JDK 1.8 HashMap 底层原理实现

1.hashMap 的特定属性

//默认初始容量为16，0000 0001 右移4位 0001 0000为16，主干数组的初始容量为16，//必须是2的倍数static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16//最大容量为int的最大值除2static final int MAXIMUM_CAPACITY = 1 << 30;//默认加载因子为0.75static final float DEFAULT_LOAD_FACTOR = 0.75f;//阈值，如果主干数组上的链表的长度大于8，链表转化为红黑树static final int TREEIFY_THRESHOLD = 8;//hash表扩容后，如果发现某一个红黑树的长度小于6，则会重新退化为链表static final int UNTREEIFY_THRESHOLD = 6;//当hashmap容量大于64时，链表才能转成红黑树static final int MIN_TREEIFY_CAPACITY = 64;//临界值=主干数组容量*负载因子int threshold;transient int modCount;//被修改的次数fast-fail机制int threshold;//临界值 当实际大小(容量*填充比)超过临界值时，会进行扩容final float loadFactor;//填充比`.

数组元素Node<K,V> 实现Map.Entry<K,V> ```java

//Node是单向链表，它实现了Map.Entry接口static class Node<k,v> implements Map.Entry<k,v> {final int hash;final K key;V value;Node<k,v> next;//构造函数Hash值 键 值 下一个节点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;}//判断两个node是否相等,若key和value都相等，返回true。可以与自身比较为truepublic 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;}}

## 红黑树```javastatic final class TreeNode<k,v> extends LinkedHashMap.Entry<k,v> {TreeNode<k,v> parent;  // 父节点TreeNode<k,v> left; //左子树TreeNode<k,v> right;//右子树TreeNode<k,v> prev;    // needed to unlink next upon deletionboolean red;    //颜色属性TreeNode(int hash, K key, V val, Node<k,v> next) {super(hash, key, val, next);}//返回当前节点的根节点final TreeNode<k,v> root() {for (TreeNode<k,v> r = this, p;;) {if ((p = r.parent) == null)return r;r = p;}}

构造方法

//构造函数1public HashMap(int initialCapacity, float loadFactor) {//指定的初始容量非负if (initialCapacity < 0)throw new IllegalArgumentException(Illegal initial capacity:  +initialCapacity);//如果指定的初始容量大于最大容量,置为最大容量if (initialCapacity > MAXIMUM_CAPACITY)initialCapacity = MAXIMUM_CAPACITY;//填充比为正if (loadFactor <= 0 || Float.isNaN(loadFactor))throw new IllegalArgumentException(Illegal load factor:  +loadFactor);this.loadFactor = loadFactor;this.threshold = tableSizeFor(initialCapacity);//新的扩容临界值}//构造函数2public HashMap(int initialCapacity) {this(initialCapacity, DEFAULT_LOAD_FACTOR);}//构造函数3public HashMap() {this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted}//构造函数4用m的元素初始化散列映射public HashMap(Map<!--? extends K, ? extends V--> m) {this.loadFactor = DEFAULT_LOAD_FACTOR;putMapEntries(m, false);}

HasMap 存值

public V put(K key, V value) {return putVal(hash(key), key, value, false, true);}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;/*如果table的在（n-1）&hash的值是空，就新建一个节点插入在该位置*/if ((p = tab[i = (n - 1) & hash]) == null)tab[i] = newNode(hash, key, value, null);/*表示有冲突,开始处理冲突*/else {Node<K,V> e;K k;/*检查第一个Node，p是不是要找的值*/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) {/*指针为空就挂在后面*/if ((e = p.next) == null) {p.next = newNode(hash, key, value, null);//如果冲突的节点数已经达到8个，看是否需要改变冲突节点的存储结构，　　　　　　　　　　　　　　　　　　　　　　　　　//treeifyBin首先判断当前hashMap的长度，如果不足64，只进行//resize，扩容table，如果达到64，那么将冲突的存储结构为红黑树if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1sttreeifyBin(tab, hash);break;}/*如果有相同的key值就结束遍历*/if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))break;p = e;}}/*就是链表上有相同的key值*/if (e != null) { // existing mapping for key，就是key的Value存在V oldValue = e.value;if (!onlyIfAbsent || oldValue == null)e.value = value;afterNodeAccess(e);return oldValue;//返回存在的Value值}}++modCount;/*如果当前大小大于门限，门限原本是初始容量*0.75*/if (++size > threshold)resize();//扩容两倍afterNodeInsertion(evict);return null;}

下面简单说下添加键值对put(key,value)的过程：
1，判断键值对数组tab[]是否为空或为null，否则以默认大小resize()；
2，根据键值key计算hash值得到插入的数组索引i，如果tab[i]==null，直接新建节点添加，否则转入3
3，判断当前数组中处理hash冲突的方式为链表还是红黑树(check第一个节点类型即可),分别处理

HashMap 取值getValue()

public V get(Object key) {Node<K,V> e;return (e = getNode(hash(key), key)) == null ? null : e.value;}final Node<K,V> getNode(int hash, Object key) {Node<K,V>[] tab;//Entry对象数组Node<K,V> first,e; //在tab数组中经过散列的第一个位置int n;K k;/*找到插入的第一个Node，方法是hash值和n-1相与，tab[(n - 1) & hash]*///也就是说在一条链上的hash值相同的if ((tab = table) != null && (n = tab.length) > 0 &&(first = tab[(n - 1) & hash]) != null) {/*检查第一个Node是不是要找的Node*/if (first.hash == hash && // always check first node((k = first.key) == key || (key != null && key.equals(k))))//判断条件是hash值要相同，key值要相同return first;/*检查first后面的node*/if ((e = first.next) != null) {if (first instanceof TreeNode)return ((TreeNode<K,V>)first).getTreeNode(hash, key);/*遍历后面的链表，找到key值和hash值都相同的Node*/do {if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))return e;} while ((e = e.next) != null);}}return null;}

get(key)方法时获取key的hash值，计算hash&(n-1)得到在链表数组中的位置first=tab[hash&(n-1)],先判断first的key是否与参数key相等，不等就遍历后面的链表找到相同的key值返回对应的Value值即可

## resize() 扩容

final Node<K,V>[] resize() {Node<K,V>[] oldTab = table;int oldCap = (oldTab == null) ? 0 : oldTab.length;int oldThr = threshold;int newCap, newThr = 0;/*如果旧表的长度不是空*/if (oldCap > 0) {if (oldCap >= MAXIMUM_CAPACITY) {threshold = Integer.MAX_VALUE;return oldTab;}/*把新表的长度设置为旧表长度的两倍，newCap=2*oldCap*/else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&oldCap >= DEFAULT_INITIAL_CAPACITY)/*把新表的门限设置为旧表门限的两倍，newThr=oldThr*2*/newThr = oldThr << 1; // double threshold}/*如果旧表的长度的是0，就是说第一次初始化表*/else if (oldThr > 0) // initial capacity was placed in thresholdnewCap = oldThr;else {               // zero initial threshold signifies using defaultsnewCap = DEFAULT_INITIAL_CAPACITY;newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);}if (newThr == 0) {float ft = (float)newCap * loadFactor;//新表长度乘以加载因子newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?(int)ft : Integer.MAX_VALUE);}threshold = newThr;@SuppressWarnings({"rawtypes","unchecked"})/*下面开始构造新表，初始化表中的数据*/Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];table = newTab;//把新表赋值给tableif (oldTab != null) {//原表不是空要把原表中数据移动到新表中/*遍历原来的旧表*/for (int j = 0; j < oldCap; ++j) {Node<K,V> e;if ((e = oldTab[j]) != null) {oldTab[j] = null;if (e.next == null)//说明这个node没有链表直接放在新表的e.hash & (newCap - 1)位置newTab[e.hash & (newCap - 1)] = e;else if (e instanceof TreeNode)((TreeNode<K,V>)e).split(this, newTab, j, oldCap);/*如果e后边有链表,到这里表示e后面带着个单链表，需要遍历单链表，将每个结点重*/else { // preserve order保证顺序////新计算在新表的位置，并进行搬运Node<K,V> loHead = null, loTail = null;Node<K,V> hiHead = null, hiTail = null;Node<K,V> next;do {next = e.next;//记录下一个结点//新表是旧表的两倍容量，实例上就把单链表拆分为两队，　　　　　　　　　　　　　　//e.hash&oldCap为偶数一队，e.hash&oldCap为奇数一对if ((e.hash & oldCap) == 0) {if (loTail == null)loHead = e;elseloTail.next = e;loTail = e;}else {if (hiTail == null)hiHead = e;elsehiTail.next = e;hiTail = e;}} while ((e = next) != null);if (loTail != null) {//lo队不为null，放在新表原位置loTail.next = null;newTab[j] = loHead;}if (hiTail != null) {//hi队不为null，放在新表j+oldCap位置hiTail.next = null;newTab[j + oldCap] = hiHead;}}}}}return newTab;}

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