An interview topic: What can go wrong with this code?

Question

A simple Calculator class that's wired up as a Spring bean in a web application:

public class Calculator { 
    int result; 

    public int addTwoNumbers(int first, int second) { 
        result = first + second; 
        return result; 
    } 
} 

What can potentially go wrong with this?

Answer 1

Multi threading problems. By default Spring beans are singletons.