1.HashMap源码阅读目标
了解具体的数据结构(hash及冲突链表、红黑树)和重要方法的具体实现(hashCode、equals、put、resize...)

2.重要方法
hashCode 与 equals都是在AbstractMap中定义的
hashCode是各元素hash的累加 h += iter.next().hashCode();
equals 1.是否是本身; 2.是否是Map实例; 3.size是否相等; 4.比较每个value
重点在于put、resize具体实现步骤:
put:
  1.tab为null或length为0 重新resize
  2.位置hash(key) & (n-1)的元素为null,则直接赋值
  3.既然对应位置的元素不为null,则要看它有什么类型(单个元素(hash无冲突)或红黑树或链表)
单个元素(新的元素如果与这个元素不相等)则要转为链表,链表则可能转为红黑树(转化规则 >= 7)
  ++modCount
  4.++size > threshold 则resize()
  remove类似(<=6则转化为链表)

 1 final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
 2                    boolean evict) {
 3         Node<K,V>[] tab; Node<K,V> p; int n, i;
 4         if ((tab = table) == null || (n = tab.length) == 0)
 5             n = (tab = resize()).length;
 6         if ((p = tab[i = (n - 1) & hash]) == null)
 7             tab[i] = newNode(hash, key, value, null);
 8         else {
 9             Node<K,V> e; K k;
10             if (p.hash == hash &&
11                 ((k = p.key) == key || (key != null && key.equals(k))))
12                 e = p;
13             else if (p instanceof TreeNode)
14                 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
15             else {
16                 for (int binCount = 0; ; ++binCount) {
17                     if ((e = p.next) == null) {
18                         p.next = newNode(hash, key, value, null);
19                         if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
20                             treeifyBin(tab, hash);
21                         break;
22                     }
23                     if (e.hash == hash &&
24                         ((k = e.key) == key || (key != null && key.equals(k))))
25                         break;
26                     p = e;
27                 }
28             }
29             if (e != null) { // existing mapping for key
30                 V oldValue = e.value;
31                 if (!onlyIfAbsent || oldValue == null)
32                     e.value = value;
33                 afterNodeAccess(e);
34                 return oldValue;
35             }
36         }
37         ++modCount;
38         if (++size > threshold)
39             resize();
40         afterNodeInsertion(evict);
41         return null;
42     }


treeifyBin: 

  链表转为红黑树,红黑树较为复杂,所以将单独另起一篇仔细研究学习


keySet/entrySet:

1 new KeySet();
2 forEach:
3     int mc = modCount;
4     for (int i = 0; i < tab.length; ++i) {
5         for (Node<K,V> e = tab[i]; e != null; e = e.next)
6             action.accept(e.key);
7     }
8     if (modCount != mc)
9         throw new ConcurrentModificationException();
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