HashMap源码分析（二）--HashMap

话不多说直接上开始

    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 初始容量为16

    
    static final int MAXIMUM_CAPACITY = 1 << 30;         

    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
     * between resizing and treeification thresholds.
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

目前知道的是，初始容量为16，最大容量为32位。

二、分析：

1.hashmap采用的是一个hashset的数组，以及在每个数组对应一个单向链表。

2.每个Enrty<key,value>以节点的方式存储在链表中。

三、具体分析：

1.节点（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;
        }
    }

（1）.key值固定，因为当key发生变化是，他的存储位置必然发生变化这样的话等于删除再重建，所以hash不允许修改key值（hash值依赖于key值），当然key值固定，hash值也就固定，所以为final。

（2）.接入Map的内部接口Entry<key,value>，并且实现了其中的常用的键值对操作方法。

（3）.next节点，由于HashMap中是单向列表不同于LinkedList的双向列表，仅仅有存放了下一个节点的地址空间，一边操作。

（4）.初始化时必须要（hash，key，value，nextnode（不过当最后节点时，为null））

2.构造方法

    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);
    }

    /**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and the default load factor (0.75).
     *
     * @param  initialCapacity the initial capacity.
     * @throws IllegalArgumentException if the initial capacity is negative.
     */
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     */
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

public HashMap(Map<? extends K, ? extends V> m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}

（1）.有上面源码可知，HashMap创建时必要的是初始长度（但是已经初始化），负荷系数（load Factor，用来显示size/capacity，满的程度）（也被初始化）

   threshord=capacity*loadFactor

2.存放其他Map

    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);
            }
        }
    }

（1）.hashMap的容量的阈值判断，前半部分都是，for循环则是将map存储。五个参数前三个与Node相同，用于node的创建

3.找到Node节点

 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 &&    //table必须不能为null，且长度大与0，且第一个值不为null
            
            (first = tab[(n - 1) & hash]) != null) {        //这个位运算的确不明白怎么找到的，应该与hash算法相关
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))       //若key值相同且hash值相同（关于或者判断前后，目的在于null值判断）
                return first;
            if ((e = first.next) != null) {                                //接下来就是向下取值了。
                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;
    }

（1）table用于接收通过hash值找到的链表，first则是第一个元素，e用于接收查找的元素。（标记在上方代码中）

（2）关于TreeNode，1.8后Java设定HashMap链表的最大程度为8，如果超出这个长度则会将链表转化为属性结构，为了提高一定效率。

4.put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;
        if ((tab = table) == null || (n = tab.length) == 0)          //判断长度是否为空
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)                   //判断hash值找到链表的首节点是否为空，如果为空则新建节点
            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))))          //判断 是否与首节点重复，key重复则覆盖，value覆盖
                e = p;
            else if (p instanceof TreeNode)                                        //判断树节点，暂时不管
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);             
            else {                                        
                for (int binCount = 0; ; ++binCount) {                                 //循环遍历链表，直到找到尾节点或者找到相同的key值覆盖
                    if ((e = p.next) == null) {                                //尾节点
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&                                       //找到覆盖
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key                  //覆盖，onlyIfAbsent可以选择是否覆盖旧的节点。
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);              //插入新的node
        return null;   
    }

（1）主要注解写在上方代码后

（2）在hashmap方法中，最为难以理解的是Node转向TreeNode，在所有添加节点的方法中都会进行判断链表下的节点数目，所以在正常看事过滤这一部分代码即可。

5.

    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;
   ｝