简简单单复习一哈HashMap

HashMap

可被序列化，线程不安全，允许null值和null键，

安全的Map

Collections.synchronizedMap()：

1/**
2  * Returns a synchronized (thread-safe) map backed by the specified map. In order to guarantee 
3  * serial access, it is critical that all access to the backing map is accomplished through the 
4  * returned map.
5  * It is imperative that the user manually synchronize on the returned map when traversing any of 
6  * its collection views via Iterator, Spliterator or Stream:
7  *      Map m = Collections.synchronizedMap(new HashMap());
8  *          ...
9  *      Set s = m.keySet();  // Needn't be in synchronized block
10  *          ...
11  *      synchronized (m) {  // Synchronizing on m, not s!
12  *          Iterator i = s.iterator(); // Must be in synchronized block
13  *          while (i.hasNext())
14  *              foo(i.next());
15  *  }
16  */
17public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
18    return new SynchronizedMap<>(m);
19}

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)

1/**
2  * The smallest table capacity for which bins may be treeified.
3  * (Otherwise the table is resized if too many nodes in a bin.)
4  * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
5  * between resizing and treeification thresholds.
6  */
7static final int MIN_TREEIFY_CAPACITY = 64;
8
9/**
10  * Replaces all linked nodes in bin at index for given hash unless
11  * table is too small, in which case resizes instead.
12  */
13final void treeifyBin(Node<K,V>[] tab, int hash) {
14        int n, index; Node<K,V> e;
15        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
16            resize();
17        else if ((e = tab[index = (n - 1) & hash]) != null) {
18            TreeNode<K,V> hd = null, tl = null;
19            do {
20                TreeNode<K,V> p = replacementTreeNode(e, null);
21                if (tl == null)
22                    hd = p;
23                else {
24                    p.prev = tl;
25                    tl.next = p;
26                }
27                tl = p;
28            } while ((e = e.next) != null);
29            if ((tab[index] = hd) != null)
30                hd.treeify(tab);
31        }
32    }

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

1    /**
2     * Associates the specified value with the specified key in this map.
3     * If the map previously contained a mapping for the key, the old
4     * value is replaced.
5     *
6     * @param key key with which the specified value is to be associated
7     * @param value value to be associated with the specified key
8     * @return the previous value associated with {@code key}, or
9     *         {@code null} if there was no mapping for {@code key}.
10     *         (A {@code null} return can also indicate that the map
11     *         previously associated {@code null} with {@code key}.)
12     */
13    public V put(K key, V value) {
14        return putVal(hash(key), key, value, false, true);
15    }
16
17    /**
18     * Implements Map.put and related methods.
19     *
20     * @param hash hash for key
21     * @param key the key
22     * @param value the value to put
23     * @param onlyIfAbsent if true, don't change existing value
24     * @param evict if false, the table is in creation mode.
25     * @return previous value, or null if none
26     */
27    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
28                   boolean evict) {
29        Node<K,V>[] tab; 
30        Node<K,V> p; 
31        int n, i;
32        if ((tab = table) == null || (n = tab.length) == 0)
33            n = (tab = resize()).length;
34        if ((p = tab[i = (n - 1) & hash]) == null)
35            tab[i] = newNode(hash, key, value, null);
36        else {
37            Node<K,V> e; K k;
38            if (p.hash == hash &&
39                ((k = p.key) == key || (key != null && key.equals(k))))
40                e = p;
41            else if (p instanceof TreeNode)
42                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
43            else {
44                for (int binCount = 0; ; ++binCount) {
45                    if ((e = p.next) == null) {
46                        p.next = newNode(hash, key, value, null);
47                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
48                            treeifyBin(tab, hash);
49                        break;
50                    }
51                    if (e.hash == hash &&
52                        ((k = e.key) == key || (key != null && key.equals(k))))
53                        break;
54                    p = e;
55                }
56            }
57            if (e != null) { // existing mapping for key
58                V oldValue = e.value;
59                if (!onlyIfAbsent || oldValue == null)
60                    e.value = value;
61                afterNodeAccess(e);
62                return oldValue;
63            }
64        }
65        ++modCount;
66        if (++size > threshold)
67            resize();
68        afterNodeInsertion(evict);
69        return null;
70    }