ConcurrentHashMap学习笔记一

记录下ConcurrentHashMap学习，记录日期2017.5.18

1.ConcurrentHashMap——实现类，接口是ConcurrentMap，线程安全，属于java.util.concurrent多线程包下

2.理解putIfAbsent 方法含义

package com.qbao.sms.common.annotation;import java.util.HashMap;import java.util.Map;import java.util.Set;import java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.ConcurrentMap;/** * Description: * * @author suoww * @date 2017/5/18 */public class Test {    public static void main(String[] args){        //测试ConcurrentHashMap putIfAbsent//        ConcurrentMap<String,Integer> concurrentMap = new ConcurrentHashMap<String, Integer>(10);        concurrentMap.put("张三", 1);        Integer value1 = concurrentMap.putIfAbsent("张三" , 10);        Integer value2 = concurrentMap.putIfAbsent("李欣" , 20);        System.out.println(value1);        System.out.println(value2);        Set<String> keySet = concurrentMap.keySet();        for (String key: keySet) {            System.out.println(key + "||" + concurrentMap.get(key));        }        System.out.println("--------------");        //HashMap        Map<String,Integer> hashMap = new HashMap<String, Integer>(10);        hashMap.put("张三", 1);        Integer value3 = hashMap.put("张三" , 10);        Integer value4 = hashMap.put("李欣" , 20);        System.out.println(value3);        System.out.println(value4);        Set<String> keySet2 = hashMap.keySet();        for (String key: keySet2) {            System.out.println(key + "||" + hashMap.get(key));        }    }}

总结：

1.和hashMap比较发现，ConcurrentHashMap通过使用putIfAbsent可以保证特定key对应的value值不会被其他线程所覆盖。putIfAbsent含义就是不存在某key value才会放进去，存在了则不会放进去。

2.无论ConcurrentHashMap还是HashMap返回值都是<K,V>中的V值