HashMap源码浅析(jdk1.8)

HashMap是以key-value键值对的形式进行存储数据的，数据结构是以数组+链表或红黑树实现。

数据结构图如下：

 

一、关键属性

HashMap初始化和方法使用的属性。

    /**     * 默认初始容量16(2的4次方)     */    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16    /**     * 最大容量(2的30次方)     */    static final int MAXIMUM_CAPACITY = 1 << 30;    /**     * 默认加载因子     */    static final float DEFAULT_LOAD_FACTOR = 0.75f;    /**     * 链表节点数大于8变成红黑树     */    static final int TREEIFY_THRESHOLD = 8;    /**     * 红黑树节点数小于6变成链表     */    static final int UNTREEIFY_THRESHOLD = 6;    /**     * 在变成红黑树前判断键值对的数量是否小于64     */    static final int MIN_TREEIFY_CAPACITY = 64;

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二、构造方法

1、HashMap(int initialCapacity, float loadFactor)，对参数进行校验并初始化容量和加载因子。

    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;        this.threshold = tableSizeFor(initialCapacity);    }        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;    }

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2、HashMap(int initialCapacity)调用第一个构造方法。

    public HashMap(int initialCapacity) {        this(initialCapacity, DEFAULT_LOAD_FACTOR);    }

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3、HashMap(Map<? extends K, ? extends V> m)，把参数map集合初始化到新集合中。

    public HashMap(Map<? extends K, ? extends V> m) {        this.loadFactor = DEFAULT_LOAD_FACTOR;        putMapEntries(m, false);    }    final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {        int s = m.size();        if (s > 0) {            if (table == null) { // pre-size                float ft = ((float)s / loadFactor) + 1.0F;                int t = ((ft < (float)MAXIMUM_CAPACITY) ?                         (int)ft : MAXIMUM_CAPACITY);                if (t > threshold)                    threshold = tableSizeFor(t);            }            else if (s > threshold)                resize();            for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {                K key = e.getKey();                V value = e.getValue();                putVal(hash(key), key, value, false, evict);            }        }    }

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4、HashMap()方法只初始化加载因子。

    public HashMap() {        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted    }

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三、主要方法

1、put(K key, V value)方法，先通过计算hash来判断新元素所在节点数组的位置，

如果位置为空则直接添加新元素放在数组节点上，如果不为空则在通过hash和key来判断新添加的元素是否

和在此数组节点上的元素有相同的key，相同则覆盖，否则在判断此节点是树节点还是普通节点，

树节点则进入红黑树的添加，普通节点进入链表的添加，链表通过循环来判断新节点是覆盖还是在尾部添加，

还是超出8个节点变成红黑树添加。

    // 添加元素或覆盖元素    public V put(K key, V value) {        return putVal(hash(key), key, value, false, true);    }    // 计算hash值，即元素所属的数组位置    static final int hash(Object key) {        int h;        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);    }        final V putVal(int hash, K key, V value, boolean onlyIfAbsent,                   boolean evict) {        Node<K,V>[] tab; Node<K,V> p; int n, i;        // 如果table为初始化或长度为0，则扩容        if ((tab = table) == null || (n = tab.length) == 0)            n = (tab = resize()).length;        // 链表第一个元素直接创建新节点并赋值        if ((p = tab[i = (n - 1) & hash]) == null)            tab[i] = newNode(hash, key, value, null);                    // 在已有链表或红黑树上添加新节点        else {            Node<K,V> e; K k;            // 如果添加的节点和原有的key相同则覆盖            if (p.hash == hash &&                ((k = p.key) == key || (key != null && key.equals(k))))                e = p;            // 如果p为红黑树则在这添加            else if (p instanceof TreeNode)                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);            // 链表这边添加            else {                // 循环链表                for (int binCount = 0; ; ++binCount) {                    // 下一个为空直接赋值                    if ((e = p.next) == null) {                        p.next = newNode(hash, key, value, null);                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st                            treeifyBin(tab, hash);                        break;                    }                    // hash和key相同退出循环                    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;        if (++size > threshold)            resize();        afterNodeInsertion(evict);        return null;    }

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2、resize()方法，对原map集合进行扩容，容量变为原来2倍。

    final Node<K,V>[] resize() {        // 保存当前数组节点        Node<K,V>[] oldTab = table;        // 保存原数组节点大小        int oldCap = (oldTab == null) ? 0 : oldTab.length;        // 保存当前阀值        int oldThr = threshold;        // 声明新数组节点大小和阀值        int newCap, newThr = 0;        // 原map有值        if (oldCap > 0) {            // 原map元素个数已达到最大值            if (oldCap >= MAXIMUM_CAPACITY) {                threshold = Integer.MAX_VALUE;                return oldTab;            }            // 原map容量2倍小于最大值且原map容量大于等于16则扩容            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&                     oldCap >= DEFAULT_INITIAL_CAPACITY)                newThr = oldThr << 1; // double threshold        }        // 只进行初始化没有添加元素的进这个        else if (oldThr > 0) // initial capacity was placed in threshold            newCap = oldThr;        // 只调用HashMap()进这个        else {               // zero initial threshold signifies using defaults            newCap = DEFAULT_INITIAL_CAPACITY;            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);        }        // 新阀值为0（只进行初始化）        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"})            // 初始化新节点数组            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];        table = newTab;        // 原节点数组不为空        if (oldTab != null) {            for (int j = 0; j < oldCap; ++j) {                Node<K,V> e;                // 原数组第一个节点不为空                if ((e = oldTab[j]) != null) {                    // 主动释放                    oldTab[j] = null;                    // 只有数组节点（此索引处只有一个节点）                    if (e.next == null)                        newTab[e.hash & (newCap - 1)] = e;                    // e为红黑树节点                    else if (e instanceof TreeNode)                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);                    // e为普通节点                    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;    }

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3、remove(Object key)方法，根据key删除元素。

    // 根据key删除元素    public V remove(Object key) {        Node<K,V> e;        return (e = removeNode(hash(key), key, null, false, true)) == null ?            null : e.value;    }        final Node<K,V> removeNode(int hash, Object key, Object value,                               boolean matchValue, boolean movable) {        Node<K,V>[] tab; Node<K,V> p; int n, index;        // map集合不为空        if ((tab = table) != null && (n = tab.length) > 0 &&            (p = tab[index = (n - 1) & hash]) != null) {            Node<K,V> node = null, e; K k; V v;            // 所删节点就在数组节点上            if (p.hash == hash &&                ((k = p.key) == key || (key != null && key.equals(k))))                node = p;            else if ((e = p.next) != null) {                // 循环红黑树                if (p instanceof TreeNode)                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);                // 普通节点                else {                    do {                        if (e.hash == hash &&                            ((k = e.key) == key ||                             (key != null && key.equals(k)))) {                            node = e;                            break;                        }                        p = e;                    } while ((e = e.next) != null);                }            }            // 找到要删除的节点进行删除            if (node != null && (!matchValue || (v = node.value) == value ||                                 (value != null && value.equals(v)))) {                if (node instanceof TreeNode)                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);                else if (node == p)                    tab[index] = node.next;                else                    p.next = node.next;                ++modCount;                --size;                afterNodeRemoval(node);                return node;            }        }        return null;    }

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4、get(Object key)方法，根据key查找元素。

    public V get(Object key) {        Node<K,V> e;        return (e = getNode(hash(key), key)) == null ? null : e.value;    }        final Node<K,V> getNode(int hash, Object key) {        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;        // map集合不为空        if ((tab = table) != null && (n = tab.length) > 0 &&            (first = tab[(n - 1) & hash]) != null) {            // 所找节点正式在数组节点上            if (first.hash == hash && // always check first node                ((k = first.key) == key || (key != null && key.equals(k))))                return first;            // 在链表或红黑树上找节点            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;    }

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