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  • JDK7之HashMap源码

    并发场景下使用HashMap的问题分析:疫苗:Java HashMap的死循环

    http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6423457

    源码来自:HashMap.java

    /*
     * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
     * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
     *
     * This code is free software; you can redistribute it and/or modify it
     * under the terms of the GNU General Public License version 2 only, as
     * published by the Free Software Foundation.  Oracle designates this
     * particular file as subject to the "Classpath" exception as provided
     * by Oracle in the LICENSE file that accompanied this code.
     *
     * This code is distributed in the hope that it will be useful, but WITHOUT
     * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
     * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
     * version 2 for more details (a copy is included in the LICENSE file that
     * accompanied this code).
     *
     * You should have received a copy of the GNU General Public License version
     * 2 along with this work; if not, write to the Free Software Foundation,
     * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
     *
     * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
     * or visit www.oracle.com if you need additional information or have any
     * questions.
     */
    
    package java.util;
    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<>(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<>(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;   }
    }
    
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  • 原文地址:https://www.cnblogs.com/liqipeng/p/7759627.html
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