jdk1.8中hashmap的扩容步骤resize

Posted 2021-05-28 小大白日志

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当hashmap第一次插入元素、元素个数达到容量阀值threshold时，都会扩容resize(),源码：

 1 final Node<K,V>[] resize() {
 2         Node<K,V>[] oldTab = table;
 3         int oldCap = (oldTab == null) ? 0 : oldTab.length;
 4         int oldThr = threshold;
 5         int newCap, newThr = 0;
 6         if (oldCap > 0) {
 7             if (oldCap >= MAXIMUM_CAPACITY) {
 8                 threshold = Integer.MAX_VALUE;
 9                 return oldTab;
10             }
11             else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
12                      oldCap >= DEFAULT_INITIAL_CAPACITY)  //注释1
13                 newThr = oldThr << 1; // double threshold
14         }
15         else if (oldThr > 0) // initial capacity was placed in threshold
16             newCap = oldThr;
17         else {               // zero initial threshold signifies using defaults
18             newCap = DEFAULT_INITIAL_CAPACITY;
19             newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
20         }
21         if (newThr == 0) {
22             float ft = (float)newCap * loadFactor;
23             newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
24                       (int)ft : Integer.MAX_VALUE);
25         }
26         threshold = newThr;
27         @SuppressWarnings({"rawtypes","unchecked"})
28             Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
29         table = newTab;
30         if (oldTab != null) {
31             for (int j = 0; j < oldCap; ++j) {  //注释2
32                 Node<K,V> e;
33                 if ((e = oldTab[j]) != null) {//注释3
34                     oldTab[j] = null;
35                     if (e.next == null)       //注释4
36                         newTab[e.hash & (newCap - 1)] = e;
37                     else if (e instanceof TreeNode) //注释5
38                         ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
39                     else { // preserve order        //注释6
40                         Node<K,V> loHead = null, loTail = null; //注释7
41                         Node<K,V> hiHead = null, hiTail = null;//注释8
42                         Node<K,V> next;
43                         do {
44                             next = e.next;
45                             if ((e.hash & oldCap) == 0) { //注释9
46                                 if (loTail == null)  //注释10
47                                     loHead = e;
48                                 else
49                                     loTail.next = e; 
50                                 loTail = e;         
51                             }
52                             else {
53                                 if (hiTail == null)//注释11
54                                     hiHead = e;
55                                 else
56                                     hiTail.next = e;
57                                 hiTail = e;
58                             }
59                         } while ((e = next) != null);
60                         if (loTail != null) {  //注释12
61                             loTail.next = null;
62                             newTab[j] = loHead;
63                         }
64                         if (hiTail != null) {//注释13
65                             hiTail.next = null;
66                             newTab[j + oldCap] = hiHead;
67                         }
68                     }
69                 }
70             }
71         }
72         return newTab;
73     }