what is the reason for the following output of overloaded function?

Question

I was asked the output of this question in an interview, though I answered correctly I am not able to figure out the exact reason

class Main{

    void hello(Object obj)
    {
        System.out.println("i am object");
    }

    void hello(String str)
    {
         System.out.println("i am String");
    }

    public static void main(String[] args)
    {
        Main m = new Main();
        m.hello(null);
    }
}

Output

i am String