1、hashMap类继承关系
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
存放示意图
由此可以看出hash值一样的节点会被存放在同一条链表上，比原始遍历equals查找效率高（hash值相等的分为一组作为一个链表，在对该链表进行遍历查找）
重要属性：
1、static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;默认容量，必须是2的指数
2、static final int MAXIMUM_CAPACITY = 1 << 30;最大容量
3、static final float DEFAULT_LOAD_FACTOR = 0.75f;加载因子
4、static final int TREEIFY_THRESHOLD = 8;
5、static final int UNTREEIFY_THRESHOLD = 6;
6、static final int MIN_TREEIFY_CAPACITY = 64;
7、transient Node<K,V>[] table;
重要方法：
1、通过key获取value
public V get(Object key) { Node<K,V> e; return (e = getNode(hash(key), key)) == null ? null : e.value; }
static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); }
final Node<K,V> getNode(int hash, Object key) { Node<K,V>[] tab; Node<K,V> first, e; int n; K k; if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) {//根据n-1*hash值找到对应key存放的节点在数组（table）中的index,如上图的first节点 if (first.hash == hash && // always check first node ((k = first.key) == key || (key != null && key.equals(k)))) return first; if ((e = first.next) != null) { if (first instanceof TreeNode) return ((TreeNode<K,V>)first).getTreeNode(hash, key); do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } while ((e = e.next) != null); 此处在链上查找对应key的node } } return null; }
2、向hashMap中存放一个node
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; if ((p = tab[i = (n - 1) & hash]) == null)//根据hash值找到对应链表的表头节点 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 &&//如果链表中存在放入的key，则在后面替换原有的value ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e;//重置起始引用 } } if (e != null) { // existing mapping for key V oldValue = e.value; if (!onlyIfAbsent || oldValue == null) e.value = value;//替换 afterNodeAccess(e); return oldValue; } } ++modCount; if (++size > threshold) resize(); afterNodeInsertion(evict); return null; }
3、根据查询map是否含有value对象的节点
public boolean containsValue(Object value) { Node<K,V>[] tab; V v; if ((tab = table) != null && size > 0) { for (int i = 0; i < tab.length; ++i) {//遍历table数组，对应找出链表头 for (Node<K,V> e = tab[i]; e != null; e = e.next) {//遍历链表 if ((v = e.value) == value || (value != null && value.equals(v))) return true; } } } return false; }
4、清空map
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;//此循环只在遍历数组，置空表头，链表其他元素并未显式置空，gc回收 } }
备注：1、必需熟悉泛型概念
2、a|b=>其结果 较大者<=result<=a+b；a&b=>其结果 0<=result<=较小者
学习资料：
http://www.cnblogs.com/devinzhang/archive/2012/01/13/2321481.html
http://www.cnblogs.com/ITtangtang/p/3948406.html
