1.7的hashMap

学习hashMap具备的基础：位运算，集成，接口的实现，数据结构，数组+链表（红黑树）
https://www.bilibili.com/video/BV1H3411q7s7?p=8
拓展:左移相当于乘以2，右移除以2。
https://www.yuque.com/shier-tcmwp/notes/vb9f90
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1.8的hashMap

资料来源：https://www.bilibili.com/video/BV1yX4y1w7ng?share_source=copy_web

  1.   public class HashMap<K, V> extends AbstractMap<K, V>
  2.             implements Map<K, V>, Cloneable, Serializable { // 1、实现了两个Map接口，重复--》多余
  3.         //        3、属性部分
  4.         static final float DEFAULT_LOAD_FACTOR = 0.75f;
  5.         final float loadFactor;
  6.         // DEFAULT_INITIAL_CAPACITY.)
  7.         int threshold;
  8.         transient Node<K,V>[] table; // 底层数组
  9.         //      2、  调用空构造器
  10.         public HashMap() {
  11.             this.loadFactor = DEFAULT_LOAD_FACTOR; // loadFactor = 0.75
  12.         }
  14.         // 4、调用有参构造器
  15.         public HashMap(int initialCapacity) { //10
  16.             this(initialCapacity, DEFAULT_LOAD_FACTOR); // this(10,0.75)
  17.         }
  19.         //5、跳到HashMap中
  20.         public HashMap(int initialCapacity, float loadFactor) { // 10,0.75
  21.             // 6、提高健壮性处理
  22.             if (initialCapacity < 0)
  23.                 throw new IllegalArgumentException("Illegal initial capacity: " +
  24.                         initialCapacity);
  25.             if (initialCapacity > MAXIMUM_CAPACITY)
  26.                 initialCapacity = MAXIMUM_CAPACITY;
  27.             if (loadFactor <= 0 || Float.isNaN(loadFactor))
  28.                 throw new IllegalArgumentException("Illegal load factor: " +
  29.                         loadFactor);
  30.             // 7、loadFactor = 0.75
  31.             this.loadFactor = loadFactor;
  32.             // 8、调用tableSizeFor方法，传入10
  33.             this.threshold = tableSizeFor(initialCapacity);
  34.         }
  36.         /**
  37.          * Returns a power of two size for the given target capacity.
  38.          * 9、tableSizeFor---》确定主数组的长度 ---》返回的是最接近参数的2的n次幂 ----》10=》16   3=》4  6=》8
  39.          */
  40.         static final int tableSizeFor(int cap) { // 10
  41.             int n = cap - 1;
  42.             n |= n >>> 1;
  43.             n |= n >>> 2;
  44.             n |= n >>> 4;
  45.             n |= n >>> 8;
  46.             n |= n >>> 16;
  47.             return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
  48.         }
  50.         // 10、调用put方法
  51.         public V put(K key, V value) {\
  52.         // 11、调用hash方法
  53.             return putVal(hash(key), key, value, false, true);
  54.         }
  56.         // 12、hash方法,返回值就是hash码
  57.         static final int hash(Object key) {
  58.             int h;
  59.             return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
  60.         }
  62.         // 13、putVal方法
  63.         final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
  64.                        boolean evict) {
  65.             Node<K,V>[] tab; Node<K,V> p; int n, i;
  66.             if ((tab = table) == null || (n = tab.length) == 0)
  67.                 // 14、调用resize方法，该方法
  68.                 n = (tab = resize()).length;
  69.             if ((p = tab[i = (n - 1) & hash]) == null)
  70.                 tab[i] = newNode(hash, key, value, null);
  71.             else {
  72.                 Node<K,V> e; K k;
  73.                 if (p.hash == hash &&
  74.                         ((k = p.key) == key || (key != null && key.equals(k))))
  75.                     e = p;
  76.                 else if (p instanceof TreeNode)
  77.                     e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
  78.                 else {
  79.                     for (int binCount = 0; ; ++binCount) {
  80.                         if ((e = p.next) == null) {
  81.                             p.next = newNode(hash, key, value, null);
  82.                             if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
  83.                                 treeifyBin(tab, hash);
  84.                             break;
  85.                         }
  86.                         if (e.hash == hash &&
  87.                                 ((k = e.key) == key || (key != null && key.equals(k))))
  88.                             break;
  89.                         p = e;
  90.                     }
  91.                 }
  92.                 if (e != null) { // existing mapping for key
  93.                     V oldValue = e.value;
  94.                     if (!onlyIfAbsent || oldValue == null)
  95.                         e.value = value;
  96.                     afterNodeAccess(e);
  97.                     return oldValue;
  98.                 }
  99.             }
  100.             ++modCount;
  101.             if (++size > threshold)
  102.                 resize();
  103.             afterNodeInsertion(evict);
  104.             return null;
  105.         }
  107.         // 15、调用resize方法
  108.         final Node<K,V>[] resize() {
  109.             Node<K,V>[] oldTab = table;
  110.             int oldCap = (oldTab == null) ? 0 : oldTab.length;
  111.             int oldThr = threshold;
  112.             int newCap, newThr = 0;
  113.             if (oldCap > 0) {
  114.                 if (oldCap >= MAXIMUM_CAPACITY) {
  115.                     threshold = Integer.MAX_VALUE;
  116.                     return oldTab;
  117.                 }
  118.                 else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
  119.                         oldCap >= DEFAULT_INITIAL_CAPACITY)
  120.                     newThr = oldThr << 1; // double threshold
  121.             }
  122.             else if (oldThr > 0) // initial capacity was placed in threshold
  123.                 newCap = oldThr;
  124.             else {               // zero initial threshold signifies using defaults
  125.                 newCap = DEFAULT_INITIAL_CAPACITY;
  126.                 newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
  127.             }
  128.             if (newThr == 0) {
  129.                 float ft = (float)newCap * loadFactor;
  130.                 newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
  131.                         (int)ft : Integer.MAX_VALUE);
  132.             }
  133.             threshold = newThr;
  134.             @SuppressWarnings({"rawtypes","unchecked"})
  135.             Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
  136.             // 16、resize方法从开始到这里都是确定table的大小。重要的地方是就是这里，table：底层的数组长度在这里确定了，为一个xxx长度的Node类型的数组
  137.             table = newTab;
  138.             if (oldTab != null) {
  139.                 for (int j = 0; j < oldCap; ++j) {
  140.                     Node<K,V> e;
  141.                     if ((e = oldTab[j]) != null) {
  142.                         oldTab[j] = null;
  143.                         if (e.next == null)
  144.                             newTab[e.hash & (newCap - 1)] = e;
  145.                         else if (e instanceof TreeNode)
  146.                             ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
  147.                         else { // preserve order
  148.                             Node<K,V> loHead = null, loTail = null;
  149.                             Node<K,V> hiHead = null, hiTail = null;
  150.                             Node<K,V> next;
  151.                             do {
  152.                                 next = e.next;
  153.                                 if ((e.hash & oldCap) == 0) {
  154.                                     if (loTail == null)
  155.                                         loHead = e;
  156.                                     else
  157.                                         loTail.next = e;
  158.                                     loTail = e;
  159.                                 }
  160.                                 else {
  161.                                     if (hiTail == null)
  162.                                         hiHead = e;
  163.                                     else
  164.                                         hiTail.next = e;
  165.                                     hiTail = e;
  166.                                 }
  167.                             } while ((e = next) != null);
  168.                             if (loTail != null) {
  169.                                 loTail.next = null;
  170.                                 newTab[j] = loHead;
  171.                             }
  172.                             if (hiTail != null) {
  173.                                 hiTail.next = null;
  174.                                 newTab[j + oldCap] = hiHead;
  175.                             }
  176.                         }
  177.                     }
  178.                 }
  179.             }
  180.             return newTab;
  181.         }
  183.     }

经典面试题：
1、装填因子，装载因子，加载因子为什么要设置为0.75。
装填因子设置为1，空间利用率得到大大的提高，很容易发生碰撞，产生链表-》查询效率低
装填因子设置为0.5，空间利用率太低了，碰撞几率也低，产生链表低-》查询效率高。
所以取了一个折中的：0.75
2、主数组的长度为什么必须为2的n次方。
原因1：h & (length - 1)等效 h % length 操作，等效的前提是：length必须是2的整数倍。
原因2：防止哈希冲突，位置冲突。

  • 验证是整数倍：

length：8
hash ：3 00000011
length-1 00000111
——————————————————与运算，相同为1，不同为0
00000011 —-》3 位置

hash ：2 00000010
length-1 00000111
——————————————————
00000010 ——-》2位置

  • 验证不是整数倍：

length：9
hash ：3 00000011
length-1 00001000
—————————————
00000000 ——》0位置

hash ：2 00000010
length-1 00001000
—————————————————
00000000 ——》0位置

3、头插法为什么有循环问题