HashMap

HashMap是基于哈希表的 Map 接口的实现。HashMap 的实例有两个参数影响其性能：初始容量（16）和加载因子（0.75）。

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 初始容量16
static final float DEFAULT_LOAD_FACTOR = 0.75f;//加载因子0.75

HashMap的特点：

1. HashMap中允许存放null键、null值
2. 线程不安全 –线程不同步
3. 与HashTable大致相同
4. Hashset是HashMap的实例 HashMap的底层是HashTable哈希表
5. 在Java中 HashTable的父类是Dictionary

HashMap的底层结构主要是链表和数组，在jdk1.8之后增加了红黑树。如果链表长度超过阀值，就把链表转成红黑树，链表长度低于6，就把红黑树转回链表。
static final int TREEIFY_THRESHOLD = 8;
static final int UNTREEIFY_THRESHOLD = 6;

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16（初始容量默认16）
    static final int MAXIMUM_CAPACITY = 1 << 30;//最大容量为2^30
    static final float DEFAULT_LOAD_FACTOR = 0.75f;//加载因子默认0.75
    static final int TREEIFY_THRESHOLD = 8;//树化阀值
    static final int UNTREEIFY_THRESHOLD = 6;//阀值
    static final int MIN_TREEIFY_CAPACITY = 64;
    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Node<K,V> next;//链表
        ...
    }
    ...
    transient Node<K,V>[] table;//数组
    ...
}

HashMap的put方法（源码）

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
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;
        //i ： key的hashcode运算后的值赋给i作为key在数组中的索引
        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;
    }

未完待续…