Why is it possible to call method on Java interface method? [Comparable]

Question

In AP Computer Science class today, I had this code:

    Comparable x = 45;
    Comparable y = 56;
    System.out.println(x.compareTo(y));

And this is valid. It prints 1 (or -1, I forget which), but it is possible to compare them.

I understand that interface variables refer to an object of a class that implements that interface, but what makes no sense to me is how an interface variable can be assigned an integer, and then have a method called on it. What object in this case is the compareTo() method being called on? Nothing was even instantiated!

Answer 1

This is called autoboxing, your primitive int type is automatically wrapped into an Integer instance, which is an object and it does implement Comparable interface.