Java 8中HashMap源码解析

HashMap是Java和Android程序员的基本功， JDK1.8对HashMap进行了优化， 你真正理解它了吗？
考虑如下问题：
1、哈希基本原理？（答：散列表、hash碰撞、链表、红黑树）
2、hashmap查询的时间复杂度， 影响因素和原理？ （答：最好O（1），最差O（n）， 如果是红黑O（logn））
3、resize如何实现的， 记住已经没有rehash了！！！（答：拉链entry根据高位bit散列到当前位置i和size+i位置）
4、为什么获取下标时用按位与&，而不是取模%？ （答：不只是&速度更快哦， 我觉得你能答上来便真正理解hashmap了。

下面来分析一下Jdk1.8中的HashMap源码

     //默认的初始化容量为16，并且必须为2的n次幂     static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;       //最大的容量为2^30       static final int MAXIMUM_CAPACITY = 1 << 30;      //默认加载因子为0.75       static final float DEFAULT_LOAD_FACTOR = 0.75f;      //****这是JDK1.8对HashMap的优化， 哈希碰撞后的链表上达到8个节点时要将链表重构为红黑树，查询的时间复杂度变为O(logN)。       static final int TREEIFY_THRESHOLD = 8;      //       static final int UNTREEIFY_THRESHOLD = 6;       static final int MIN_TREEIFY_CAPACITY = 64;

HashMap节点类声明

 static class Node<K,V> implements Map.Entry<K,V> {        final int hash;        final K key;        V value;        Node<K,V> next;        Node(int hash, K key, V value, Node<K,V> next) {            this.hash = hash;            this.key = key;            this.value = value;            this.next = next;        }        public final K getKey()        { return key; }        public final V getValue()      { return value; }        public final String toString() { return key + "=" + value; }        public final int hashCode() {            return Objects.hashCode(key) ^ Objects.hashCode(value);        }        public final V setValue(V newValue) {            V oldValue = value;            value = newValue;            return oldValue;        }        public final boolean equals(Object o) {            if (o == this)                return true;            if (o instanceof Map.Entry) {                Map.Entry<?,?> e = (Map.Entry<?,?>)o;                if (Objects.equals(key, e.getKey()) &&                    Objects.equals(value, e.getValue()))                    return true;            }            return false;        }    }

计算key的hashcode方法：

 static final int hash(Object key) {        int h;        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);    }

HashMap中的节点数组transient Node<K,V>[] table;transient Set<Map.Entry<K,V>> entrySet;//容量transient int size;//修改次数，其中指的是map结构的变化，put等是不需要改变的transient int modCount;//阈值int threshold;//加载因子final float loadFactor;

 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;        //jdk1.7中的阈值是size*loadFactory,而jdk1.8中的阈值        //        this.threshold = tableSizeFor(initialCapacity);    }    //如给定10，返回2的4次方16. static final int tableSizeFor(int cap) {        int n = cap - 1;        n |= n >>> 1;        n |= n >>> 2;        n |= n >>> 4;        n |= n >>> 8;        n |= n >>> 16;        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;    }

 public V put(K key, V value) {        return putVal(hash(key), key, value, false, true);    }    /**     * Implements Map.put and related methods     *     * @param hash hash for key     * @param key the key     * @param value the value to put     * @param onlyIfAbsent if true, don't change existing value     * @param evict if false, the table is in creation mode.     * @return previous value, or null if none     */    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,                   boolean evict) {        Node<K,V>[] tab; Node<K,V> p; int n, i;        if ((tab = table) == null || (n = tab.length) == 0)            n = (tab = resize()).length;        if ((p = tab[i = (n - 1) & hash]) == null)            tab[i] = newNode(hash, key, value, null);        else {            Node<K,V> e; K k;            if (p.hash == hash &&                ((k = p.key) == key || (key != null && key.equals(k))))                e = p;            else if (p instanceof TreeNode)                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);            else {                for (int binCount = 0; ; ++binCount) {                    if ((e = p.next) == null) {                        p.next = newNode(hash, key, value, null);                        //如果链表上超过八个Node，那么进行重构，重构成红黑树                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st                            treeifyBin(tab, hash);                        break;                    }                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k))))                        break;                    p = e;                }            }            if (e != null) { // existing mapping for key                V oldValue = e.value;                if (!onlyIfAbsent || oldValue == null)                    e.value = value;                afterNodeAccess(e);                return oldValue;            }        }        ++modCount;        //如果size大于阈值，那么进行扩容操作，        if (++size > threshold)            resize();        afterNodeInsertion(evict);        return null;    }    //resize()    final Node<K,V>[] resize() {        Node<K,V>[] oldTab = table;        int oldCap = (oldTab == null) ? 0 : oldTab.length;        int oldThr = threshold;        int newCap, newThr = 0;        if (oldCap > 0) {            if (oldCap >= MAXIMUM_CAPACITY) {                threshold = Integer.MAX_VALUE;                return oldTab;            }            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&                     oldCap >= DEFAULT_INITIAL_CAPACITY)                newThr = oldThr << 1; // double threshold        }        else if (oldThr > 0) // initial capacity was placed in threshold            newCap = oldThr;        else {               // zero initial threshold signifies using defaults            newCap = DEFAULT_INITIAL_CAPACITY;            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);        }        if (newThr == 0) {            float ft = (float)newCap * loadFactor;            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?                      (int)ft : Integer.MAX_VALUE);        }        threshold = newThr;        @SuppressWarnings({"rawtypes","unchecked"})            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];        table = newTab;        if (oldTab != null) {            for (int j = 0; j < oldCap; ++j) {                Node<K,V> e;                if ((e = oldTab[j]) != null) {                    oldTab[j] = null;                    if (e.next == null)                        newTab[e.hash & (newCap - 1)] = e;                    else if (e instanceof TreeNode)                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);                    else { // preserve order                        Node<K,V> loHead = null, loTail = null;                        Node<K,V> hiHead = null, hiTail = null;                        Node<K,V> next;                        do {                            next = e.next;                            if ((e.hash & oldCap) == 0) {                                if (loTail == null)                                    loHead = e;                                else                                    loTail.next = e;                                loTail = e;                            }                            else {                                if (hiTail == null)                                    hiHead = e;                                else                                    hiTail.next = e;                                hiTail = e;                            }                        } while ((e = next) != null);                        if (loTail != null) {                            loTail.next = null;                            newTab[j] = loHead;                        }                        if (hiTail != null) {                            hiTail.next = null;                            newTab[j + oldCap] = hiHead;                        }                    }                }            }        }        return newTab;    }

public V get(Object key) {        Node<K,V> e;        return (e = getNode(hash(key), key)) == null ? null : e.value;    }    /**     * Implements Map.get and related methods     *     * @param hash hash for key     * @param key the key     * @return the node, or null if none     */    //查找时间复杂度最好为O(1),最差为O(n),如果重构成红黑树复杂 度为O(logn)    final Node<K,V> getNode(int hash, Object key) {        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;        if ((tab = table) != null && (n = tab.length) > 0 &&            (first = tab[(n - 1) & hash]) != null) {            if (first.hash == hash && // always check first node                ((k = first.key) == key || (key != null && key.equals(k))))                return first;            if ((e = first.next) != null) {                if (first instanceof TreeNode)                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);                do {                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k))))                        return e;                } while ((e = e.next) != null);            }        }        return null;    } /**         * Calls find for root node.         */        final TreeNode<K,V> getTreeNode(int h, Object k) {            return ((parent != null) ? root() : this).find(h, k, null);        }