ConcurrentHashMap的putIfAbsent

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

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