HashMap源码

/* *  @(#)HashMap.java1.73 07/03/13 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */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 * @version 1.73, 03/13/07 * @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 volatile 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 (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext(); ) {            Map.Entry<? extends K, ? extends V> e = i.next();            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 (Iterator<? extends Map.Entry<? extends K, ? extends V>> i = m.entrySet().iterator(); i.hasNext(); ) {            Map.Entry<? extends K, ? extends V> e = i.next();            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 stuffs.defaultWriteObject();// Write out number of bucketss.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 stuffs.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 HashMapfor (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;   }}