 596         } else {
 597             return new Enumerator<>(type, true);
 598         }
 599     }
 600 
 601     // Views
 602 
 603     /**
 604      * Each of these fields are initialized to contain an instance of the
 605      * appropriate view the first time this view is requested.  The views are
 606      * stateless, so there's no reason to create more than one of each.
 607      */
 608     private transient volatile Set<K> keySet = null;
 609     private transient volatile Set<Map.Entry<K,V>> entrySet = null;
 610     private transient volatile Collection<V> values = null;
 611 
 612     /**
 613      * Returns a {@link Set} view of the keys contained in this map.
 614      * The set is backed by the map, so changes to the map are
 615      * reflected in the set, and vice-versa.  If the map is modified
 616      * while an iteration over the set is in progress (except through
 617      * the iterator's own <tt>remove</tt> operation), the results of
 618      * the iteration are undefined.  The set supports element removal,
 619      * which removes the corresponding mapping from the map, via the
 620      * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
 621      * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
 622      * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
 623      * operations.
 624      *
 625      * @since 1.2
 626      */
 627     public Set<K> keySet() {
 628         if (keySet == null)
 629             keySet = Collections.synchronizedSet(new KeySet(), this);
 630         return keySet;
 631     }
 632 
 633     private class KeySet extends AbstractSet<K> {
 634         public Iterator<K> iterator() {
 635             return getIterator(KEYS);
 636         }
 637         public int size() {
 638             return count;
 639         }
 640         public boolean contains(Object o) {
 641             return containsKey(o);
 642         }
 643         public boolean remove(Object o) {
 644             return Hashtable.this.remove(o) != null;
 645         }
 646         public void clear() {
 647             Hashtable.this.clear();
 648         }
 649     }
 650 
 651     /**
 652      * Returns a {@link Set} view of the mappings contained in this map.
 653      * The set is backed by the map, so changes to the map are
 654      * reflected in the set, and vice-versa.  If the map is modified
 655      * while an iteration over the set is in progress (except through
 656      * the iterator's own <tt>remove</tt> operation, or through the
 657      * <tt>setValue</tt> operation on a map entry returned by the
 658      * iterator) the results of the iteration are undefined.  The set
 659      * supports element removal, which removes the corresponding
 660      * mapping from the map, via the <tt>Iterator.remove</tt>,
 661      * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
 662      * <tt>clear</tt> operations.  It does not support the
 663      * <tt>add</tt> or <tt>addAll</tt> operations.
 664      *
 665      * @since 1.2
 666      */
 667     public Set<Map.Entry<K,V>> entrySet() {
 668         if (entrySet==null)
 669             entrySet = Collections.synchronizedSet(new EntrySet(), this);
 670         return entrySet;
 671     }
 672 
 673     private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
 674         public Iterator<Map.Entry<K,V>> iterator() {
 675             return getIterator(ENTRIES);
 676         }
 677 
 678         public boolean add(Map.Entry<K,V> o) {
 679             return super.add(o);
 680         }
 681 
 682         public boolean contains(Object o) {
 683             if (!(o instanceof Map.Entry))
 684                 return false;
 685             Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
 686             Object key = entry.getKey();
 687             Entry<?,?>[] tab = table;
 688             int hash = key.hashCode();
 689             int index = (hash & 0x7FFFFFFF) % tab.length;
 690 
 691             for (Entry<?,?> e = tab[index]; e != null; e = e.next)
 692                 if (e.hash==hash && e.equals(entry))
 693                     return true;
 694             return false;
 695         }
 696 
 697         public boolean remove(Object o) {
 698             if (!(o instanceof Map.Entry))
 699                 return false;
 700             Map.Entry<?,?> entry = (Map.Entry<?,?>) o;
 701             Object key = entry.getKey();
 702             Entry<?,?>[] tab = table;
 703             int hash = key.hashCode();
 704             int index = (hash & 0x7FFFFFFF) % tab.length;
 705 
 706             @SuppressWarnings("unchecked")
 707             Entry<K,V> e = (Entry<K,V>)tab[index];
 708             for(Entry<K,V> prev = null; e != null; prev = e, e = e.next) {
 709                 if (e.hash==hash && e.equals(entry)) {
 710                     modCount++;
 711                     if (prev != null)
 712                         prev.next = e.next;
 713                     else
 714                         tab[index] = e.next;
 715 
 716                     count--;
 717                     e.value = null;
 718                     return true;
 719                 }
 720             }
 721             return false;
 722         }
 723 
 724         public int size() {
 725             return count;
 726         }
 727 
 728         public void clear() {
 729             Hashtable.this.clear();
 730         }
 731     }
 732 
 733     /**
 734      * Returns a {@link Collection} view of the values contained in this map.
 735      * The collection is backed by the map, so changes to the map are
 736      * reflected in the collection, and vice-versa.  If the map is
 737      * modified while an iteration over the collection is in progress
 738      * (except through the iterator's own <tt>remove</tt> operation),
 739      * the results of the iteration are undefined.  The collection
 740      * supports element removal, which removes the corresponding
 741      * mapping from the map, via the <tt>Iterator.remove</tt>,
 742      * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
 743      * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
 744      * support the <tt>add</tt> or <tt>addAll</tt> operations.
 745      *
 746      * @since 1.2
 747      */
 748     public Collection<V> values() {
 749         if (values==null)
 750             values = Collections.synchronizedCollection(new ValueCollection(),
 751                                                         this);
 752         return values;
 753     }
 754 
 755     private class ValueCollection extends AbstractCollection<V> {
 756         public Iterator<V> iterator() {
 757             return getIterator(VALUES);
 758         }
 759         public int size() {
 760             return count;
 761         }
 762         public boolean contains(Object o) {
 763             return containsValue(o);
 764         }
 765         public void clear() {
 766             Hashtable.this.clear();
 767         }
 768     }
 769 
 770     // Comparison and hashing
 771 
 772     /**
 773      * Compares the specified Object with this Map for equality,
 774      * as per the definition in the Map interface.
 775      *
 776      * @param  o object to be compared for equality with this hashtable
 777      * @return true if the specified Object is equal to this Map
 778      * @see Map#equals(Object)
 779      * @since 1.2
 780      */
 781     public synchronized boolean equals(Object o) {
 782         if (o == this)
 783             return true;
 784 
 785         if (!(o instanceof Map))
 786             return false;
 787         Map<?,?> t = (Map<?,?>) o;
 788         if (t.size() != size())
 789             return false;
 790 
 791         try {
 792             Iterator<Map.Entry<K,V>> i = entrySet().iterator();
 793             while (i.hasNext()) {
 794                 Map.Entry<K,V> e = i.next();
 795                 K key = e.getKey();
 796                 V value = e.getValue();
 797                 if (value == null) {
 798                     if (!(t.get(key)==null && t.containsKey(key)))
 799                         return false;
 800                 } else {
 801                     if (!value.equals(t.get(key)))
 802                         return false;
 803                 }
 804             }
 805         } catch (ClassCastException unused)   {
 806             return false;
 807         } catch (NullPointerException unused) {
 808             return false;
 809         }
 810 
 811         return true;
 812     }
 813 
 814     /**
 815      * Returns the hash code value for this Map as per the definition in the
 816      * Map interface.
 817      *
 818      * @see Map#hashCode()
 819      * @since 1.2
 820      */
 821     public synchronized int hashCode() {
 822         /*
 823          * This code detects the recursion caused by computing the hash code
 824          * of a self-referential hash table and prevents the stack overflow
 825          * that would otherwise result.  This allows certain 1.1-era
 826          * applets with self-referential hash tables to work.  This code
 827          * abuses the loadFactor field to do double-duty as a hashCode
 828          * in progress flag, so as not to worsen the space performance.
 829          * A negative load factor indicates that hash code computation is
 830          * in progress.
 831          */
 832         int h = 0;
 833         if (count == 0 || loadFactor < 0)
 834             return h;  // Returns zero
 835 
 836         loadFactor = -loadFactor;  // Mark hashCode computation in progress
 837         Entry<?,?>[] tab = table;
 838         for (int i = 0; i < tab.length; i++)
 839             for (Entry<?,?> e = tab[i]; e != null; e = e.next)
 840                 h += e.key.hashCode() ^ e.value.hashCode();




 841         loadFactor = -loadFactor;  // Mark hashCode computation complete
 842 
 843         return h;
 844     }
 845 
 846     /**
 847      * Save the state of the Hashtable to a stream (i.e., serialize it).
 848      *
 849      * @serialData The <i>capacity</i> of the Hashtable (the length of the
 850      *             bucket array) is emitted (int), followed by the
 851      *             <i>size</i> of the Hashtable (the number of key-value
 852      *             mappings), followed by the key (Object) and value (Object)
 853      *             for each key-value mapping represented by the Hashtable
 854      *             The key-value mappings are emitted in no particular order.
 855      */
 856     private void writeObject(java.io.ObjectOutputStream s)
 857             throws IOException {
 858         Entry<Object, Object> entryStack = null;
 859 
 860         synchronized (this) {
 861             // Write out the length, threshold, loadfactor
 862             s.defaultWriteObject();
 863 
 864             // Write out length, count of elements
 865             s.writeInt(table.length);
 866             s.writeInt(count);
 867 
 868             // Stack copies of the entries in the table
 869             for (int index = 0; index < table.length; index++) {
 870                 Entry<?,?> entry = table[index];
 871 
 872                 while (entry != null) {
 873                     entryStack =
 874                         new Entry<>(0, entry.key, entry.value, entryStack);
 875                     entry = entry.next;
 876                 }
 877             }
 878         }
 879 
 880         // Write out the key/value objects from the stacked entries
 881         while (entryStack != null) {
 882             s.writeObject(entryStack.key);
 883             s.writeObject(entryStack.value);
 884             entryStack = entryStack.next;
 885         }
 886     }
 887 
 888     /**
 889      * Reconstitute the Hashtable from a stream (i.e., deserialize it).
 890      */
 891     private void readObject(java.io.ObjectInputStream s)
 892          throws IOException, ClassNotFoundException
 893     {
 894         // Read in the length, threshold, and loadfactor
 895         s.defaultReadObject();
 896 




 897         // Read the original length of the array and number of elements
 898         int origlength = s.readInt();
 899         int elements = s.readInt();
 900 
 901         // Compute new size with a bit of room 5% to grow but
 902         // no larger than the original size.  Make the length
 903         // odd if it's large enough, this helps distribute the entries.
 904         // Guard against the length ending up zero, that's not valid.
 905         int length = (int)(elements * loadFactor) + (elements / 20) + 3;
 906         if (length > elements && (length & 1) == 0)
 907             length--;
 908         if (origlength > 0 && length > origlength)
 909             length = origlength;
 910         Entry<?,?>[] table = new Entry<?,?>[length];

 911         count = 0;
 912 
 913         // Read the number of elements and then all the key/value objects
 914         for (; elements > 0; elements--) {
 915             @SuppressWarnings("unchecked")
 916                 K key = (K)s.readObject();
 917             @SuppressWarnings("unchecked")
 918                 V value = (V)s.readObject();
 919             // synch could be eliminated for performance
 920             reconstitutionPut(table, key, value);
 921         }
 922         this.table = table;
 923     }
 924 
 925     /**
 926      * The put method used by readObject. This is provided because put
 927      * is overridable and should not be called in readObject since the
 928      * subclass will not yet be initialized.
 929      *
 930      * <p>This differs from the regular put method in several ways. No
 931      * checking for rehashing is necessary since the number of elements
 932      * initially in the table is known. The modCount is not incremented
 933      * because we are creating a new instance. Also, no return value
 934      * is needed.
 935      */
 936     private void reconstitutionPut(Entry<?,?>[] tab, K key, V value)
 937         throws StreamCorruptedException
 938     {
 939         if (value == null) {
 940             throw new java.io.StreamCorruptedException();
 941         }
 942         // Makes sure the key is not already in the hashtable.
 943         // This should not happen in deserialized version.
 944         int hash = key.hashCode();
 945         int index = (hash & 0x7FFFFFFF) % tab.length;
 946         for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
 947             if ((e.hash == hash) && e.key.equals(key)) {
 948                 throw new java.io.StreamCorruptedException();
 949             }
 950         }
 951         // Creates the new entry.
 952         @SuppressWarnings("unchecked")
 953             Entry<K,V> e = (Entry<K,V>)tab[index];
 954         tab[index] = new Entry<>(hash, key, value, e);
 955         count++;
 956     }
 957 
 958     /**
 959      * Hashtable collision list.
 960      */
 961     private static class Entry<K,V> implements Map.Entry<K,V> {
 962         int hash;
 963         K key;
 964         V value;
 965         Entry<K,V> next;
 966 
 967         protected Entry(int hash, K key, V value, Entry<K,V> next) {
 968             this.hash = hash;
 969             this.key = key;
 970             this.value = value;
 971             this.next = next;
 972         }
 973 
 974         @SuppressWarnings("unchecked")
 975         protected Object clone() {
 976             return new Entry<>(hash, key, value,
 977                                   (next==null ? null : (Entry<K,V>) next.clone()));
 978         }
 979 
 980         // Map.Entry Ops
 981 
 982         public K getKey() {
 983             return key;
 984         }
 985 
 986         public V getValue() {
 987             return value;
 988         }
 989 
 990         public V setValue(V value) {
 991             if (value == null)
 992                 throw new NullPointerException();
 993 
 994             V oldValue = this.value;
 995             this.value = value;
 996             return oldValue;
 997         }
 998 
 999         public boolean equals(Object o) {
1000             if (!(o instanceof Map.Entry))
1001                 return false;
1002             Map.Entry<?,?> e = (Map.Entry<?,?>)o;
1003 
1004             return (key==null ? e.getKey()==null : key.equals(e.getKey())) &&
1005                (value==null ? e.getValue()==null : value.equals(e.getValue()));
1006         }
1007 
1008         public int hashCode() {
1009             return hash ^ (value==null ? 0 : value.hashCode());
1010         }
1011 
1012         public String toString() {
1013             return key.toString()+"="+value.toString();
1014         }
1015     }
1016 
1017     // Types of Enumerations/Iterations
1018     private static final int KEYS = 0;
1019     private static final int VALUES = 1;
1020     private static final int ENTRIES = 2;
1021 
1022     /**
1023      * A hashtable enumerator class.  This class implements both the
1024      * Enumeration and Iterator interfaces, but individual instances
1025      * can be created with the Iterator methods disabled.  This is necessary
1026      * to avoid unintentionally increasing the capabilities granted a user
1027      * by passing an Enumeration.
1028      */
1029     private class Enumerator<T> implements Enumeration<T>, Iterator<T> {
1030         Entry<?,?>[] table = Hashtable.this.table;
1031         int index = table.length;
1032         Entry<?,?> entry = null;
1033         Entry<?,?> lastReturned = null;
1034         int type;
1035 
1036         /**
1037          * Indicates whether this Enumerator is serving as an Iterator
1038          * or an Enumeration.  (true -> Iterator).
1039          */
1040         boolean iterator;
1041 
1042         /**
1043          * The modCount value that the iterator believes that the backing
1044          * Hashtable should have.  If this expectation is violated, the iterator
1045          * has detected concurrent modification.
1046          */
1047         protected int expectedModCount = modCount;
1048 
1049         Enumerator(int type, boolean iterator) {
1050             this.type = type;
1051             this.iterator = iterator;
1052         }
1053 
1054         public boolean hasMoreElements() {
1055             Entry<?,?> e = entry;
1056             int i = index;
1057             Entry<?,?>[] t = table;
1058             /* Use locals for faster loop iteration */
1059             while (e == null && i > 0) {
1060                 e = t[--i];
1061             }
1062             entry = e;
1063             index = i;
1064             return e != null;
1065         }
1066 
1067         @SuppressWarnings("unchecked")
1068         public T nextElement() {
1069             Entry<?,?> et = entry;
1070             int i = index;
1071             Entry<?,?>[] t = table;
1072             /* Use locals for faster loop iteration */
1073             while (et == null && i > 0) {
1074                 et = t[--i];
1075             }
1076             entry = et;
1077             index = i;
1078             if (et != null) {
1079                 Entry<?,?> e = lastReturned = entry;
1080                 entry = e.next;
1081                 return type == KEYS ? (T)e.key : (type == VALUES ? (T)e.value : (T)e);
1082             }
1083             throw new NoSuchElementException("Hashtable Enumerator");
1084         }
1085 
1086         // Iterator methods
1087         public boolean hasNext() {
1088             return hasMoreElements();
1089         }
1090 
1091         public T next() {
1092             if (modCount != expectedModCount)
1093                 throw new ConcurrentModificationException();
1094             return nextElement();
1095         }
1096 
1097         public void remove() {
1098             if (!iterator)
1099                 throw new UnsupportedOperationException();
1100             if (lastReturned == null)
1101                 throw new IllegalStateException("Hashtable Enumerator");
1102             if (modCount != expectedModCount)
1103                 throw new ConcurrentModificationException();
1104 
1105             synchronized(Hashtable.this) {
1106                 Entry<?,?>[] tab = Hashtable.this.table;
1107                 int index = (lastReturned.hash & 0x7FFFFFFF) % tab.length;
1108 
1109                 @SuppressWarnings("unchecked")
1110                 Entry<K,V> e = (Entry<K,V>)tab[index];
1111                 for(Entry<K,V> prev = null; e != null; prev = e, e = e.next) {
1112                     if (e == lastReturned) {
1113                         modCount++;
1114                         expectedModCount++;
1115                         if (prev == null)
1116                             tab[index] = e.next;
1117                         else
1118                             prev.next = e.next;
1119                         count--;
1120                         lastReturned = null;
1121                         return;
1122                     }
1123                 }
1124                 throw new ConcurrentModificationException();
1125             }
1126         }
1127     }
1128 }
--- EOF ---