Add new exception in existing method without affecting legacy code

Question

I was asked this question in an interview.

I have a method say public int add(int i, int j) and this method is already being used by many clients.

Now i have to make an update (may be some enhancement) to the add method which creates a scenario where i have to throw an exception. How can i make the existing clients to continue use the add() method without code change from their end? [Interviewer gave a hint: Clients may or may not use whatever new enhancement I made in add method]

First, I thought of overloading add, wrapping add in a new add method which throws exception. Then i thought of throwing Exception as RuntimException from add...

But none of them accepted as a correct approach.

Any pattern or design approach i am missing?

Answer 1

Approach 1: Make use of Wrapper Class Integer

public class B {
    public int add(int i, int j) {
        return 0;
    }

    public int add(Integer i, Integer j) throws Exception {
        return 0;
    }
}

Approach 2: Make use of Overriding

You can take the advantage of overriding method can choose not to throw exception at all.

What you can do is to declare a Parent class which will have method with exception and child class which does not have the exception and will override method from parent. Now, when you want clients to use add with exception pass reference with type A, otherwise pass reference with type B:

class A { // <---New Class
    public int add(int i, int j) throws Exception { // <-- Method with Exception
        return 0;
    }
}

class B extends A { // <----Original Class
    @Override
    public int add(int i, int j) { // <--- Original Method
        return 0;
    }
}