HashMap ( Java 8)
HashTable是早起java提供的基于hash表的实现，不允许存放null键和值，是同步的，影响开销，不太被推荐。
HashMap行为上和HashTable差不多，不是同步的，允许键和值为null，通过put(),get()来存取数据。
一、默认属性值：
这里摘出了重要属性的默认值：
// 默认容量是16，而且如果自定义容量必须上2的幂 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 //最大容量是1073741824 static final int MAXIMUM_CAPACITY = 1 << 30; //默认负载 因子是0.75，少了可能会频繁扩容，多了可能会影响效率，默认值比较适合大多数场景 static final float DEFAULT_LOAD_FACTOR = 0.75f; //树化阀值为8，链表长度大于等于8就会转化成红黑树 static final int TREEIFY_THRESHOLD = 8; //链表长度小于6就会由数退化为链表 static final int UNTREEIFY_THRESHOLD = 6;
二、构造方法
HashMap 的构造方法：
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); } public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } public HashMap() { this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted }
构造方法可以允许你自定义初始容量，但是HashMap会通过tableSizeFor(int cap)方法去讲容量转化为最接近2的幂的值
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; }
三、put方法
HashMap的PUT方法：主要调用了putVal()方法
public V put(K key, V value) { return putVal(hash(key), key, value, false, true); }
putVal()方法，标注了一些说明：
1 final V putVal(int hash, K key, V value, boolean onlyIfAbsent, 2 boolean evict) { 3 Node<K,V>[] tab; Node<K,V> p; int n, I; 4 // 如果table为null或者长度为0，调用resize()进行初始化为16 5 if ((tab = table) == null || (n = tab.length) == 0) 6 //n为当前数组的长度 7 n = (tab = resize()).length; 8 //将n-1和key的hash值相与值赋给i，如果当前数组第i位没有值，则将此数据插入到这个索引位置 9 if ((p = tab[i = (n - 1) & hash]) == null) 10 tab[i] = newNode(hash, key, value, null); 11 //如果当前索引位置有数据，则新建一个节点，放在上一个节点后面 12 else { 13 Node<K,V> e; K k; 14 //如果key相同，说明这次操作是修改操作，将val值修改 15 if (p.hash == hash && 16 ((k = p.key) == key || (key != null && key.equals(k)))) 17 e = p; 18 //如果p是树节点的话，则按照树节点进行插入 19 else if (p instanceof TreeNode) 20 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); 21 //遍历节点，将新节点插入到链表尾部 22 else { 23 for (int binCount = 0; ; ++binCount) { 24 if ((e = p.next) == null) { 25 p.next = newNode(hash, key, value, null); 26 //如果节点数大于等于8，则进行树化操作 27 if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st 28 treeifyBin(tab, hash); 29 break; 30 } 31 //如果插入到节点和原有节点key相同，修改原有节点 32 if (e.hash == hash && 33 ((k = e.key) == key || (key != null && key.equals(k)))) 34 break; 35 p = e; 36 } 37 } 38 //e不为空说明是一次修改操作，将当前节点e的value替换为新的 39 if (e != null) { // existing mapping for key 40 V oldValue = e.value; 41 if (!onlyIfAbsent || oldValue == null) 42 e.value = value; 43 afterNodeAccess(e); 44 return oldValue; 45 } 46 } 47 ++modCount; 48 //如果数组大小大于阀值，则扩容 49 if (++size > threshold) 50 resize(); 51 afterNodeInsertion(evict); 52 return null; 53 }
resize()方法：
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) { //极限的设定 if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; } //扩容后未达到极限，新的数组扩容为两倍，阈值也扩至原来两倍 else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold } //构造方法里自定义数组大小 else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; //默认的容量16，阈值0.75*16=12 else { // zero initial threshold signifies using defaults newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); } //如果阈值为0（自定义数组大小）设置阈值为负载因子*容量 if (newThr == 0) { float ft = (float)newCap * loadFactor; newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE); } //阈值为新计算的值 threshold = newThr; // ··· return newTab; }
treeifyBin()方法：
final void treeifyBin(Node<K,V>[] tab, int hash) { int n, index; Node<K,V> e; //如果桶容量大小为达到最小树化容量（64）时，则扩容 if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY) resize(); //如果当前数组位置不为空，转化为树 else if ((e = tab[index = (n - 1) & hash]) != null) { TreeNode<K,V> hd = null, tl = null; do { TreeNode<K,V> p = replacementTreeNode(e, null); if (tl == null) hd = p; else { p.prev = tl; tl.next = p; } tl = p; } while ((e = e.next) != null); if ((tab[index] = hd) != null) hd.treeify(tab); } }
Clear()方法：将数组内清空
public void clear() { Node<K,V>[] tab; modCount++; if ((tab = table) != null && size > 0) { size = 0; for (int i = 0; i < tab.length; ++i) tab[i] = null; } }
get()方法，调用getNode()方法：
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; Node<K,V> first, e; int n; K k; //数组不为空 if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) { //检查第一个节点的值是否对的上 if (first.hash == hash && // always check first node ((k = first.key) == key || (key != null && key.equals(k)))) return first; //第一个值对不上，看看有没有下一个节点 if ((e = first.next) != null) { //如果第一个节点上树节点，则去树中查找 if (first instanceof TreeNode) return ((TreeNode<K,V>)first).getTreeNode(hash, key); //如果是链表，则遍历全部链表，看是否有该key值 do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } while ((e = e.next) != null); } } //没有返回null return null; }
containsKey():同样是去调用getNode()
public boolean containsKey(Object key) { return getNode(hash(key), key) != null; }
