HashMap的知识点整理

/**
 * Hash table based implementation of the <tt>Map</tt> interface.  This
 * implementation provides all of the optional map operations, and permits
 * <tt>null</tt> values and the <tt>null</tt> key.  (The <tt>HashMap</tt>
 * class is roughly equivalent to <tt>Hashtable</tt>, except that it is
 * unsynchronized and permits nulls.)  This class makes no guarantees as to
 * the order of the map; in particular, it does not guarantee that the order
 * will remain constant over time.
 */

//默认的初始容量为 16
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
//最大的容量上限为 2^30
static final int MAXIMUM_CAPACITY = 1 << 30;
//默认的负载因子为 0.75
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//变成树型结构的临界值为 8
static final int TREEIFY_THRESHOLD = 8;
//恢复链式结构的临界值为 6
static final int UNTREEIFY_THRESHOLD = 6;
//哈希表
transient Node<K,V>[] table;
//哈希表中键值对的个数
transient int size;
//哈希表被修改的次数
transient int modCount;
//它是通过 capacity*load factor 计算出来的，当 size 到达这个值时，
就会进行扩容操作
int threshold;
//负载因子
final float loadFactor;
//当哈希表的大小超过这个阈值，才会把链式结构转化成树型结构，否则仅采
取扩容来尝试减少冲突
static final int MIN_TREEIFY_CAPACITY = 64;

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;
    }

A hash table supporting full concurrency of retrievals and
 * high expected concurrency for updates. This class obeys the
 * same functional specification as {@link java.util.Hashtable}, and
 * includes versions of methods corresponding to each method of
 * {@code Hashtable}. However, even though all operations are
 * thread-safe, retrieval operations do <em>not</em> entail locking,
 * and there is <em>not</em> any support for locking the entire table
 * in a way that prevents all access.  This class is fully
 * interoperable with {@code Hashtable} in programs that rely on its
 * thread safety but not on its synchronization details.