JDK1.8源码hashMap

一、概念

允许key为null，value为null；线程不安全；不保证映射的顺序；迭代 collection 视图所需的时间与 HashMap 实例的“容量”（桶的数量）及其大小（键-值映射关系数）成比例；HashMap 的实例有两个参数影响其性能：初始容量 和加载因子。容量 是哈希表中桶的数量，初始容量只是哈希表在创建时的容量。加载因子是哈希表在其容量自动增加之前可以达到多满的一种尺度。当哈希表中的条目数超出了加载因子与当前容量的乘积时，则要对该哈希表进行 rehash 操作（即重建内部数据结构），从而哈希表将具有大约两倍的桶数；加载因子默认是0.75；存储过多的相同的key，降低hash table性能；

线程不安全的，想要线程安全，最好的方法是在创建的时候使用下面方法：

 Map m = Collections.synchronizedMap(new HashMap(...));

关于迭代器和LinkedList一样！

JDK 1.8 以前 HashMap 的实现是 数组+链表，即使哈希函数取得再好，也很难达到元素百分百均匀分布。

当 HashMap 中有大量的元素都存放到同一个桶中时，这个桶下有一条长长的链表，这个时候 HashMap 就相当于一个单链表，假如单链表有 n 个元素，遍历的时间复杂度就是 O(n)，完全失去了它的优势。

针对这种情况，JDK 1.8 中引入了红黑树（查找时间复杂度为 O(logn)）来优化这个问题！

二、属性和构造方法：

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

    private static final long serialVersionUID = 362498820763181265L;

    /**
     * The default initial capacity - MUST be a power of two.
     */
    //默认初始容量-必须是2的指数。2的4次方=16次方
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    //最大容量，2的30次方=1073741824
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
    //负载系数=0.75
    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.
     */
    //一个桶的树化阈值：8，当桶中元素个数超过这个值时，需要使用红黑树节点替换链表节点
    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.
     */
    //一个树的链表还原阈值：6，当扩容时，桶中元素个数小于这个值，就会把树形的桶元素 还原（切分）为链表结构
    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.
     */
    //hash表的最小树形化容量：64，1，当哈希表中的容量大于这个值时，表中的桶才能进行树形化；否则桶内元素太多时会扩容，而不是树形化，
        //为了避免进行扩容、树形化选择的冲突，这个值不能小于 4 * TREEIFY_THRESHOLD
    static final int MIN_TREEIFY_CAPACITY = 64;

    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash; //对key的hashcode值进行hash运算后得到的值，存储在Entry，避免重复计算
        final K key;
        V value;
        Node<K,V> next; //存储指向下一个Entry的引用，单链表结构

        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;
        }

        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;
        }
    }

    //key的hash值
    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));
    }

    /**
     * Returns a power of two size for the given target capacity.
     */
    //返回给定的容量的2的指数
    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;
    }

    /* ---------------- 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.)
     */
    //初始化数组 
    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;

    /**
     * The number of key-value mappings contained in this map.
     */
    //实际存储的key-value键值对的个数 
    transient int size;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient int modCount;

    /**
     * The next size value at which to resize (capacity * load factor).
     *
     * @serial
     */
    // (The javadoc description is true upon serialization.
    // Additionally, if the table array has not been allocated, this
    // field holds the initial array capacity, or zero signifying
    // DEFAULT_INITIAL_CAPACITY.)
    //阈值，当table == {}时，该值为初始容量（初始容量默认为16）；
    //当table被填充了，也就是为table分配内存空间后，threshold一般为 capacity*loadFactory。
    int threshold;

    //负载系数,代表了table的填充度有多少，默认是0.75
    final float loadFactor;

    //创建一个空的HashMap用指定的容量和负载系数
    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)//不能大于最大值
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        this.loadFactor = loadFactor;
        this.threshold = tableSizeFor(initialCapacity);
    }

    //创建一个空的HashMap用指定的容量和默认的负载系数（0.75） 
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    //创建一个空的HashMap用默认的容量（16）和默认的负载系数（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);
    }

三、扩容机制

put方法和resize()方法：

关于扩容的理解可以参考博文 ：https://blog.csdn.net/qq_27093465/article/details/52270519

                                                   https://blog.csdn.net/u013494765/article/details/77837338

                                                   https://www.cnblogs.com/chengxiao/p/6059914.html

我理解为什么HashMap的数组长度一定是2的次幂？很大一部分原因是：为了均匀分布table数据和充分利用空间，减少hash冲突，快速查询

   /**
     * 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 数组是空
                float ft = ((float)s / loadFactor) + 1.0F;
                int t = ((ft < (float)MAXIMUM_CAPACITY) ?
                         (int)ft : MAXIMUM_CAPACITY);
                if (t > threshold)//大于map的实际可容量就扩大
                    threshold = tableSizeFor(t);
            }
            else if (s > threshold)
                resize(); //扩容
            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);
            }
        }
    }
    
      /**
     * 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;
        if ((tab = table) == null || (n = tab.length) == 0) //数组未初始化
            n = (tab = resize()).length;  //数组设置初始值
        if ((p = tab[i = (n - 1) & hash]) == null) //计算脚标i，数组[i]是否是null，(n - 1) & hash计量把数组分布均匀的填满
            tab[i] = newNode(hash, key, value, null); //将key，value放到数组[i]
        else { //hash冲突时
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k)))) //判断key是否是同一个key
                e = p;
            else if (p instanceof TreeNode) //如果是tree节点 
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        //在链表头部插入key，value
                        p.next = newNode(hash, key, value, null);
                        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;
    }
    
      /**
     * 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
     */
    //以默认容量大小16，默认负载系数0.75为例
    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;//原数组
        int oldCap = (oldTab == null) ? 0 : oldTab.length;//原数组长度 16
        int oldThr = threshold;//原临界值 12
        int newCap, newThr = 0;
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {//原数组长度大于最大容量(1073741824) 
                //则将threshold设为Integer.MAX_VALUE=2147483647，接近MAXIMUM_CAPACITY的两倍
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }            
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&  //newCap=32
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                     // 新数组长度 是原数组长度的2倍，
                newThr = oldThr << 1; // double threshold 临界值也扩大为原来2倍 24
        }
        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; //新临界值 24
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; //创建以长度32的新数组
        table = newTab;
        //如果原来数组有数据，则将原数据复制到新数组中
        if (oldTab != null) {
            // 根据容量进行循环整个数组，将非空元素进行复制
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                // 获取数组的第j个元素
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null; //原数组j位置置为null，交给GC
                    // 如果链表只有一个，则进行直接赋值
                    if (e.next == null)
                        // e.hash & (newCap - 1) 确定元素存放位置
                        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;
    }

 剩下的一些方法：

putIfAbsent：value不为空并且不改变现存的值

computeIfAbsent：如果key已存在，返回oldVlaue；不存在创建，返回新创建value

computeIfPresent：如果key不存在，返回null；如果已存在，value为null则删除此节点，不为null替换节点value并返回此value。

compute：如果key不存在，新建key进行存储；如果key存在，value为null则删除此节点，不为null替换节点value并返回此value。