一、简介
- HashMap的源码咱们之前解读过，数组加链表，链表过长时裂变为红黑树。主动扩容机制没细说，明天具体看一下
二、扩容机制
先说论断：
- hashmap的容量都是2的倍数，比方2，4，8，16，32，64 ...
- 每次扩容都是扩一倍，2到4 ，4到8，8到16， 16到32 等等
- 扩容因子：默认是0.75，也能够指定一个小数
- 扩容工夫点：当容器内的元素数量达到：容量*扩容因子 开始扩容
三、源码剖析
（1）先看构造函数
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16static final float DEFAULT_LOAD_FACTOR = 0.75f;public HashMap() { this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted}默认的构造函数指定了扩容因子：0.75, 默认容量是16
public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR);}指定初始容量，默认扩容因子：0.75
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);}同时指定初始容量和扩容因子
/** * The next size value at which to resize (capacity * load factor). * * @serial */int threshold;- 留神这个变量：下一个要扩容的值，扩容容量，容量*扩容因子
- 看这一句：this.threshold = tableSizeFor(initialCapacity);
/** * Returns a power of two size for the given target capacity. */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;}- 这个办法是取给定值四舍五入之后的2的倍数，比方3—->4 ,15->16, 27->32
- 至此筹备工作就做好了，上面看put办法
（2）put办法
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; // ① 最开始table为null, 调用resize（）办法 if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; 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 1st treeifyBin(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 key V oldValue = e.value; if (!onlyIfAbsent || oldValue == null) e.value = value; afterNodeAccess(e); return oldValue; } } ++modCount; // ② 完结的时候判断容量是不是大于扩容容量，大于则调用resize办法 if (++size > threshold) resize(); afterNodeInsertion(evict); return null;}- ① 最开始table为null, 调用resize（）办法
- ② 完结的时候判断容量是不是大于扩容容量，大于则调用resize（）办法
- 看resize（）办法
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;}- 先剖析第一种状况：Map map = new HashMap();
走最初一个分支 , 容量为16，扩容容量为12
else { newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);}- 剖析第二种状况：Map map = new HashMap(20);
- 走第二个分支，后面剖析过，threshold = tableSizeFor(20) 为 32
- 新容量newcap = oldThr 为32
// 容量else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr;- 新扩容容量newThr = newCap * loadFactor 为 24
// 扩容容量if (newThr == 0) { float ft = (float)newCap * loadFactor; newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE);}threshold = newThr;- 剖析第三种状况：下面的map曾经插入24个元素，新插入一个要扩容
- 走第一个分支，oldCap=32，oldThr=24
- 扩容：newCap = oldCap << 1 为64
- 扩扩容容量newThr = oldThr << 1 为48
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}- 最初是复制元素到新的table
- 单个元素间接复制
- 如果是树，调用树的复制办法
- 如果是链表，循环链表复制
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