HashMap优点总结及源码分析

HashMap优点总结:

可存储不同类型的数据：使用泛型来定义键和值的类型，兼容所有数据类型
高效的查找和插入操作：通过key的hash映射，实现快速的查找和插入操作。时间复杂度基本为O(1)
灵活的容量调整：可根据数据量增长自行动态扩容。当容量过大时，HashMap会自动进行缩容，从而提高空间利用率
节点转换：当链表中的节点个数超过8个时，结构会转换为红黑树，以减少平均查找时间，提高插入和删除操作的性能。
可存储大量数据：HashMap的存储空间只受限于计算机的内存大小。
支持快速迭代：HashMap提供了快速的迭代方法，可以方便地遍历所有的键值对。
优秀的算法1：表容量保证是2的幂数，容量-1做与运算的散列效果更好，同时便于计算扩容后元素位置，可以提高扩容效率
优秀的算法2：高效的容量值优化算法，借用位运算高效运算。输入容量-1，之后将后面的每一个二进制位都转为1，最后值+1，保证最终结果一定是大于等于当前值的最小2的幂
优秀的算法3：key的hash运算，key的hashcode值向右位运算，纳入高位影响，更好的实现扩散效果,同时减少hash碰撞
迭代器快速失败：HashMap的迭代器使用快速失败机制，通过modCount属性监测并发修改，即在迭代时如果有其他线程并发地修改了HashMap的结构（例如添加或删除键值对），那么迭代器会立即抛出ConcurrentModificationException异常，避免出现数据不一致的情况。

好的设计思想：根据不同数据量，灵活切换多样合适的数据结构，如Redis的数据结构中也有体现

源码解析：

HashMap类

public class HashMap<K,V> extends AbstractMap<K,V>implements Map<K,V>, Cloneable, Serializable {

泛型容纳所有类型的key,value包括null

属性解析

/*** 版本序列号，可保证当类增减属性后，依旧可以反序列化成功* 位运算，提醒大家hash表大小一定是2的幂*/
private static final long serialVersionUID = 362498820763181265L;
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16/*** 总数量，默认最大容量；最大容量一定是2的幂数* 好处1：2的幂值-1做与运算散列效果更好* 好处2：便于计算扩容后元素位置可以提高扩容效率*/
static final int MAXIMUM_CAPACITY = 1 << 30;/*** 构造方法不指定时的默认负载因子* 达到总容量的75% 即resize扩容*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;/*** 链表长度至少达到8 结构才能转为红黑树*/
static final int TREEIFY_THRESHOLD = 8;/*** 树的节点数减到6 退化为链表*/
static final int UNTREEIFY_THRESHOLD = 6;/*** 数组的长度至少达到64，链表才能转为红黑树*/
static final int MIN_TREEIFY_CAPACITY = 64;

Node解析

Node存储实际key,value数据的结构，TreeNode的父类

    static class Node<K,V> implements Map.Entry<K,V> {final int hash;final K key;V value;Node<K,V> next;Node(int hash, K key, V value, Node<K,V> next) {this.hash = hash;this.key = key;this.value = value;this.next = next;}public final K getKey()        { return key; }public final V getValue()      { return value; }public final String toString() { return key + "=" + value; }public final int hashCode() {return Objects.hashCode(key) ^ Objects.hashCode(value);}public final V setValue(V newValue) {V oldValue = value;value = newValue;return oldValue;}public final boolean equals(Object o) {if (o == this)return true;if (o instanceof Map.Entry) {Map.Entry<?,?> e = (Map.Entry<?,?>)o;if (Objects.equals(key, e.getKey()) &&Objects.equals(value, e.getValue()))return true;}return false;}}

key的hash计算方式解析

    static final int hash(Object key) {int h;// 向右位运算，纳入高位影响，更好的实现扩散效果,同时减少hash碰撞，^运算结果不同为1相同为0return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);}

tableSizeFor表扩容容量值优化方法解析

为保证key更好的分散效果及更佳的扩容效率，保证最大容量始终是2的幂

	/*** 将输入容量规范化为2的幂          			       * 算法逻辑：位运算效率更高。输入容量-1，之后将后面的每一个二进制位都转为1，最后值+1，保证最终结果一定是大于等于当前值的最小2的幂	*/static final int tableSizeFor(int cap) {int n = cap - 1;n |= n >>> 1;n |= n >>> 2;n |= n >>> 4;n |= n >>> 8;n |= n >>> 16;return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;}

Fields 其他属性解析

 /* ---------------- Fields -------------- *//*** 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 修饰，序列化会忽略此属性*/transient Node<K,V>[] table;/*** Holds cached entrySet(). Note that AbstractMap fields are used* for keySet() and values().*/transient Set<Map.Entry<K,V>> entrySet;/*** The number of key-value mappings contained in this map.*/transient int size;/*** The number of times this HashMap has been structurally modified* Structural modifications are those that change the number of mappings in* the HashMap or otherwise modify its internal structure (e.g.,* rehash).  This field is used to make iterators on Collection-views of* the HashMap fail-fast.  (See ConcurrentModificationException).* 在HashMap中用于记录结构修改次数，以支持Fail-Fast机制并防止并发修改导致的不一致操作。*/transient int modCount;/*** The next size value at which to resize (capacity * load factor).** @serial*/// (The javadoc description is true upon serialization.// Additionally, if the table array has not been allocated, this// field holds the initial array capacity, or zero signifying// DEFAULT_INITIAL_CAPACITY.)int threshold;/*** The load factor for the hash table.** @serial*/final float loadFactor;

put方法解析

第一次初始化hash表是在第一次调用put的时候

插入，更新，获取都是大体思路相同

判断表是否为null或长度length=0
定位到hash桶中key的位置
判断头结点是否存在并进行处理
判断为树结构并进行处理
判断为链表结构并进行处理

/*** Associates the specified value with the specified key in this map.* If the map previously contained a mapping for the key, the old* value is replaced.** @param key key with which the specified value is to be associated* @param value value to be associated with the specified key* @return the previous value associated with <tt>key</tt>, or*         <tt>null</tt> if there was no mapping for <tt>key</tt>.*         (A <tt>null</tt> return can also indicate that the map*         previously associated <tt>null</tt> with <tt>key</tt>.)*/public V put(K key, V value) {return putVal(hash(key), key, value, false, true);}/*** Implements Map.put and related methods** @param hash hash for key* @param key the key* @param value the value to put* @param onlyIfAbsent if true, don't change existing value* @param evict if false, the table is in creation mode.* @return previous value, or null if none*/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;// hash桶定位计算方法(n - 1) & hash。模运算也可以做到，但是没有位运算效率高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 1sttreeifyBin(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 keyV oldValue = e.value;if (!onlyIfAbsent || oldValue == null)e.value = value;afterNodeAccess(e);return oldValue;}}++modCount;if (++size > threshold)resize();afterNodeInsertion(evict);return null;}