ConcurrentHashMap的putIfAbsent方法

转自：http://www.cnblogs.com/gaoxing/p/4271749.html

这个方法在key不存在的时候加入一个值,如果key存在就不放入，等价：

   if (!map.containsKey(key)) 
      return map.put(key, value);
  else
       return map.get(key);

 

public class Test {
    public staticvoid main(String[] args) {
        ConcurrentHashMap<String,String> map=new ConcurrentHashMap<String,String>();
        String temp=map.putIfAbsent("a", "a1");
        System.out.println(map.get("a"));

        System.out.println("temp======"+ temp);
        temp=map.putIfAbsent("a","a2");
        System.out.println(map.get("a"));

        System.out.println("temp======"+ temp);
        temp=map.putIfAbsent("a","a3");
        System.out.println(map.get("a"));

        System.out.println("temp======"+ temp);
    }
}

运行结果：

a1
temp======null
a1
temp======a1

a1
temp======a1