String.valueOf 空指针

Question

    发现一个奇怪的问题，求大神解释

            TreeMap<String,Object> ttt = new TreeMap<String,Object>();
    if(ttt.get("xxxx") == null){
        System.out.println(String.valueOf(ttt.get("xxxx")));  //程序运行这里没报错
    }

    System.out.println(String.valueOf(null));  //程序运行这里报错了

Answer 1

主要还是String.valueOf函数的处理
https://stackoverflow.com/questions/3131865/why-does-string-valueofnull-throw-a-nullpointerexception