HashMap

可被序列化，线程不安全，允许null值和null键，

安全的Map

Collections.synchronizedMap()：

/**
  * Returns a synchronized (thread-safe) map backed by the specified map. In order to guarantee 
  * serial access, it is critical that all access to the backing map is accomplished through the 
  * returned map.
  * It is imperative that the user manually synchronize on the returned map when traversing any of 
  * its collection views via Iterator, Spliterator or Stream:
  *      Map m = Collections.synchronizedMap(new HashMap());
  *          ...
  *      Set s = m.keySet();  // Needn't be in synchronized block
  *          ...
  *      synchronized (m) {  // Synchronizing on m, not s!
  *          Iterator i = s.iterator(); // Must be in synchronized block
  *          while (i.hasNext())
  *              foo(i.next());
  *  }
  */
public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
    return new SynchronizedMap<>(m);
}

ConcurrentHashMap：

默认负载因子为0.75，

static final float DEFAULT_LOAD_FACTOR = 0.75f;

默认初始大小为16，

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

当阈值超过16*0.75=12时，会进行扩增操作，每次扩展会进行2的幂扩展，如果当前容量超过了默认容量大小时，最大容量会变成Integer的最大值

最大容量

static final int MAXIMUM_CAPACITY = 1 << 30;

当容量大于等于64时，HashMap会将列表转化为树(bin的值必须大于2且至少为8)

/**
  * 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;

/**
  * Replaces all linked nodes in bin at index for given hash unless
  * table is too small, in which case resizes instead.
  */
final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
                TreeNode<K,V> p = replacementTreeNode(e, null);
                if (tl == null)
                    hd = p;
                else {
                    p.prev = tl;
                    tl.next = p;
                }
                tl = p;
            } while ((e = e.next) != null);
            if ((tab[index] = hd) != null)
                hd.treeify(tab);
        }
    }

在进行put操作时，如果已经有相同的key，则会将其值进行替换。如果hash值相同会进行内存地址比较，如果地址不相同，则会继续进行非空和equals值比较

    /**
     * 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 {@code key}, or
     *         {@code null} if there was no mapping for {@code key}.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with {@code key}.)
     */
    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)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            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))))
                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);
                        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;
    }