集合之HashMap

/**
     * 默认初始容量，默认为2的4次方 = 16，2的n次方是为了加快hash计算速度，；；减少hash冲突，，，h & (length-1)，，1111111
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * 最大容量，默认为2的30次方，
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * 负载因子(load factor)，它用来衡量哈希表的 空/满 程度，一定程度上也可以体现查询的效率，
     * 计算公式为:负载因子 = 总键值对数 / 箱子个数
     * 负载因子越大，意味着哈希表越满，越容易导致冲突，性能也就越低。
     * 因此，一般来说，当负载因子大于某个常数(可能是 1，或者 0.75 等)时，哈希表将自动扩容
     *
     * 默认负载因子，默认为0.75
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * 如果哈希函数不合理，即使扩容也无法减少箱子中链表的长度，
     * 因此 Java 的处理方案是当链表太长时，转换成红黑树。
     * 这个值表示当某个箱子中，链表长度大于 8 时，有可能会转化成树
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     *  在哈希表扩容时，如果发现链表长度小于 6，则会由树重新退化为链表
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     *在转变成树之前，还会有一次判断，只有键值对数量大于 64 才会发生转换。
     *这是为了避免在哈希表建立初期，多个键值对恰好被放入了同一个链表中而导致不必要的转化
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

    /**
     * HashMap内部类实现了Map的内部类Entry,用于存储K,V
     */
    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Node<K,V> next;//当遇到哈希冲突时，可以形成单列表结构

        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;
        }

        /**
         * 重写equals方法用于判断两个K-V映射是否相等
         * @param o
         * @return
         */
        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;
        }
    }

    /* ---------------- Static utilities -------------- */

    /**
     * Computes key.hashCode() and spreads (XORs) higher bits of hash
     * to lower.  Because the table uses power-of-two masking, sets of
     * hashes that vary only in bits above the current mask will
     * always collide. (Among known examples are sets of Float keys
     * holding consecutive whole numbers in small tables.)  So we
     * apply a transform that spreads the impact of higher bits
     * downward. There is a tradeoff between speed, utility, and
     * quality of bit-spreading. Because many common sets of hashes
     * are already reasonably distributed (so don't benefit from
     * spreading), and because we use trees to handle large sets of
     * collisions in bins, we just XOR some shifted bits in the
     * cheapest possible way to reduce systematic lossage, as well as
     * to incorporate impact of the highest bits that would otherwise
     * never be used in index calculations because of table bounds.
     */
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

    /**
     * Returns x's Class if it is of the form "class C implements
     * Comparable<C>", else null.
     */
    static Class<?> comparableClassFor(Object x) {
        if (x instanceof Comparable) {
            Class<?> c; Type[] ts, as; Type t; ParameterizedType p;
            if ((c = x.getClass()) == String.class) // bypass checks
                return c;
            if ((ts = c.getGenericInterfaces()) != null) {
                for (int i = 0; i < ts.length; ++i) {
                    if (((t = ts[i]) instanceof ParameterizedType) &&
                        ((p = (ParameterizedType)t).getRawType() ==
                         Comparable.class) &&
                        (as = p.getActualTypeArguments()) != null &&
                        as.length == 1 && as[0] == c) // type arg is c
                        return c;
                }
            }
        }
        return null;
    }

    /**
     * Returns k.compareTo(x) if x matches kc (k's screened comparable
     * class), else 0.
     */
    @SuppressWarnings({"rawtypes","unchecked"}) // for cast to Comparable
    static int compareComparables(Class<?> kc, Object k, Object x) {
        return (x == null || x.getClass() != kc ? 0 :
                ((Comparable)k).compareTo(x));
    }

    /**
     * 返回一个2的倍数的数  最接近cap的.
     */
    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;
    }

    /* ---------------- Fields -------------- */

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * K-v键值对映射元素的个数
     */
    transient int size;

    /**
     *Hash表结构性修改次数，用于实现迭代器快速失败行为
     */
    transient int modCount;

    /**
     * 当前Hash表的下一次扩容的容量阀值.
     */
    int threshold;

    /**
     * 当前Hash表的负载因子 
     *
     * @serial
     */
    final float loadFactor;

    /* ---------------- Public operations -------------- */

    /**
     *制定初始容量和负载因子的构造方法
     */
    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
    }

    /**
     * Constructs a new <tt>HashMap</tt> with the same mappings as the
     * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with
     * default load factor (0.75) and an initial capacity sufficient to
     * hold the mappings in the specified <tt>Map</tt>.
     *
     * @param   m the map whose mappings are to be placed in this map
     * @throws  NullPointerException if the specified map is null
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

    /**
     * Implements Map.putAll and Map constructor
     *
     * @param m the map
     * @param evict false when initially constructing this map, else
     * true (relayed to method afterNodeInsertion).
     */
    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);
            }
        }
    }

    /**
     * Returns the number of key-value mappings in this map.
     *
     * @return the number of key-value mappings in this map
     */
    public int size() {
        return size;
    }

    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     *
     * @return <tt>true</tt> if this map contains no key-value mappings
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

    /**
     * Implements Map.get and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @return the node, or null if none
     */
    final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        /**
         * 1：将tab指向table，并校验tab不为空（即table不为空）
         * 2：n赋值为table的长度，校验n>0（说明有K-V键值对元素）
         * 3：将tab的(n - 1) & hash元素赋值给first，校验first不为空
         * 以上条件全部符合，说明key可能存在，需要遍历first连接的整个链表（也可能是红黑树）
         * 
         */
        if ((tab = table) != null 
        		&& (n = tab.length) > 0 
        		&&(first = tab[(n - 1) & hash]) != null)
        {
        	/**
        	 * 从首节点开始匹配
        	 * 1）校验哈希值是否一样
        	 * 2）校验key是否相等，此步骤分以下两种情况
        	 * 	1：key的引用相等
        	 * 	2：key不为null且key的内容一样（如果一个自定义类进行比较，必须重写hashCode和equals方法）
        	 */
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            /**
             * 首节点不是，就遍历链表或红黑树
             */
            if ((e = first.next) != null) {
            	//红黑树
                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;
    }

    /**
     * Returns <tt>true</tt> if this map contains a mapping for the
     * specified key.
     *
     * @param   key   The key whose presence in this map is to be tested
     * @return <tt>true</tt> if this map contains a mapping for the specified
     * key.
     */
    public boolean containsKey(Object key) {
        return getNode(hash(key), key) != null;
    }

    /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    /**
     * Implements Map.put and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to put
     * @param onlyIfAbsent if true, don't change existing value
     * @param evict if false, the table is in creation mode.
     * @return previous value, or null if none
     */
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        /**
         * 1：将tab指向table，并校验tab不为空（即table不为空）
         * 2：n赋值为table的长度，校验n>0（说明有K-V键值对元素）
         * 符合以上任一条件，说明当前数组没被初始化，需要初始化
         */
        if ((tab = table) == null || (n = tab.length) == 0)
        	/**
        	 * 初始化数组并赋值给tab
        	 * 将tab的长度赋值给n
        	 */
            n = (tab = resize()).length;
        /**
         * tab的(n - 1) & hash位置元素不空，就将数据插入到此位置（插入一个包含K-V键值对的Node节点）
         */
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
        	/**
        	 * 说明哈希冲突了，需要通过链表或红黑树（链表长度大于某个阀值会转为红黑树）解决
        	 * 找到hash和key都相同的那个节点
        	 */
            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)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);//将链表转成红黑树
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            //已存在K的映射
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;//覆盖value
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        //size大于扩容阀值时进行扩容
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

    /**
     * Initializes or doubles table size.  If null, allocates in
     * accord with initial capacity target held in field threshold.
     * Otherwise, because we are using power-of-two expansion, the
     * elements from each bin must either stay at same index, or move
     * with a power of two offset in the new table.
     *
     * @return the table
     */
    final Node<K,V>[] resize() {
    	//指向旧的数组
        Node<K,V>[] oldTab = table;
        //旧数组的容量
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        //旧的扩容阀值
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
        	//如果旧的数组容量大于MAXIMUM_CAPACITY，就将扩容阀值设为Integer.MAX_VALUE，返回旧的数组，不进行扩容操作
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            /**
             * 1：将newCap设为旧数组容量的两倍
             * 2：newCap小于MAXIMUM_CAPACITY 且 oldCap不小于DEFAULT_INITIAL_CAPACITY
             */
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
            	//扩容阀值设为旧阀值两倍
                newThr = oldThr << 1; // double threshold
        }
        //只是初始化了扩容阀值，没有创建数组
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
        	/**
        	 * 说明数组、阀值都没有初始化
        	 * 	容量设为默认16
        	 *  阀值=16*0.75
        	 */
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        /**
         * 出现newThr = 0的条件
         * 1）oldCap > 0 且 (oldCap*2 >= MAXIMUM_CAPACITY或oldCap <DEFAULT_INITIAL_CAPACITY)
         * 2) oldThr有值
         */
        if (newThr == 0) {
        	//新容量*负载因子得出一个阀值
            float ft = (float)newCap * loadFactor;
            /**
             * 新容量小于MAXIMUM_CAPACITY且新得出阀值<MAXIMUM_CAPACITY 新阀值就是ft
             * 	否则新阀值为Integer.MAX_VALUE
             */
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        //设置扩容阀值为新阀值
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
        //创建一个新的数组，大小为新容量
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        //旧数组不为空时，需要将数据拷贝到新数组内
        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;
                            /**
                             * (e.hash & oldCap)=0表示就算数组扩容之后，通过hash计算出来的索引还是一样
                             * 不需要调整位置
                             */
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            /**
                             * 需要调整索引
                             * 新索引=原索引+oldCap
                             */
                            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;
    }