HashMap

HashMap

• 结构: 数组+链表(红黑树)

• 放数据:

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

• key的hash(): key==null,其hash为0,不为null其值为key的高16位异或key的低16位(h右移位16的值),这样会把这个hashcode的位的用上,因为接下来通过hash值判断数组的位置时使用的是01111这样的与运算,如果直接使用hashcode会使得hashcode的高位没有意义

  hashcode与equals的关系: equals相等则hashcode一定相等,反之不一定,因为两个key相同的键值对数据就是hashcode相同但是equals不同

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

• 链表节点:

    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;
        }
  

• 数组大小: 默认16,最大值1 << 30

    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;	
    static final int MAXIMUM_CAPACITY = 1 << 30;
  

• 数组扩容: 条件: 当前数组的容量被使用了0.75倍之后进行2倍扩容

    static final float DEFAULT_LOAD_FACTOR = 0.75f;
  

• 链表转变: 当链表节点增加大于等于8时转换成红黑树,减少到6时有红黑树转换成普通链表

    static final int TREEIFY_THRESHOLD = 8; 
    static final int UNTREEIFY_THRESHOLD = 6;
  

• putVal

     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;//通过resize()初始化或者加倍数组大小
       
    	//节点为空
    	if ((p = tab[i = (n - 1) & hash]) == null)//用01111与hash进行与运算(计算机执行比较快,目的与取模一样)
            tab[i] = newNode(hash, key, value, null);//当该数组位置没有元素,直接把Node节点赋给该数组位置
  
        //节点非空
        else { 
            Node<K,V> e; K k;			
            
    		//添加元素的key == p.key
    		if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
  
    		//添加元素的key != p.key,放入红黑树
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            
    		//添加元素的key != p.key,放入普通链表
    		else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // 节点大于等于8时由链表转换为红黑树
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k))))//遍历链表时,存在相同key节点
                        break;
                    p = e;
                }
            }
    		//上面的for循环如果发现相同的key的节点就把e置为改节点,否则置为null
            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)//扩容: size为数组所使用的大小,threshold为0.75*数组容量
            resize();
        afterNodeInsertion(evict);
        return null;
  

  为什么数组两倍扩容? ---->因为需要使用数组长度(n)-1进行与计算,需要生成后位都是1的数,初始容量为16(10000),两倍扩容后就可以产生这个效果(例:10000-1=01111,100000000-1=011111111)

• 初始化或者加倍数组大小

    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) { //已经达到最大数组,不扩容了
                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) {
    		//遍历数组
            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;
    				//拆分红黑树
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
    				//拆分链表
                    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) {//链表节点迁移只有两种可能,原地不动和往后移oldCap
                                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;
    }
  

  数组扩容后节点迁移

    扩容前:	0x0101 & 01111 = 0101
  
    扩容后:    0x0101 & 11111 = x0101
  

  x只能为0或1,当为0时节点保持不动,当为1时,节点位置增加了10000也就是oldCap(扩容前数组的容量)

• 设置初始容量

  initialCapacity = (存储元素个数 / 负载因子) + 1

• null值(<<阿里巴巴Java开发手册>>)