ConcurrentHashMap的putIfAbsent

這個方法在key不存在的時候加入一個值,如果key存在就不放入，等價：

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

測試代碼：

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"));
    }
}

結果為

gaoxing
gaoxing
gaoxing

 

ConcurrentHashMap的putIfAbsent用來做緩沖相當不錯，多線程安全的