1. HashMap要点：
1.1 基本数据结构：
采用 数组+链表/平衡二叉查找树 的组合形式，所有键值对都以Entry<K,V>形式存储（每put进一个键值对，就会实例化一个Entry<K, V>）。
- 数组：Entry<K,V>数组，以hash( key.hashCode() ) 为数组索引。即计算键值的hash值，以此为索引存储键值对Entry<K, V>。数组长度总是2的n次方（这与hash有关，后边会讲）
- 链表：如果hash()方法算出的hash值相同，在该索引处，建立链表，存储hash值相同的键值对。
- 平衡二叉查找树：如果该当前索引处是一个链表，且插入当前键值对后，该链表处元素个数超过阈值（8），则把当前索引处的链表转换为一个平衡二叉查找树；如果当前索引处是一个树，则直接直接插入当前键值对
JDK1.8版本中table的定义（Node<K, V>实现了接口Entry<K,V>）：
/** * The table, initialized on first use, and resized as * necessary. When allocated, length is always a power of two. * (We also tolerate length zero in some operations to allow * bootstrapping mechanics that are currently not needed.) */ transient Node<K,V>[] table; //static class Node<K,V> implements Map.Entry<K,V>
table初值为16，扩充时，每次扩原来的两倍。下边是对table初值的定义（java中基本数据类型的字长是固定的，平台独立。如int总是4bytes的，因此table索引最大是整型的最大值。）：
1 /** 2 * The default initial capacity - MUST be a power of two. 3 */ 4 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 5 6 /** 7 * The maximum capacity, used if a higher value is implicitly specified 8 * by either of the constructors with arguments. 9 * MUST be a power of two <= 1<<30. 10 */ 11 static final int MAXIMUM_CAPACITY = 1 << 30; 12 13 /** 14 * The bin count threshold for using a tree rather than list for a 15 * bin. Bins are converted to trees when adding an element to a 16 * bin with at least this many nodes. The value must be greater 17 * than 2 and should be at least 8 to mesh with assumptions in 18 * tree removal about conversion back to plain bins upon 19 * shrinkage. 20 */ 21 static final int TREEIFY_THRESHOLD = 8; //每个索引处链表元素个数的阈值，超过则用二叉查找树存
1.2 哈希算法
JDK1.8版本的源码如下：
/** * Computes key.hashCode() and spreads (XORs) higher bits of hash * to lower. Because the table uses power-of-two masking, sets of * hashes that vary only in bits above the current mask will * always collide. (Among known examples are sets of Float keys * holding consecutive whole numbers in small tables.) So we * apply a transform that spreads the impact of higher bits * downward. There is a tradeoff between speed, utility, and * quality of bit-spreading. Because many common sets of hashes * are already reasonably distributed (so don't benefit from * spreading), and because we use trees to handle large sets of * collisions in bins, we just XOR some shifted bits in the * cheapest possible way to reduce systematic lossage, as well as * to incorporate impact of the highest bits that would otherwise * never be used in index calculations because of table bounds. */ static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); }
即将key.hashCode()的高16位与低16位按位异或的结果作为键值的hash值。
有几点：
- key.hashCode()：hashCode()是Object类的本地方法。键值对象类可以重写自己的hashCode()方法，例如Integer类的hashCode()方法仅返回其int值。
- hash()：为避免key.hashCode()获得的hash值重复太多，如Integer类的就很简单，所以就又加了一层hash。但是也可能原来的hashCode()方法就已经很好，所以衡量效率和效果之后，采用尽量简单的方法来获取新的hash值。
- h>>>16：这里hash值的结果是将高16位与低16位异或，并存储在低16位中。即hash值是16位bit值，这与1.1中介绍的table长度是相关的，因为table索引就是hash值。
1.3 主要基本操作
1.3.1 put()
- 索引处没有键值对，直接插入为链表
- 否则，如果树中或链表中有相同key值，则更新原来的value值，并返回旧value值
- 否则，如果是链表则插入链表，插入后超过长度阈值，则转为二叉查找树；如果是树，则插入树
JDK1.8版本源码如下：
1 public V put(K key, V value) { 2 return putVal(hash(key), key, value, false, true); 3 } 4 5 /** 6 * Implements Map.put and related methods 7 * 8 * @param hash hash for key 9 * @param key the key 10 * @param value the value to put 11 * @param onlyIfAbsent if true, don't change existing value 12 * @param evict if false, the table is in creation mode. 13 * @return previous value, or null if none 14 */ 15 final V putVal(int hash, K key, V value, boolean onlyIfAbsent, 16 boolean evict) { 17 Node<K,V>[] tab; Node<K,V> p; int n, i;
// 如果目前table为空或0，就对table初始化resize()， 18 if ((tab = table) == null || (n = tab.length) == 0) 19 n = (tab = resize()).length;
//如果当前key值的hash索引处没有值，直接在该索引处存储键值对 20 if ((p = tab[i = (n - 1) & hash]) == null) 21 tab[i] = newNode(hash, key, value, null);
//如果当前key值的hash索引处有值 22 else { 23 Node<K,V> e; K k;
//如果当前索引处的key值和待插入的key值相同（即待插入键值对与bin内第一个键值对的键值相同）----即使是不同对象，值相同就可以（因为采用了equals()方法判断） 24 if (p.hash == hash && 25 ((k = p.key) == key || (key != null && key.equals(k)))) 26 e = p;
//否则，如果当前bin内是以树节点的形式存储，把该节点插入到这个树中 27 else if (p instanceof TreeNode) 28 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //如果树中已经有这个节点，返回这个节点之前的value；如果没有返回null
//否则，即当前bin内以链表形式存储，遍历链表，插入该节点 29 else { 30 for (int binCount = 0; ; ++binCount) { 31 if ((e = p.next) == null) { 32 p.next = newNode(hash, key, value, null); 33 if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st //如果插入节点之后，元素个数超过阈值，就把链表转换为二叉查找树 34 treeifyBin(tab, hash); 35 break; 36 } 37 if (e.hash == hash && 38 ((k = e.key) == key || (key != null && key.equals(k)))) 39 break; 40 p = e; 41 } 42 }
//如果该索引处有键值相同的键值对，那么更新原有键值对的value为新的value，返回旧的value 43 if (e != null) { // existing mapping for key 44 V oldValue = e.value; 45 if (!onlyIfAbsent || oldValue == null) 46 e.value = value; 47 afterNodeAccess(e); 48 return oldValue; 49 } 50 } 51 ++modCount; 52 if (++size > threshold) 53 resize(); 54 afterNodeInsertion(evict); 55 return null; 56 }
1.3.2 pop
- 如果hashmap中有当前的key，返回其对应的value，否则返回null
JDK1.8版本源码如下：
1 /** 2 * Returns the value to which the specified key is mapped, 3 * or {@code null} if this map contains no mapping for the key. 4 * 5 * <p>More formally, if this map contains a mapping from a key 6 * {@code k} to a value {@code v} such that {@code (key==null ? k==null : 7 * key.equals(k))}, then this method returns {@code v}; otherwise 8 * it returns {@code null}. (There can be at most one such mapping.) 9 * 10 * <p>A return value of {@code null} does not <i>necessarily</i> 11 * indicate that the map contains no mapping for the key; it's also 12 * possible that the map explicitly maps the key to {@code null}. 13 * The {@link #containsKey containsKey} operation may be used to 14 * distinguish these two cases. 15 * 16 * @see #put(Object, Object) 17 */ 18 public V get(Object key) { 19 Node<K,V> e; 20 return (e = getNode(hash(key), key)) == null ? null : e.value; 21 }
