Do interfaces have toString method?

How is it possible to call toString method using the reference variable of interface My, which does not have a toString method?

interface My
{
void show();
String toHi();
}
class C implements My
{
public void show(){
System.out.println("show\n");
}
public String toString(){
return "HELLO"; 
}
public String toHi(){
return "Hi"; 
}

public static void main(String[] args) 
{
My m=new C();
String s=m.toString();
System.out.println(s);
}
}