HashMap.java

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    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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    * 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
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    * You should have received a copy of the GNU General Public License version
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   //package java.util;
   import java.io;
   
   /**
    * Hash table based implementation of the <tt>Map</tt> interface.  This
    * implementation provides all of the optional map operations, and permits
    * <tt>null</tt> values and the <tt>null</tt> key.  (The <tt>HashMap</tt>
    * class is roughly equivalent to <tt>Hashtable</tt>, except that it is
    * unsynchronized and permits nulls.)  This class makes no guarantees as to
    * the order of the map; in particular, it does not guarantee that the order
    * will remain constant over time.
    *
    * <p>This implementation provides constant-time performance for the basic
    * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
    * disperses the elements properly among the buckets.  Iteration over
    * collection views requires time proportional to the "capacity" of the
    * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
    * of key-value mappings).  Thus, it's very important not to set the initial
    * capacity too high (or the load factor too low) if iteration performance is
    * important.
    *
    * <p>An instance of <tt>HashMap</tt> has two parameters that affect its
    * performance: <i>initial capacity</i> and <i>load factor</i>.  The
    * <i>capacity</i> is the number of buckets in the hash table, and the initial
    * capacity is simply the capacity at the time the hash table is created.  The
    * <i>load factor</i> is a measure of how full the hash table is allowed to
    * get before its capacity is automatically increased.  When the number of
    * entries in the hash table exceeds the product of the load factor and the
    * current capacity, the hash table is <i>rehashed</i> (that is, internal data
    * structures are rebuilt) so that the hash table has approximately twice the
    * number of buckets.
    *
    * <p>As a general rule, the default load factor (.75) offers a good tradeoff
    * between time and space costs.  Higher values decrease the space overhead
    * but increase the lookup cost (reflected in most of the operations of the
    * <tt>HashMap</tt> class, including <tt>get</tt> and <tt>put</tt>).  The
    * expected number of entries in the map and its load factor should be taken
    * into account when setting its initial capacity, so as to minimize the
    * number of rehash operations.  If the initial capacity is greater
    * than the maximum number of entries divided by the load factor, no
    * rehash operations will ever occur.
    *
    * <p>If many mappings are to be stored in a <tt>HashMap</tt> instance,
    * creating it with a sufficiently large capacity will allow the mappings to
    * be stored more efficiently than letting it perform automatic rehashing as
    * needed to grow the table.
    *
    * <p><strong>Note that this implementation is not synchronized.</strong>
    * If multiple threads access a hash map concurrently, and at least one of
    * the threads modifies the map structurally, it <i>must</i> be
    * synchronized externally.  (A structural modification is any operation
    * that adds or deletes one or more mappings; merely changing the value
    * associated with a key that an instance already contains is not a
    * structural modification.)  This is typically accomplished by
    * synchronizing on some object that naturally encapsulates the map.
    *
    * If no such object exists, the map should be "wrapped" using the
    * {@link Collections#synchronizedMap Collections.synchronizedMap}
    * method.  This is best done at creation time, to prevent accidental
    * unsynchronized access to the map:<pre>
    *   Map m = Collections.synchronizedMap(new HashMap(...));</pre>
    *
    * <p>The iterators returned by all of this class's "collection view methods"
    * are <i>fail-fast</i>: if the map is structurally modified at any time after
    * the iterator is created, in any way except through the iterator's own
    * <tt>remove</tt> method, the iterator will throw a
    * {@link ConcurrentModificationException}.  Thus, in the face of concurrent
    * modification, the iterator fails quickly and cleanly, rather than risking
    * arbitrary, non-deterministic behavior at an undetermined time in the
    * future.
    *
    * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
    * as it is, generally speaking, impossible to make any hard guarantees in the
    * presence of unsynchronized concurrent modification.  Fail-fast iterators
    * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
    * Therefore, it would be wrong to write a program that depended on this
    * exception for its correctness: <i>the fail-fast behavior of iterators
    * should be used only to detect bugs.</i>
    *
    * <p>This class is a member of the
    * <a href="{@docRoot}/../technotes/guides/collections/index.html">
    * Java Collections Framework</a>.
    *
    * @param <K> the type of keys maintained by this map
    * @param <V> the type of mapped values
    *
    * @author  Doug Lea
    * @author  Josh Bloch
    * @author  Arthur van Hoff
    * @author  Neal Gafter
    * @see     Object#hashCode()
    * @see     Collection
    * @see     Map
    * @see     TreeMap
    * @see     Hashtable
    * @since   1.2
    */
   
   public class HashMap<K,V>
       extends AbstractMap<K,V>
       implements Map<K,V>, Cloneable, Serializable
   {
   
       /**
        * The default initial capacity - MUST be a power of two.
        */
       static final int DEFAULT_INITIAL_CAPACITY = 16;
   
       /**
        * The maximum capacity, used if a higher value is implicitly specified
        * by either of the constructors with arguments.
        * MUST be a power of two <= 1<<30.
        */
       static final int MAXIMUM_CAPACITY = 1 << 30;
   
       /**
        * The load factor used when none specified in constructor.
        */
       static final float DEFAULT_LOAD_FACTOR = 0.75f;
   
       /**
        * The table, resized as necessary. Length MUST Always be a power of two.
        */
       transient Entry[] table;
   
       /**
        * The number of key-value mappings contained in this map.
        */
       transient int size;
   
       /**
        * The next size value at which to resize (capacity * load factor).
        * @serial
        */
       int threshold;
   
       /**
        * The load factor for the hash table.
        *
        * @serial
        */
       final float loadFactor;
   
       /**
        * The number of times this HashMap has been structurally modified
        * Structural modifications are those that change the number of mappings in
        * the HashMap or otherwise modify its internal structure (e.g.,
        * rehash).  This field is used to make iterators on Collection-views of
        * the HashMap fail-fast.  (See ConcurrentModificationException).
        */
       transient int modCount;
   
       /**
        * Constructs an empty <tt>HashMap</tt> with the specified initial
        * capacity and load factor.
        *
        * @param  initialCapacity the initial capacity
        * @param  loadFactor      the load factor
        * @throws IllegalArgumentException if the initial capacity is negative
        *         or the load factor is nonpositive
        */
       public HashMap(int initialCapacity, float loadFactor) {
           if (initialCapacity < 0)
               throw new IllegalArgumentException("Illegal initial capacity: " +
                                                  initialCapacity);
           if (initialCapacity > MAXIMUM_CAPACITY)
               initialCapacity = MAXIMUM_CAPACITY;
           if (loadFactor <= 0 || Float.isNaN(loadFactor))
               throw new IllegalArgumentException("Illegal load factor: " +
                                                  loadFactor);
   
           // Find a power of 2 >= initialCapacity
           int capacity = 1;
           while (capacity < initialCapacity)
               capacity <<= 1;
   
           this.loadFactor = loadFactor;
           threshold = (int)(capacity * loadFactor);
           table = new Entry[capacity];
           init();
       }
   
       /**
        * Constructs an empty <tt>HashMap</tt> with the specified initial
        * capacity and the default load factor (0.75).
        *
        * @param  initialCapacity the initial capacity.
        * @throws IllegalArgumentException if the initial capacity is negative.
        */
       public HashMap(int initialCapacity) {
           this(initialCapacity, DEFAULT_LOAD_FACTOR);
       }
   
       /**
        * Constructs an empty <tt>HashMap</tt> with the default initial capacity
        * (16) and the default load factor (0.75).
        */
       public HashMap() {
           this.loadFactor = DEFAULT_LOAD_FACTOR;
           threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
           table = new Entry[DEFAULT_INITIAL_CAPACITY];
           init();
       }
   
       /**
        * Constructs a new <tt>HashMap</tt> with the same mappings as the
        * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with
        * default load factor (0.75) and an initial capacity sufficient to
        * hold the mappings in the specified <tt>Map</tt>.
        *
        * @param   m the map whose mappings are to be placed in this map
        * @throws  NullPointerException if the specified map is null
        */
       public HashMap(Map<? extends K, ? extends V> m) {
           this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                         DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
           putAllForCreate(m);
       }
   
       // internal utilities
   
       /**
        * Initialization hook for subclasses. This method is called
        * in all constructors and pseudo-constructors (clone, readObject)
        * after HashMap has been initialized but before any entries have
        * been inserted.  (In the absence of this method, readObject would
        * require explicit knowledge of subclasses.)
        */
       void init() {
       }
   
       /**
        * Applies a supplemental hash function to a given hashCode, which
        * defends against poor quality hash functions.  This is critical
        * because HashMap uses power-of-two length hash tables, that
        * otherwise encounter collisions for hashCodes that do not differ
        * in lower bits. Note: Null keys always map to hash 0, thus index 0.
        */
       static int hash(int h) {
           // This function ensures that hashCodes that differ only by
           // constant multiples at each bit position have a bounded
           // number of collisions (approximately 8 at default load factor).
           h ^= (h >>> 20) ^ (h >>> 12);
           return h ^ (h >>> 7) ^ (h >>> 4);
       }
   
       /**
        * Returns index for hash code h.
        */
       static int indexFor(int h, int length) {
           return h & (length-1);
       }
   
       /**
        * Returns the number of key-value mappings in this map.
        *
        * @return the number of key-value mappings in this map
        */
       public int size() {
           return size;
       }
   
       /**
        * Returns <tt>true</tt> if this map contains no key-value mappings.
        *
        * @return <tt>true</tt> if this map contains no key-value mappings
        */
       public boolean isEmpty() {
           return size == 0;
       }
   
       /**
        * Returns the value to which the specified key is mapped,
        * or {@code null} if this map contains no mapping for the key.
        *
        * <p>More formally, if this map contains a mapping from a key
        * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
        * key.equals(k))}, then this method returns {@code v}; otherwise
        * it returns {@code null}.  (There can be at most one such mapping.)
        *
        * <p>A return value of {@code null} does not <i>necessarily</i>
        * indicate that the map contains no mapping for the key; it's also
        * possible that the map explicitly maps the key to {@code null}.
        * The {@link #containsKey containsKey} operation may be used to
        * distinguish these two cases.
        *
        * @see #put(Object, Object)
        */
       public V get(Object key) {
           if (key == null)
               return getForNullKey();
           int hash = hash(key.hashCode());
           for (Entry<K,V> e = table[indexFor(hash, table.length)];
                e != null;
                e = e.next) {
               Object k;
               if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
                   return e.value;
           }
           return null;
       }
   
       /**
        * Offloaded version of get() to look up null keys.  Null keys map
        * to index 0.  This null case is split out into separate methods
        * for the sake of performance in the two most commonly used
        * operations (get and put), but incorporated with conditionals in
        * others.
        */
       private V getForNullKey() {
           for (Entry<K,V> e = table[0]; e != null; e = e.next) {
               if (e.key == null)
                   return e.value;
           }
           return null;
       }
   
       /**
        * Returns <tt>true</tt> if this map contains a mapping for the
        * specified key.
        *
        * @param   key   The key whose presence in this map is to be tested
        * @return <tt>true</tt> if this map contains a mapping for the specified
        * key.
        */
       public boolean containsKey(Object key) {
           return getEntry(key) != null;
       }
   
       /**
        * Returns the entry associated with the specified key in the
        * HashMap.  Returns null if the HashMap contains no mapping
        * for the key.
        */
       final Entry<K,V> getEntry(Object key) {
           int hash = (key == null) ? 0 : hash(key.hashCode());
           for (Entry<K,V> e = table[indexFor(hash, table.length)];
                e != null;
                e = e.next) {
               Object k;
               if (e.hash == hash &&
                   ((k = e.key) == key || (key != null && key.equals(k))))
                   return e;
           }
           return null;
       }
   
   
       /**
        * Associates the specified value with the specified key in this map.
        * If the map previously contained a mapping for the key, the old
        * value is replaced.
        *
        * @param key key with which the specified value is to be associated
        * @param value value to be associated with the specified key
        * @return the previous value associated with <tt>key</tt>, or
        *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
        *         (A <tt>null</tt> return can also indicate that the map
        *         previously associated <tt>null</tt> with <tt>key</tt>.)
        */
       public V put(K key, V value) {
           if (key == null)
               return putForNullKey(value);
           int hash = hash(key.hashCode());
           int i = indexFor(hash, table.length);
           for (Entry<K,V> e = table[i]; e != null; e = e.next) {
               Object k;
               if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                   V oldValue = e.value;
                   e.value = value;
                   e.recordAccess(this);
                   return oldValue;
               }
           }
   
           modCount++;
           addEntry(hash, key, value, i);
           return null;
       }
   
       /**
        * Offloaded version of put for null keys
        */
       private V putForNullKey(V value) {
           for (Entry<K,V> e = table[0]; e != null; e = e.next) {
               if (e.key == null) {
                   V oldValue = e.value;
                   e.value = value;
                   e.recordAccess(this);
                   return oldValue;
               }
           }
           modCount++;
           addEntry(0, null, value, 0);
           return null;
       }
   
       /**
        * This method is used instead of put by constructors and
        * pseudoconstructors (clone, readObject).  It does not resize the table,
        * check for comodification, etc.  It calls createEntry rather than
        * addEntry.
        */
       private void putForCreate(K key, V value) {
           int hash = (key == null) ? 0 : hash(key.hashCode());
           int i = indexFor(hash, table.length);
   
           /**
            * Look for preexisting entry for key.  This will never happen for
            * clone or deserialize.  It will only happen for construction if the
            * input Map is a sorted map whose ordering is inconsistent w/ equals.
            */
           for (Entry<K,V> e = table[i]; e != null; e = e.next) {
               Object k;
               if (e.hash == hash &&
                   ((k = e.key) == key || (key != null && key.equals(k)))) {
                   e.value = value;
                   return;
               }
           }
   
           createEntry(hash, key, value, i);
       }
   
       private void putAllForCreate(Map<? extends K, ? extends V> m) {
           for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
               putForCreate(e.getKey(), e.getValue());
       }
   
       /**
        * Rehashes the contents of this map into a new array with a
        * larger capacity.  This method is called automatically when the
        * number of keys in this map reaches its threshold.
        *
        * If current capacity is MAXIMUM_CAPACITY, this method does not
        * resize the map, but sets threshold to Integer.MAX_VALUE.
        * This has the effect of preventing future calls.
        *
        * @param newCapacity the new capacity, MUST be a power of two;
        *        must be greater than current capacity unless current
        *        capacity is MAXIMUM_CAPACITY (in which case value
        *        is irrelevant).
        */
       void resize(int newCapacity) {
           Entry[] oldTable = table;
           int oldCapacity = oldTable.length;
           if (oldCapacity == MAXIMUM_CAPACITY) {
               threshold = Integer.MAX_VALUE;
               return;
           }
   
           Entry[] newTable = new Entry[newCapacity];
           transfer(newTable);
           table = newTable;
           threshold = (int)(newCapacity * loadFactor);
       }
   
       /**
        * Transfers all entries from current table to newTable.
        */
       void transfer(Entry[] newTable) {
           Entry[] src = table;
           int newCapacity = newTable.length;
           for (int j = 0; j < src.length; j++) {
               Entry<K,V> e = src[j];
               if (e != null) {
                   src[j] = null;
                   do {
                       Entry<K,V> next = e.next;
                       int i = indexFor(e.hash, newCapacity);
                       e.next = newTable[i];
                       newTable[i] = e;
                       e = next;
                   } while (e != null);
               }
           }
       }
   
       /**
        * Copies all of the mappings from the specified map to this map.
        * These mappings will replace any mappings that this map had for
        * any of the keys currently in the specified map.
        *
        * @param m mappings to be stored in this map
        * @throws NullPointerException if the specified map is null
        */
       public void putAll(Map<? extends K, ? extends V> m) {
           int numKeysToBeAdded = m.size();
           if (numKeysToBeAdded == 0)
               return;
   
           /*
            * Expand the map if the map if the number of mappings to be added
            * is greater than or equal to threshold.  This is conservative; the
            * obvious condition is (m.size() + size) >= threshold, but this
            * condition could result in a map with twice the appropriate capacity,
            * if the keys to be added overlap with the keys already in this map.
            * By using the conservative calculation, we subject ourself
            * to at most one extra resize.
            */
           if (numKeysToBeAdded > threshold) {
               int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
               if (targetCapacity > MAXIMUM_CAPACITY)
                   targetCapacity = MAXIMUM_CAPACITY;
               int newCapacity = table.length;
               while (newCapacity < targetCapacity)
                   newCapacity <<= 1;
               if (newCapacity > table.length)
                   resize(newCapacity);
           }
   
           for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
               put(e.getKey(), e.getValue());
       }
   
       /**
        * Removes the mapping for the specified key from this map if present.
        *
        * @param  key key whose mapping is to be removed from the map
        * @return the previous value associated with <tt>key</tt>, or
        *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
        *         (A <tt>null</tt> return can also indicate that the map
        *         previously associated <tt>null</tt> with <tt>key</tt>.)
        */
       public V remove(Object key) {
           Entry<K,V> e = removeEntryForKey(key);
           return (e == null ? null : e.value);
       }
   
       /**
        * Removes and returns the entry associated with the specified key
        * in the HashMap.  Returns null if the HashMap contains no mapping
        * for this key.
        */
       final Entry<K,V> removeEntryForKey(Object key) {
           int hash = (key == null) ? 0 : hash(key.hashCode());
           int i = indexFor(hash, table.length);
           Entry<K,V> prev = table[i];
           Entry<K,V> e = prev;
   
           while (e != null) {
               Entry<K,V> next = e.next;
               Object k;
               if (e.hash == hash &&
                   ((k = e.key) == key || (key != null && key.equals(k)))) {
                   modCount++;
                   size--;
                   if (prev == e)
                       table[i] = next;
                   else
                       prev.next = next;
                   e.recordRemoval(this);
                   return e;
               }
               prev = e;
               e = next;
           }
   
           return e;
       }
   
       /**
        * Special version of remove for EntrySet.
        */
       final Entry<K,V> removeMapping(Object o) {
           if (!(o instanceof Map.Entry))
               return null;
   
           Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
           Object key = entry.getKey();
           int hash = (key == null) ? 0 : hash(key.hashCode());
           int i = indexFor(hash, table.length);
           Entry<K,V> prev = table[i];
           Entry<K,V> e = prev;
   
           while (e != null) {
               Entry<K,V> next = e.next;
               if (e.hash == hash && e.equals(entry)) {
                   modCount++;
                   size--;
                   if (prev == e)
                       table[i] = next;
                   else
                       prev.next = next;
                   e.recordRemoval(this);
                   return e;
               }
               prev = e;
               e = next;
           }
   
           return e;
       }
   
       /**
        * Removes all of the mappings from this map.
        * The map will be empty after this call returns.
        */
       public void clear() {
           modCount++;
           Entry[] tab = table;
           for (int i = 0; i < tab.length; i++)
               tab[i] = null;
           size = 0;
       }
   
       /**
        * Returns <tt>true</tt> if this map maps one or more keys to the
        * specified value.
        *
        * @param value value whose presence in this map is to be tested
        * @return <tt>true</tt> if this map maps one or more keys to the
        *         specified value
        */
       public boolean containsValue(Object value) {
           if (value == null)
               return containsNullValue();
   
           Entry[] tab = table;
           for (int i = 0; i < tab.length ; i++)
               for (Entry e = tab[i] ; e != null ; e = e.next)
                   if (value.equals(e.value))
                       return true;
           return false;
       }
   
       /**
        * Special-case code for containsValue with null argument
        */
       private boolean containsNullValue() {
           Entry[] tab = table;
           for (int i = 0; i < tab.length ; i++)
               for (Entry e = tab[i] ; e != null ; e = e.next)
                   if (e.value == null)
                       return true;
           return false;
       }
   
       /**
        * Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and
        * values themselves are not cloned.
        *
        * @return a shallow copy of this map
        */
       public Object clone() {
           HashMap<K,V> result = null;
           try {
               result = (HashMap<K,V>)super.clone();
           } catch (CloneNotSupportedException e) {
               // assert false;
           }
           result.table = new Entry[table.length];
           result.entrySet = null;
           result.modCount = 0;
           result.size = 0;
           result.init();
           result.putAllForCreate(this);
   
           return result;
       }
   
       static class Entry<K,V> implements Map.Entry<K,V> {
           final K key;
           V value;
           Entry<K,V> next;
           final int hash;
   
           /**
            * Creates new entry.
            */
           Entry(int h, K k, V v, Entry<K,V> n) {
               value = v;
               next = n;
               key = k;
               hash = h;
           }
   
           public final K getKey() {
               return key;
           }
   
           public final V getValue() {
               return value;
           }
   
           public final V setValue(V newValue) {
               V oldValue = value;
               value = newValue;
               return oldValue;
           }
   
           public final boolean equals(Object o) {
               if (!(o instanceof Map.Entry))
                   return false;
               Map.Entry e = (Map.Entry)o;
               Object k1 = getKey();
               Object k2 = e.getKey();
               if (k1 == k2 || (k1 != null && k1.equals(k2))) {
                   Object v1 = getValue();
                   Object v2 = e.getValue();
                   if (v1 == v2 || (v1 != null && v1.equals(v2)))
                       return true;
               }
               return false;
           }
   
           public final int hashCode() {
               return (key==null   ? 0 : key.hashCode()) ^
                      (value==null ? 0 : value.hashCode());
           }
   
           public final String toString() {
               return getKey() + "=" + getValue();
           }
   
           /**
            * This method is invoked whenever the value in an entry is
            * overwritten by an invocation of put(k,v) for a key k that's already
            * in the HashMap.
            */
           void recordAccess(HashMap<K,V> m) {
           }
   
           /**
            * This method is invoked whenever the entry is
            * removed from the table.
            */
           void recordRemoval(HashMap<K,V> m) {
           }
       }
   
       /**
        * Adds a new entry with the specified key, value and hash code to
        * the specified bucket.  It is the responsibility of this
        * method to resize the table if appropriate.
        *
        * Subclass overrides this to alter the behavior of put method.
        */
       void addEntry(int hash, K key, V value, int bucketIndex) {
           Entry<K,V> e = table[bucketIndex];
           table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
           if (size++ >= threshold)
               resize(2 * table.length);
       }
   
       /**
        * Like addEntry except that this version is used when creating entries
        * as part of Map construction or "pseudo-construction" (cloning,
        * deserialization).  This version needn't worry about resizing the table.
        *
        * Subclass overrides this to alter the behavior of HashMap(Map),
        * clone, and readObject.
        */
       void createEntry(int hash, K key, V value, int bucketIndex) {
           Entry<K,V> e = table[bucketIndex];
           table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
           size++;
       }
   
       private abstract class HashIterator<E> implements Iterator<E> {
           Entry<K,V> next;        // next entry to return
           int expectedModCount;   // For fast-fail
           int index;              // current slot
           Entry<K,V> current;     // current entry
   
           HashIterator() {
               expectedModCount = modCount;
               if (size > 0) { // advance to first entry
                   Entry[] t = table;
                   while (index < t.length && (next = t[index++]) == null)
                       ;
               }
           }
   
           public final boolean hasNext() {
               return next != null;
           }
   
           final Entry<K,V> nextEntry() {
               if (modCount != expectedModCount)
                   throw new ConcurrentModificationException();
               Entry<K,V> e = next;
               if (e == null)
                   throw new NoSuchElementException();
   
               if ((next = e.next) == null) {
                   Entry[] t = table;
                   while (index < t.length && (next = t[index++]) == null)
                       ;
               }
               current = e;
               return e;
           }
   
           public void remove() {
               if (current == null)
                   throw new IllegalStateException();
               if (modCount != expectedModCount)
                   throw new ConcurrentModificationException();
               Object k = current.key;
               current = null;
               HashMap.this.removeEntryForKey(k);
               expectedModCount = modCount;
           }
   
       }
   
       private final class ValueIterator extends HashIterator<V> {
           public V next() {
               return nextEntry().value;
           }
       }
   
       private final class KeyIterator extends HashIterator<K> {
           public K next() {
               return nextEntry().getKey();
           }
       }
   
       private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
           public Map.Entry<K,V> next() {
               return nextEntry();
           }
       }
   
       // Subclass overrides these to alter behavior of views' iterator() method
       Iterator<K> newKeyIterator()   {
           return new KeyIterator();
       }
       Iterator<V> newValueIterator()   {
           return new ValueIterator();
       }
       Iterator<Map.Entry<K,V>> newEntryIterator()   {
           return new EntryIterator();
       }
   
   
       // Views
   
       private transient Set<Map.Entry<K,V>> entrySet = null;
   
       /**
        * Returns a {@link Set} view of the keys contained in this map.
        * The set is backed by the map, so changes to the map are
        * reflected in the set, and vice-versa.  If the map is modified
        * while an iteration over the set is in progress (except through
        * the iterator's own <tt>remove</tt> operation), the results of
        * the iteration are undefined.  The set supports element removal,
        * which removes the corresponding mapping from the map, via the
        * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
        * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
        * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
        * operations.
        */
       public Set<K> keySet() {
           Set<K> ks = keySet;
           return (ks != null ? ks : (keySet = new KeySet()));
       }
   
       private final class KeySet extends AbstractSet<K> {
           public Iterator<K> iterator() {
               return newKeyIterator();
           }
           public int size() {
               return size;
           }
           public boolean contains(Object o) {
               return containsKey(o);
           }
           public boolean remove(Object o) {
               return HashMap.this.removeEntryForKey(o) != null;
           }
           public void clear() {
               HashMap.this.clear();
           }
       }
   
       /**
        * Returns a {@link Collection} view of the values contained in this map.
        * The collection is backed by the map, so changes to the map are
        * reflected in the collection, and vice-versa.  If the map is
        * modified while an iteration over the collection is in progress
        * (except through the iterator's own <tt>remove</tt> operation),
        * the results of the iteration are undefined.  The collection
        * supports element removal, which removes the corresponding
        * mapping from the map, via the <tt>Iterator.remove</tt>,
        * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
        * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
        * support the <tt>add</tt> or <tt>addAll</tt> operations.
        */
       public Collection<V> values() {
           Collection<V> vs = values;
           return (vs != null ? vs : (values = new Values()));
       }
   
       private final class Values extends AbstractCollection<V> {
           public Iterator<V> iterator() {
               return newValueIterator();
           }
           public int size() {
               return size;
           }
           public boolean contains(Object o) {
               return containsValue(o);
           }
           public void clear() {
               HashMap.this.clear();
           }
       }
   
       /**
        * Returns a {@link Set} view of the mappings contained in this map.
        * The set is backed by the map, so changes to the map are
        * reflected in the set, and vice-versa.  If the map is modified
        * while an iteration over the set is in progress (except through
        * the iterator's own <tt>remove</tt> operation, or through the
        * <tt>setValue</tt> operation on a map entry returned by the
        * iterator) the results of the iteration are undefined.  The set
        * supports element removal, which removes the corresponding
        * mapping from the map, via the <tt>Iterator.remove</tt>,
        * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
        * <tt>clear</tt> operations.  It does not support the
        * <tt>add</tt> or <tt>addAll</tt> operations.
        *
        * @return a set view of the mappings contained in this map
        */
       public Set<Map.Entry<K,V>> entrySet() {
           return entrySet0();
       }
   
       private Set<Map.Entry<K,V>> entrySet0() {
           Set<Map.Entry<K,V>> es = entrySet;
           return es != null ? es : (entrySet = new EntrySet());
       }
   
       private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
           public Iterator<Map.Entry<K,V>> iterator() {
               return newEntryIterator();
           }
           public boolean contains(Object o) {
               if (!(o instanceof Map.Entry))
                   return false;
               Map.Entry<K,V> e = (Map.Entry<K,V>) o;
               Entry<K,V> candidate = getEntry(e.getKey());
               return candidate != null && candidate.equals(e);
           }
           public boolean remove(Object o) {
               return removeMapping(o) != null;
           }
           public int size() {
               return size;
           }
           public void clear() {
               HashMap.this.clear();
           }
       }
   
       /**
        * Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,
        * serialize it).
        *
        * @serialData The <i>capacity</i> of the HashMap (the length of the
        *             bucket array) is emitted (int), followed by the
        *             <i>size</i> (an int, the number of key-value
        *             mappings), followed by the key (Object) and value (Object)
        *             for each key-value mapping.  The key-value mappings are
        *             emitted in no particular order.
        */
       private void writeObject(java.io.ObjectOutputStream s)
           throws IOException
       {
           Iterator<Map.Entry<K,V>> i =
               (size > 0) ? entrySet0().iterator() : null;
   
           // Write out the threshold, loadfactor, and any hidden stuff
           s.defaultWriteObject();
   
           // Write out number of buckets
           s.writeInt(table.length);
   
           // Write out size (number of Mappings)
           s.writeInt(size);
   
           // Write out keys and values (alternating)
           if (i != null) {
               while (i.hasNext()) {
                   Map.Entry<K,V> e = i.next();
                   s.writeObject(e.getKey());
                   s.writeObject(e.getValue());
               }
           }
       }
   
       private static final long serialVersionUID = 362498820763181265L;
   
       /**
        * Reconstitute the <tt>HashMap</tt> instance from a stream (i.e.,
        * deserialize it).
        */
       private void readObject(java.io.ObjectInputStream s)
            throws IOException, ClassNotFoundException
       {
           // Read in the threshold, loadfactor, and any hidden stuff
           s.defaultReadObject();
   
           // Read in number of buckets and allocate the bucket array;
           int numBuckets = s.readInt();
           table = new Entry[numBuckets];
   
           init();  // Give subclass a chance to do its thing.
   
           // Read in size (number of Mappings)
           int size = s.readInt();
   
           // Read the keys and values, and put the mappings in the HashMap
           for (int i=0; i<size; i++) {
               K key = (K) s.readObject();
               V value = (V) s.readObject();
               putForCreate(key, value);
           }
       }
   
       // These methods are used when serializing HashSets
       int   capacity()     { return table.length; }
       float loadFactor()   { return loadFactor;   }
   }