Simple Calculator

package ProcessingFiles; //Does not catch any errors... May make a new version which utilises collections with try catch.

import java.util.Scanner;

public class App {
    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        System.out.println("Enter your first value: ");
        String firstValue = input.nextLine();
        System.out.println("Enter the second value: ");
        String secondValue = input.nextLine();
        System.out.println("Enter a chosen operator between +, -, /, *: ");
        String op = input.nextLine();

        outPut(firstValue, secondValue, op);

    }
    //Bad recursion practice. May lead to stack overflow.
    public static void outPut(String val1, String val2, String op) {
        double result = calc(val1, val2, op);

        if (result == -9999) {
            System.out.println("You've entered an invalid operator");
        }
        else System.out.println("Result: " + result);

        Scanner input = new Scanner(System.in);
        System.out.println("Would you like to go again? Enter yes or no: ");
        String answer = input.nextLine();

        if(answer.equals("yes")) {
            System.out.println("Enter your first value: ");
            String firstValue = input.nextLine();
            System.out.println("Enter the second value: ");
            String secondValue = input.nextLine();
            System.out.println("Enter a chosen operator between +, -, /, *: ");
            String operator = input.nextLine();

            outPut(firstValue, secondValue, operator);
        }
        else System.out.println("Thank you.");
    }

    public static double calc(String val1, String val2, String op) {
        int firstValue = Integer.parseInt(val1);
        int secondValue = Integer.parseInt(val2);
        if (op.equals("+"))
            return firstValue + secondValue;
        else if (op.equals("-"))
            return firstValue - secondValue;
        else if (op.equals("*"))
            return firstValue * secondValue;
        else if (op.equals("/"))
            return firstValue / secondValue;
        else
            return -9999;
    }

}