HashMap

2018-07-03

常用属性

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; //默认大小16
static final float DEFAULT_LOAD_FACTOR = 0.75f; //默认负载因子
static final int TREEIFY_THRESHOLD = 8;//默认树化值
/** hashMap 的基本元素Node **/
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;
        }
    }
/** 说明HashMap是数组，类型是Node 不能序列化**/
 transient Node<K,V>[] table;

transient int modCount;//hashMap 修改次数
int threshold;//resize 的值
final float loadFactor;//负载因子

get

public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

对Key进行hash后，调用getNode(）方法

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;
    }

first = tab[(n - 1) & hash] ：通过key的hash值定位到这个元素在数组的为位置first

在判断首节点是不是要查找的内容。

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//判断是否为查找对象的方法：先比较hash值，然后key相等或者key的equal方法相等
if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))

接着判断Node的类型是不是TreeNode，如果是调用getTreeNode(hash, key)方法。

否则遍历Node节点，进行查找。否则返回null。

看一下TreeNode的查找方法：getTreeNode(hash, key)

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final TreeNode<K,V> getTreeNode(int h, Object k) {
            return ((parent != null) ? root() : this).find(h, k, null);
        }

这个方法是来找到TreeNode的根节点，也就是parent为空的节点

final TreeNode<K,V> find(int h, Object k, Class<?> kc) {
            TreeNode<K,V> p = this;
            do {
                int ph, dir; K pk;
                TreeNode<K,V> pl = p.left, pr = p.right, q;
                if ((ph = p.hash) > h)
                    p = pl;
                else if (ph < h)
                    p = pr;
                else if ((pk = p.key) == k || (k != null && k.equals(pk)))
                    return p;
                else if (pl == null)
                    p = pr;
                else if (pr == null)
                    p = pl;
                else if ((kc != null ||
                          (kc = comparableClassFor(k)) != null) &&
                         (dir = compareComparables(kc, k, pk)) != 0)
                    p = (dir < 0) ? pl : pr;
                else if ((q = pr.find(h, k, kc)) != null)
                    return q;
                else
                    p = pl;
            } while (p != null);
            return null;
        }

判断当前节点的hash值比key的hash大，从左节点查找。查找成功的条件hash值相等，然后判断key相等或者key的equal方法相等。和Node的条件是一样的。找不到放回null

put

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public V put(K key, V value) {
       return putVal(hash(key), key, value, false, true);
   }

和get方法一样，也对key重新hash了，接着调用了putVal（）方法。

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                  boolean evict) {
       Node<K,V>[] tab; Node<K,V> p; int n, i;
      //当table为空或者长度为0调用resize方法
       if ((tab = table) == null || (n = tab.length) == 0)
           n = (tab = resize()).length;
      //算出key在table数组的位置，为空则新建一个Node
       if ((p = tab[i = (n - 1) & hash]) == null)
           tab[i] = newNode(hash, key, value, null);
       else {
           Node<K,V> e; K k;
           //首先判断第一个Node节点是否符合：hash值相等后，key或者key的equal方法相等
           if (p.hash == hash &&
               ((k = p.key) == key || (key != null && key.equals(k))))
               e = p;
           //判断节点类型是不是TreeNode ，如果是调用putTreeVal方法。
           else if (p instanceof TreeNode)
               e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
           else {
               //递归查找Node，直到Node为空，新建一个Node
               for (int binCount = 0; ; ++binCount) {
                   if ((e = p.next) == null) {
                       p.next = newNode(hash, key, value, null);
                       //当Node长度为7，要添加一个新Node是，进行树化
                       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;
       if (++size > threshold)
           resize();
       afterNodeInsertion(evict);
       return null;
   }

put方法:如果节点Node就新建一个节点，Node如果和put的key符合相等条件，则覆盖该Node。如果Node的长度为7，新建一个Node后，就会被树化。树化的过程比较复杂。

static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
        TreeNode<K,V> parent;  // red-black tree links
        TreeNode<K,V> left;
        TreeNode<K,V> right;
        TreeNode<K,V> prev;    // needed to unlink next upon deletion
        boolean red;
        TreeNode(int hash, K key, V val, Node<K,V> next) {
            super(hash, key, val, next);
        }

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
            //这里说明了hashmap没有默认容量为16，负载因子为0.75
            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"})
           //构建新的数组
            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;
    }

table为null，容量为16，负载因子为0.75。否则：如果原始容量的size的两倍大于16，则容器为原来的两倍，threshold为原来的两倍。否则容量为两倍，threshold不变。然后用新的容量构建数组和链表。