Invoking parent class method without changing code

Consider the following code snippet below.

class X {
    public String toString() {
        return "Hi";
    }
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public class Main {
    public static void main(String[] args) {
        Object obj = new X();
        System.out.println(obj.toString());
    }
}

How do I invoke the toString() inside Object class now, without changing the code? Or what I ask is not possible?

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From the outside, you can't - that would violate encapsulation. (Imagine toString() were really a method to mutate the state of the object, and the subclass wanted to enforce some constraints - you shouldn't be able to skip those constraints.) You can do it from within X itself, e.g.

public String toString() {
    return super.toString() + "Hi";
}