简易HashMap实现
为了更好的理解HashMap线程不安全的根源,这里提供了HashMap的简易实现:
package map.test; import org.apache.commons.lang3.StringUtils; /** * Author:yepei@meituan.com * Date:2017/7/5 * Time:15:45 * ------------------------------------ * Desc: */ public class HashMp { private transient Node[] entries;//数组 private transient float loadFactor; private transient int size; public static void main(String[] args) { HashMp map = new HashMp(2, 0.75f); map.put(3, "A"); System.out.println(map); map.put(5, "B"); map.put(7, "C"); map.put(11, "D"); map.put(9, "E"); map.put(12, "F"); map.put(17, "G"); map.put(22, "H"); map.put(24, "I"); map.put(25, "I"); System.out.println(map); String s1 = map.get(22); String s2 = map.get(25); String s3 = map.get(0); } public HashMp(int capacity, float loadFactor) { this.entries = new Node[capacity]; this.loadFactor = loadFactor; } public HashMp() { this(16, 0.75f); } public String put(int key, String value) { Node n = new Node(key, value); //算Hash值,为简单起见,hash值 int i = n.getIndex(entries.length); Integer thisKey = key; //查找是否有重复: hashCode相等,且equals方法相等的 for (Node old = entries[i]; old != null; old = old.next) { Integer oldKey; //hash相同,且equals返回true,证明有重复值插入,直接替换掉旧值 if (old.getHash() == n.getHash() && ((oldKey = old.key) == key || thisKey.equals(oldKey))) { String oldValue = old.value; old.value = value;//替换成新值 return oldValue;//返回旧值 } } //该key不存在,需要增加一个结点 addEntry(n); return null; } public String get(int key) { int idx = new Node(key, null).getIndex(entries.length); Integer thisKey = key; //查找是否有重复: hashCode相等,且equals方法相等的 for (Node old = entries[idx]; old != null; old = old.next) { Integer oldKey = old.key; if (thisKey == oldKey || thisKey.equals(oldKey)) { return old.value; } } return null; } private void addEntry(Node n) { int oldLen = entries.length; int targetIdx = n.getIndex(oldLen); n.next = entries[targetIdx]; entries[targetIdx] = n;//插入到链首 if (size++ >= oldLen * loadFactor) { resize(2 * oldLen); } } private void resize(int newCapacity) { Node[] old = entries; Node[] newMap = new Node[newCapacity]; for (int j = 0; j < old.length; j++) { Node e = old[j]; if (e == null) { continue; } old[j] = null;//老表中的元素置为Null //注意:该过程会将同一个bucket中的元素逆置,因为总是将元素插在了新表的头部。 //但不是就地逆置,因为将元素逐个copy到了新的链表中 do { Node next = e.next;//暂存尾链表(去掉head的链表) int i = e.getIndex(newCapacity);//得到将在新表中存储的位置 e.next = newMap[i];//切断指针,重新指向新表的表头(可能为null) newMap[i] = e;//将e插入到新链表头部 e = next;//重新将e赋值为尾链表的头,对尾链表继续“切断指针,插入新表头” } while (e != null); } entries = newMap; } @Override public String toString() { StringBuilder sb = new StringBuilder("["); int i = 0; for (Node n : entries) { sb.append(i++).append("{").append(n).append("}, "); } return StringUtils.removeEnd(sb.toString(), ", ") + "]"; } static class Node { int key; String value; Node next; Node(int key, String value) { this.key = key; this.value = value; } int getIndex(int tableLength) { return getHash() % tableLength; } //为方便起见,hash值直接取该对象的key int getHash() { return key; } @Override public String toString() { return "(" + key + "," + value + ")——>" + next; } } }
参考
HashMap源码解读:http://www.xiaomager.com/category/program/java/hashmap
Hashcode生成原理:http://www.cnblogs.com/godtrue/p/6395098.html
HashMap存在的三大并发问题:https://my.oschina.net/xianggao/blog/393990