ConcurrentHashMap不支持null

    @override
    public void run(string... args) throws exception {
        concurrenthashmap<string, string> concurrenthashmap = new concurrenthashmap<>();
        concurrenthashmap.put(null, "1");//npe
        concurrenthashmap.put("1", null);//npe
        concurrenthashmap.get(null);//npe
        hashmap<string, string> hashmap = new hashmap<>();
        hashmap.put(null, "1");//ok
        hashmap.put("1", null);//ok
        system.out.println(hashmap.get(null));//"1"
        system.out.println(hashmap.get("1"));//null
    }