HashMap源码分析

本文基于JDK1.7.0_79的版本进行分析. 注释比较详细. 若有不明白的地方可以指出, 我再进行细化.

public class HashMap<K,V>    extends AbstractMap<K,V>    implements Map<K,V>, Cloneable, Serializable{    /**     * 默认容量为16     */    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16    /**     * 最大容量为2^30. 超过最大容量则, 自动转换为2^30     */    static final int MAXIMUM_CAPACITY = 1 << 30;    /**     * 默认负载因子为0.75     */    static final float DEFAULT_LOAD_FACTOR = 0.75f;    /**     * 空的Entry数组, 初始化使用     */    static final Entry<?,?>[] EMPTY_TABLE = {};    /**     * Entry数组, 用于存放HashMap中的数据, 其大小必须为2的幂     * HashMap采用Hash表的方式存放数据, 若出现hash碰撞, 则采用单向链表的方式进行存储.     * 具体的实现方式在下面介绍     */    transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;    /**     * Map的实际大小     */    transient int size;    // 阀值, 当size超过该阀值时, 自动扩容量.    int threshold;    /**     * 负载因子     */    final float loadFactor;    /**     * HashMap的修改次数. 仅用于采用iterator进行遍历时, 若发现modCount不同, 则直接报错. 即fail-fast模式.     */    transient int modCount;    /**     * 默认构造函数.     * 默认容量为16     * 默认负载因子为0.75     * 默认的阀值是16*0.75=12     * 即当HashMap的size>12时, 则会自动扩充容量. 扩充后的容量=当前容量*2     * 在JDK1.7中HashMap中table在第一次put数据时才会被初始化, 具体初始化过程后面进行详细介绍     */    public HashMap() {        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);    }    /**     * 带有初始容量及负载因子的构造器     */    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);        this.loadFactor = loadFactor;        threshold = initialCapacity;        init();    }    /**     * 在HashMap子类中, 可以通过该方法进行一些初始化或扩展. 熟称Hook(钩子).     * 因为是包访问权限, 我们是用不到了. 有兴趣的可以看下LinkedHashMap中的实现.     */    void init() {    }    /**     * 带有初始容量的构造器     */    public HashMap(int initialCapacity) {        this(initialCapacity, DEFAULT_LOAD_FACTOR);    }    /**     * 我们来先看下put方法. 方法中涉及的其他方法将按照调用顺序逐个进行说明.     */    public V put(K key, V value) {        //在put时进行初始化        //因为HashMap是非线程安全的, 所以如果在多线程中使用可能会导致数据被初始化多次.        //最终导致数据丢失.        if (table == EMPTY_TABLE) {            inflateTable(threshold);        }        //key==null时, 把该值put到table[0]中        //在HashMap中仅存在一个key=null的数据        if (key == null)            return putForNullKey(value);        //rehash, 重新计算key的hash值        int hash = hash(key);        //找到hash在table中的位置(索引)        int i = indexFor(hash, table.length);        //因为Hash表中的数据是采用单向链表的方式存储的, 所以这里通过索引取出该单向链表,        //然后逐个比对.        for (Entry<K,V> e = table[i]; e != null; e = e.next) {            Object k;            //值已经存在, 则替换value            //判断方式为:            // 1) 先判断hash值是否相同. 注: hash值相同key不一定相同            // 2) 再判断key是否相等            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {                V oldValue = e.value;                e.value = value;                //空方法, 只在put时调用.                e.recordAccess(this);                return oldValue;            }        }        //modCount++, 记录数据修改次数. 方便做fail-fast        modCount++;        //若不存在该实体, 则创建新的Entry        addEntry(hash, key, value, i);        return null;    }    /**     * 往table中put key==null的值.     * 一个HashMap中只存在一个key==null的值, 且存放在table[0]中     */    private V putForNullKey(V value) {        for (Entry<K,V> e = table[0]; e != null; e = e.next) {            //key已经存在, 则替换.            if (e.key == null) {                V oldValue = e.value;                e.value = value;                e.recordAccess(this);                return oldValue;            }        }        //创建新的Entry        modCount++;        addEntry(0, null, value, 0);        return null;    }    /**     * 初始化table     */    private void inflateTable(int toSize) {        // 找到大于或等于toSize的2的幂, 具体查找方法看下方法roundUpToPowerOf2.        //有时候在面试时会被问到, 如果已知有18条数据, 那么在初始化HashMap时长度给定多少合适?        //如果面试官只关心长度, 那么你回答长度在17-32之间都可以. 可能他更想要的答案时32吧.        int capacity = roundUpToPowerOf2(toSize);        //而在JDK6中是通过不停的左移得到capacity的. 所以在JDK7中对此做了优化.        //int capacity = 1;        //while (capacity < initialCapacity)        //    capacity <<= 1;        //初始化threshold        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);        //初始化table        table = new Entry[capacity];        //对String的特殊处理, 将放在最后进行介绍        //因为这一块是Jdk7中的尝试性方法, 在Jdk8中已经去掉了, 所以将不进行详细说明        initHashSeedAsNeeded(capacity);    }    /**     * 返回大于number的2的幂.     * if number>=2^30 return 2^30     * if number=1 return 1 即2^0     * else return 最接近number的2^x     */    private static int roundUpToPowerOf2(int number) {        // assert number >= 0 : "number must be non-negative";        return number >= MAXIMUM_CAPACITY                ? MAXIMUM_CAPACITY                : (number > 1)                //这里是避免number正好是2的幂. 所以采用number-1再左移一位的方式                ? Integer.highestOneBit((number - 1) << 1)                : 1;    }    /**     * 此方法为Integer中的方法, 加进来只是为了方便说明.     * 该方法的返回结果为: 取输入值的最高位的1, 后面全部补0     */    // public static int highestOneBit(int i) {    //     // 这里以8为例. 8的二进制: 0000 1000    //     i |= (i >>  1); //0000 1000 | 0000 0100 = 0000 1100    //     i |= (i >>  2); //0000 1100 | 0000 0011 = 0000 1111    //     i |= (i >>  4); //0000 1111 | 0000 1111 = 0000 1111    //     i |= (i >>  8); //0000 1111 | 0000 1111 = 0000 1111    //     i |= (i >> 16); //0000 1111 | 0000 1111 = 0000 1111    //     //可以看出上面这些 右移 异或 等操作其实就是把最高位右移31位    //     //最后的结果就是经过处理变成从最高位的1开始, 之后全部为1    //     return i - (i >>> 1); //0000 1111 - 0000 0111 = 15 - 7 = 8    // }    /**     * 对key进行rehash, 让hash值更为散列, 尽量避免hash碰撞.     */    final int hash(Object k) {        //这一块主要时为String类型进行特殊的hash处理,        //新的哈希算法可能会严重影响高并发、多线程代码. 所以在Jdk8中已经弃用.        //有兴趣的可以参考: http://www.importnew.com/7656.html        int h = hashSeed;        if (0 != h && k instanceof String) {            return sun.misc.Hashing.stringHash32((String) k);        }        h ^= k.hashCode();        // 主要是为了让高位低位都参与计算, 让hash值更散列.        // 在此不做详细介绍. 想了解的可以参照: http://www.iteye.com/topic/709945        h ^= (h >>> 20) ^ (h >>> 12);        return h ^ (h >>> 7) ^ (h >>> 4);    }    /**     * Returns index for hash code h.     * 计算hash在table中的位置.     * 通过该方法可以看出为什么要求table的length必须等于2的幂     * 1111 1111 & 0000 0010 = 0000 0010.     * 说白了就是把hash值与length-1取余     */    static int indexFor(int h, int length) {        // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";        return h & (length-1);    }    /**     * 添加Entry, 当size>=threshold, 并且bucketIndex有值时进行resize.     */    void addEntry(int hash, K key, V value, int bucketIndex) {        //当size>=threshold(阀值), 并且要加入的位置有值时进行容量扩充        //比如: new HashMap(), 默认的threshold=16*0.75=12        //也就是说当put第12个数据, 并且该位置上有值时进行resize        if ((size >= threshold) && (null != table[bucketIndex])) {            resize(2 * table.length);            hash = (null != key) ? hash(key) : 0;            bucketIndex = indexFor(hash, table.length);        }        createEntry(hash, key, value, bucketIndex);    }    /**     * 创建Entry, 并把Entry放到单向链表尾部     */    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++;    }    /**     * 扩充table的大小. 并把原有数据转移到新的table中     */    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];        //把oldTable中的数据转移到newTable中        transfer(newTable, initHashSeedAsNeeded(newCapacity));        table = newTable;        //重新设置threshold        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);    }    /**     * 从oldTable中把数据转移到newTable中     * rehash: 默认为true, 通过该方法获取initHashSeedAsNeeded(newCapacity), 该方法是配合测试String的新的hash策略使用.     */    void transfer(Entry[] newTable, boolean rehash) {        int newCapacity = newTable.length;        //遍历table中的每个Entry        for (Entry<K,V> e : table) {            while(null != e) {                //取出e.next, 方便后面做数据交换                Entry<K,V> next = e.next;                if (rehash) { //是否需要重新hash                    e.hash = null == e.key ? 0 : hash(e.key);                }                //重新计算Entry在table中的位置                int i = indexFor(e.hash, newCapacity);                //进行数据交换.                //1. 把newTable[i]对应的Entry赋值给e.next. 即把e拼接到newTable[i]对应的链表中                e.next = newTable[i];                //2. 把e赋值给newTable[i]. 即: 把拼接后的newTable[i]放回到newTable中.                newTable[i] = e;                //3. 把next赋值给e, 继续往下遍历oldTable中的数据                e = next;            }        }    }    /**     * putAll与put差不多. 在此不做赘述. 可以参考put的说明     */    public void putAll(Map<? extends K, ? extends V> m) {        int numKeysToBeAdded = m.size();        if (numKeysToBeAdded == 0)            return;        if (table == EMPTY_TABLE) {            inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));        }        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());    }    /**     * 这个构造方法会在初始化时对table进行初始化. 初始化过程不在赘述     */    public HashMap(Map<? extends K, ? extends V> m) {        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,                      DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);        inflateTable(threshold);        putAllForCreate(m);    }    /**     * 通过key取值     */    public V get(Object key) {        if (key == null)            return getForNullKey();        Entry<K,V> entry = getEntry(key);        return null == entry ? null : entry.getValue();    }    /**     * 从table[0]的Entry中取key=null的数据.     * 有时我们会通过map.get(key) == null来判断map中是否存在指定的数据,     * 一般情况下是不会有问题的. 但是如果正好key=null, value=null.     * 此时通过get(key)就无法区分map中是否真正存在这条数据了.     * 因为get(key)=null是有多种情况的:     *      1. value不存在     *      2. map为空     *      3. key不存在     * 所以判断数据是否存在还是用containsKey比较靠谱.     */    private V getForNullKey() {        if (size == 0) {            return null;        }        //null的key默认放在table[0]中        for (Entry<K,V> e = table[0]; e != null; e = e.next) {            if (e.key == null)                return e.value;        }        return null;    }    /**     * 获取指定key对应的Entry     */    final Entry<K,V> getEntry(Object key) {        if (size == 0) {            return null;        }        //null的hash默认为0.        int hash = (key == null) ? 0 : hash(key);        //循环table[indexFor(hash, table.length)]        //通过e.hash == hash && (key==e.key || key.equals(e.key)), 找到相应的entry        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;    }    /**     * 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.     *     * 目前该方法仅在构造器中使用. 该方法不会resize table.     */    private void putForCreate(K key, V value) {        int hash = null == key ? 0 : hash(key);        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);    }    //仅在构造器中使用.    private void putAllForCreate(Map<? extends K, ? extends V> m) {        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())            putForCreate(e.getKey(), e.getValue());    }    /**     * 通过key移除数据     */    public V remove(Object key) {        Entry<K,V> e = removeEntryForKey(key);        return (e == null ? null : e.value);    }    /**     * 通过key移除数据     */    final Entry<K,V> removeEntryForKey(Object key) {        if (size == 0) {            return null;        }        int hash = (key == null) ? 0 : hash(key);        //取key在table中的位置        int i = indexFor(hash, table.length);        //取出数据, 方便后面进行操作        Entry<K,V> prev = table[i];        Entry<K,V> e = prev;        while (e != null) {            //先取出e.next            Entry<K,V> next = e.next;            Object k;            if (e.hash == hash &&                ((k = e.key) == key || (key != null && key.equals(k)))) {                //设置modCount                modCount++;                size--;                //如果删除的元素在table[i]的第一个位置. 在需要把table[i]指给next;                //如果删除的元素在table[i]的其他位置. 则把table[i].next=next;                if (prev == e)                    table[i] = next;                else                    prev.next = next;                //Entry中的Hook方法. 方面子类进行扩展                e.recordRemoval(this);                return e;            }            //没找到, 则继续遍历            prev = e;            e = next;        }        return e;    }    /**     * 通过Entry移除数据, 根据entry.equals方法进行判断     */    final Entry<K,V> removeMapping(Object o) {        if (size == 0 || !(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);        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--;                //如果删除的元素在table[i]的第一个位置. 在需要把table[i]指给next;                //如果删除的元素在table[i]的其他位置. 则把table[i].next=next;                if (prev == e)                    table[i] = next;                else                    prev.next = next;                e.recordRemoval(this);                return e;            }            prev = e;            e = next;        }        return e;    }    /**     * 返回map的实际长度     */    public int size() {        return size;    }    /**     * 判断map是否为空     */    public boolean isEmpty() {        return size == 0;    }    /**     * 是否包含指定的key     */    public boolean containsKey(Object key) {        return getEntry(key) != null;    }    /**     * 清空map     */    public void clear() {        //需要注意table只会不断变大, 不会自动变小. 即使清空也只是清空里面的内容        //所以对于一个比较大的map对象, 清空后最好把该对象设置为null. 方便JVM回收.        modCount++;        Arrays.fill(table, null);        size = 0;    }    /**     * 判断map中是否包含指定的value.     * 当找到一个相等的value即返回true.     */    public boolean containsValue(Object value) {        //containsValue是遍历全部数据, 效率不高        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;    }    /**     * 是否存在value=null的数据     * 当找到一个value==null的数据即返回true.     */    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;        }        if (result.table != EMPTY_TABLE) {            result.inflateTable(Math.min(                (int) Math.min(                    size * Math.min(1 / loadFactor, 4.0f),                    // we have limits...                    HashMap.MAXIMUM_CAPACITY),               table.length));        }        result.entrySet = null;        result.modCount = 0;        result.size = 0;        result.init();        result.putAllForCreate(this);        return result;    }    //单向链表的数据结构    static class Entry<K,V> implements Map.Entry<K,V> {        final K key;        V value;        Entry<K,V> next;        int hash; //key的hash值        /**         * 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();            //两个Entry的key相等, 并且value也相等            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 Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());        }        public final String toString() {            return getKey() + "=" + getValue();        }        /**         * put时调用         */        void recordAccess(HashMap<K,V> m) {        }        /**         * remove时调用         */        void recordRemoval(HashMap<K,V> m) {        }    }    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() {            //在创建iterator时把map中的modCount赋值给expectedModCount            //当遍历过程中发现expectedModCount != modCount时直接报错            //出现这种情况, 则说明有其他线程操作了(put/remove)该map            //所以在遍历hashMap时建议使用iterator, 避免出现数据处理异常            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) //fail-fast                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;    /**     * 返回所有的key     */    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();        }    }    /**     * 返回所有的values     */    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();        }    }    /**     * 返回所有的Entry     */    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();        }    }    /**     * 下面的方法不再做重复复述. 有兴趣的可以参考: http://www.importnew.com/7656.html     */    static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;    /**     * holds values which can't be initialized until after VM is booted.     */    private static class Holder {        /**         * Table capacity above which to switch to use alternative hashing.         */        static final int ALTERNATIVE_HASHING_THRESHOLD;        static {            String altThreshold = java.security.AccessController.doPrivileged(                new sun.security.action.GetPropertyAction(                    "jdk.map.althashing.threshold"));            int threshold;            try {                threshold = (null != altThreshold)                        ? Integer.parseInt(altThreshold)                        : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;                // disable alternative hashing if -1                if (threshold == -1) {                    threshold = Integer.MAX_VALUE;                }                if (threshold < 0) {                    throw new IllegalArgumentException("value must be positive integer.");                }            } catch(IllegalArgumentException failed) {                throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);            }            ALTERNATIVE_HASHING_THRESHOLD = threshold;        }    }    /**     * A randomizing value associated with this instance that is applied to     * hash code of keys to make hash collisions harder to find. If 0 then     * alternative hashing is disabled.     */    transient int hashSeed = 0;    /**     * Initialize the hashing mask value. We defer initialization until we     * really need it.     */    final boolean initHashSeedAsNeeded(int capacity) {        boolean currentAltHashing = hashSeed != 0;        boolean useAltHashing = sun.misc.VM.isBooted() &&                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);        boolean switching = currentAltHashing ^ useAltHashing;        if (switching) {            hashSeed = useAltHashing                ? sun.misc.Hashing.randomHashSeed(this)                : 0;        }        return switching;    }}