Java Object with static variable instance inside Stateless Bean

Question

Is it ok to have an object inside of EAR like the Calculator class to be used as a utility for other stateless classes?

Is it a bad design? If so what appropriate approach should be applied?

    @Stateless
    class A{
        public void sumForA(){
            System.out.println("SUM IS : "+ (Calculator.getInstance().add(4+6)));
        }
    }

    @Stateless
    class B{
        public void sumForB(){
            System.out.println("SUM IS : "+(Calculator.getInstance().add(1+2)));
        }   
    }

    public class Calculator{

        static{
            INSTANCE=new Calculator();
        }

        private static INSTANCE;

        public Calculator getInstance(){
            return INSTANCE;
        }

        public int add(int x,int y){
            return x+y;
        }
    }

Answer 1

First, there is no such name "static variable instance", there is instance variables and static variables, you can find an example here: Java Static vs Instance.

Second, regarding your Calculator class, you need to mark the getInstance() method as static beacause you are calling it directly. And, you seem trying to use the singleton pattern, I suggest you take a look at this SO question: What is an efficient way to implement a singleton pattern in Java?

Third, in your example there is no static variable in the statless bean, and to make it simple: you are only invoking a method in the Class Calculator which has static members. So why not?! you are using your utility class inside your method, it doesn't matter if it's a stateless bean or any kind of beans (EJB session beans, CDI / JSF beans, Spring Components ... ).