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

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?

What are the important topics for coding interview?

This is probably the most important topic for coding interviews and you should prepare it well. At a bare minimum, you should know about the array, linked list, binary trees, binary search tree, self-balanced trees like AVL tree, hash table(or map or dictionaries as known in Java and Python) stack, queue, and graph.

Multi threading problems. By default Spring beans are singletons.