疑问：null是String 还是Object？

public class Main {
    public static void main(String[] args)
    {

        new Main().addOne(null);

    }
    public void addOne(Object o)
    {
        System.out.println("haha");
    }
    public void addOne(String x)
    {
        System.out.println("haha111");
    }
}