private void evaluateOperator() throws BinaryOperatorException {
        BinaryOperator operator = expressionStack.operatorStack.pop();
        if (2 > expressionStack.operandStack.size()) {
            throw new BinaryOperatorException("binary operator wasn't calculated");
        }
        BigDecimal operandFirst = expressionStack.operandStack.pop();
        BigDecimal operandSecond = expressionStack.operandStack.pop();
        expressionStack.operandStack.push(operator.evaluate(operandFirst, operandSecond));
    }