Test2.java

public class Main {

    private static class Calculator {

        private Calculator() {

        }

        public int calc(String string) {
            String [] instance = string.split(" ");
            Operation operation = Builder(instance[1]);

            int digit1 = Integer.parseInt(instance[0]);
            int digit2 = Integer.parseInt(instance[2]);
            return operation.calculate(digit1, digit2);
        }

        public static Operation Builder(String operation) {
            switch (operation){
                case "+":
                    return new Plus();
                case "-":
                    return new Minus();
                case "*":
                    return new Mult();
                case "/":
                    return new Split();
                default:
                    throw new RuntimeException("Operation not yet implemented");
            }
        }

        private interface Operation {
            int calculate(int digit1, int digit2);
        }

        private static class Plus implements Operation {
            @Override
            public int calculate(int digit1, int digit2) {
                return digit1 + digit2;
            }
        }

        private static class Minus implements Operation {
            @Override
            public int calculate(int digit1, int digit2) {
                return digit1 - digit2;
            }
        }

        private static class Mult implements Operation {
            @Override
            public int calculate(int digit1, int digit2) {
                return digit1 * digit2;
            }
        }

        private static class Split implements Operation {
            @Override
            public int calculate(int digit1, int digit2) {
                return digit1 / digit2;
            }
        }
    }


        public static void main(String[] args) {
            Calculator calculator = new Calculator();
            if (calculator.calc("2 + 2") != 4) throw new RuntimeException();
            if (calculator.calc("2 - 2") != 0) throw new RuntimeException();
            if (calculator.calc("2 * 2") != 4) throw new RuntimeException();
            if (calculator.calc("2 / 2") != 1) throw new RuntimeException();
            System.out.print("Done");
        }

}