PostfixCalculator

    protected void evaluateOperator(String operator){
        LinkedList<Double> result = new LinkedList<>();
        //Grabs the number of arguments the operator needs from the operatorMap
        for (int i = 0; i < operatorMap.get(operator).numArgs(); i++) {
            result.add(stack.pop());
            System.out.println(result.get(i));
        }
        stack.push(operatorMap.get(operator).eval(result));
    }


    public static class Add implements Operator {
        /**
         * @return
         */
        @Override
        public int numArgs() {
            return 2;
        }

        @Override
        public double eval(List<Double> args) {
            return args.get(0) + args.get(1);
        }
    }