HashMap(jdk1.8)

1.类声明：

public class HashMap<K,V> extends AbstractMap<K,V>    implements Map<K,V>, Cloneable, Serializable {}

2.变量：

//默认初始化容量    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16    //最大容量    static final int MAXIMUM_CAPACITY = 1 << 30;    //默认加载因子     static final float DEFAULT_LOAD_FACTOR = 0.75f;     //对于table里面的每个列表，从链表结构转为红黑树结构的阈值     static final int TREEIFY_THRESHOLD = 8;     //对于table里面的每个列表，从红黑树结构转为列表结构的阈值     static final int UNTREEIFY_THRESHOLD = 6;     //最小树化容量。只有当表的容量达到这个阈值，才能从列表结构转为树结构     static final int MIN_TREEIFY_CAPACITY = 64;     //当改变了表的映射数目（put方法增加了新的Node节点）或者修改了表结构（remove(),clear()）的时候，进行+1，若是对map进行迭代的时候，发生modCount不是期望的modCount,即被修改了，则抛出ConcurrentModificationException异常      transient int modCount;

3.方法：

//找到不小于cap的最小的2的n次方，这个算法之精辟令人叹为观止。具体可见参考文章1。通过右移使得cap-1表示的二进制数每一位都变成1。最后加1得到所需数。  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;    }    //put方法      public V put(K key, V value) {        return putVal(hash(key), key, value, false, true);    }    //若onlyIfAbsent为true，不会覆盖掉旧值，若为false,会覆盖掉旧值     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)//如果是TreeNode节点                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);            else {//如果是链表节点                for (int binCount = 0; ; ++binCount) {                    if ((e = p.next) == null) {                        p.next = newNode(hash, key, value, null);//往链表尾部插入节点，jdk7是往链表头部插入节点。                        //当链表数量大于树化阈值时，进行树化操作，对应的当remove的时候会有一个链表化的操作                        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;                }            }            //如果找到了旧的数据，则根据onlyIfAbsent值决定是否更新            if (e != null) { // existing mapping for key                V oldValue = e.value;                if (!onlyIfAbsent || oldValue == null)                    e.value = value;                afterNodeAccess(e);                return oldValue;            }        }        ++modCount;//当增加了新的node时，修改次数+1        if (++size > threshold)            resize();        afterNodeInsertion(evict);        return null;    }    //往树结构中插入数据     final TreeNode<K,V> putTreeVal(HashMap<K,V> map, Node<K,V>[] tab,                                       int h, K k, V v) {            Class<?> kc = null;            boolean searched = false;            TreeNode<K,V> root = (parent != null) ? root() : this;            for (TreeNode<K,V> p = root;;) {                int dir, ph; K pk;                if ((ph = p.hash) > h)                    dir = -1;                else if (ph < h)                    dir = 1;                else if ((pk = p.key) == k || (k != null && k.equals(pk)))                    return p;                else if ((kc == null &&                          (kc = comparableClassFor(k)) == null) ||                         (dir = compareComparables(kc, k, pk)) == 0) {                    if (!searched) {                        TreeNode<K,V> q, ch;                        searched = true;                        if (((ch = p.left) != null &&                             (q = ch.find(h, k, kc)) != null) ||                            ((ch = p.right) != null &&                             (q = ch.find(h, k, kc)) != null))                            return q;                    }                    dir = tieBreakOrder(k, pk);                }                TreeNode<K,V> xp = p;                if ((p = (dir <= 0) ? p.left : p.right) == null) {                    Node<K,V> xpn = xp.next;                    TreeNode<K,V> x = map.newTreeNode(h, k, v, xpn);                    if (dir <= 0)                        xp.left = x;                    else                        xp.right = x;                    xp.next = x;                    x.parent = x.prev = xp;                    if (xpn != null)                        ((TreeNode<K,V>)xpn).prev = x;                    moveRootToFront(tab, balanceInsertion(root, x));                    return null;                }            }        }

参考文章：1.http://blog.csdn.net/fan2012huan/article/details/51097331
2.http://blog.csdn.net/crazy1235/article/details/75579654