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干了这杯Java之HashMap

类:

public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable继承自AbstractMap 实现了Map,Cloneable,Serializable接口可以被序列化可以被Clone

//默认初始容量为16static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16//最大容量static final int MAXIMUM_CAPACITY = 1 << 30//默认扩容因子static final float DEFAULT_LOAD_FACTOR = 0.75f//红黑树转链表的阀值static final int TREEIFY_THRESHOLD = 8;//链表转红黑树阀值static final int UNTREEIFY_THRESHOLD = 6;//存储方式由链表转成红黑树的容量的最小阈值static final int MIN_TREEIFY_CAPACITY = 64;

初始化

Map<String, Object> map = new HashMap<>();public HashMap(int initialCapacity, float loadFactor) { if (initialCapacity < 0) throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity); if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; if (loadFactor <= 0 || Float.isNaN(loadFactor)) throw new IllegalArgumentException("Illegal load factor: " + loadFactor); this.loadFactor = loadFactor; this.threshold = tableSizeFor(initialCapacity);}new HashMap<>()的时候,会进入HashMap(int initialCapacity, float loadFactor)方法initialCapacity为0xBloadFactor默认为0.75第一步判断initialCapacity的值是不是小于0或大于1 << 30,第二步判断loadFactor是不是大于0和是否为浮点数,第三步设置实例的loadFactor为0.75,第四步设置容量为16

tableSizeFor

//返回大于或等于cap且为2的幂的数值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;}cap-1:如果cap不减去1,cap=2的幂时,结果为cap的2倍,不符合预期

hash方法

static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);}hashCode的高位16bit与低16bit参与异或运算null放在第0位

Node对象

put方法

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) //扩容,第一次扩容长度16 n = (tab = resize()).length; //(n - 1) & hash的长度必定在0-n之间 //(n - 1) & hash等于hash % n,与运算比取模快 //n - 1的2进制为01111...,N为2的次方 if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null); else { Node<K,V> e; K k; //hash和equals或=相同时,替换 if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p; else if (p instanceof TreeNode)//节点为树节点的时候,在红黑树上添加 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { //链表 for (int binCount = 0; ; ++binCount) { //添加在链表末尾 if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); //当binCount>=7的时候转换为红黑树 if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st treeifyBin(tab, hash); break; } //已经存在 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; //下一个对象 p = e; } } //已经存在key if (e != null) { V oldValue = e.value; if (!onlyIfAbsent || oldValue == null) e.value = value; afterNodeAccess(e); return oldValue; } } ++modCount; //扩容 if (++size > threshold) resize(); afterNodeInsertion(evict); return null;}取模 x % 2^n == x & (2^n - 1)

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; if (oldCap > 0) { if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; } else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold } else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; else { // zero initial threshold signifies using defaults newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); } if (newThr == 0) { float ft = (float)newCap * loadFactor; newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE); } threshold = newThr; @SuppressWarnings({"rawtypes","unchecked"}) Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; table = newTab; if (oldTab != null) { for (int j = 0; j < oldCap; ++j) { Node<K,V> e; if ((e = oldTab[j]) != null) { oldTab[j] = null; if (e.next == null) newTab[e.hash & (newCap - 1)] = e; else if (e instanceof TreeNode) ((TreeNode<K,V>)e).split(this, newTab, j, oldCap); else { // preserve order Node<K,V> loHead = null, loTail = null; Node<K,V> hiHead = null, hiTail = null; Node<K,V> next; do { next = e.next; if ((e.hash & oldCap) == 0) { if (loTail == null) loHead = e; else loTail.next = e; loTail = e; } else { if (hiTail == null) hiHead = e; else hiTail.next = e; hiTail = e; } } while ((e = next) != null); if (loTail != null) { loTail.next = null; newTab[j] = loHead; } if (hiTail != null) { hiTail.next = null; newTab[j + oldCap] = hiHead; } } } } } return newTab;}

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