Try - Catch in if - else if instruction

I was able to implement the function of the try - catch for the variable choice and it works great. I have a problem with variable stopnie. I want to check if this is numerical value. I tried to throw it in the try catch, unfortunately without success

class Task {

public static void main(String[] args) {
    Scanner user_input = new Scanner (System.in);
    System.out.println("Pick 1 to convert Fahrenheit to Celsius");
    System.out.println("Pick 2 to convert Ceslius to Fahrenheit");
    int choice = 0;
    double stopnie = 0.0;
    double convert = 0.0;
    DecimalFormat df = new DecimalFormat("0.00", new DecimalFormatSymbols(Locale.US));
    boolean loop = true;

while (loop) 
{
    try 
    {
        choice = user_input.nextInt();
        loop = false;
    } 
    catch (Exception e) 
    {
        System.out.println("Bad value");
        System.out.println("Try again");
        user_input.next();

    }
}

    if(choice == 1) 
    { 
        System.out.println("Let me know Celsius value");
        stopnie = user_input.nextDouble();
        convert = stopnie/1.8-35;
        System.out.println(stopnie + " C " + " = " + df.format(convert) + " F");
    }

    else if (choice == 2) 
    {
        System.out.println("Let me know Fahrenheit value");
        stopnie = user_input.nextDouble();
        convert = stopnie*1.8+35;
        System.out.println(stopnie + " F " + " = " + convert + " C");

    }

    else 
    {
        System.out.println("Bad value");
    }

}   

}