Continuity Issue in Java Calculator

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I am new to Java, and I’m doing a calculator APP.

I’m trying to get my calculator to keep prompting the user for the correct answer (typing Y or N) after the else statement following ‘Invalid Input’.
I want the program to continue with the calculations after the correct input is finally entered.

I have played around with an embedded while loop, but ended up with an infinite loop or a loop that terminates with no resolution. The code is below.

import java.util.Scanner;
class Calculate {
    public static void.main(String[] args) {
            System.out.println("Welcome To Calculator!");
            System.out.println("*************************************************************************");
            Scanner userinput = new Scanner(System.in);
            double num1, num2;
            String choice;
            boolean youDecide = true;
            while(youDecide == true) {
                System.out.println("Please enter a number: ");
                num1 = userinput.nextDouble();
                System.out.println("Please enter an available operator (+, -, *, /): ");
                char operator = userinput.next().charAt(0);
                System.out.println("Please enter another number: ");
                num2 = userinput.nextDouble();
                double output;      
                switch(operator) 
                {
                case '+':
                    output = num1 + num2;
                    break;
                
                case '-':
                    output = num1 - num2;
                    break;
                
                case '*':
                    output = num1 * num2;
                    break;
                
                case '/':
                    output = num1 / num2;
                        if(num2 == 0)
                            System.out.println("Math error! A number cannot be divided by zero.");
                    break;
                default:
                    System.out.println("Invalid input. Please enter an available operator, i.e (+, -, *, /): ");
                    return;
                }
                System.out.println("*************************************************************************");
                System.out.println("The answer is: " + "n" + output);
                System.out.println("*************************************************************************");
                System.out.println("Would you like to calculate again?");
                System.out.println("Please enter Y for yes, or N for no");
                choice = userinput.next();
            
                if(choice.equalsIgnoreCase("Y")) {
                    System.out.println("Okay. Let's continue!");
                    System.out.println("*************************************************************************");
                    youDecide = true;
                }
                else if(choice.equalsIgnoreCase("N")) {
                    System.out.println("*************************************************************************");
                    System.out.println("Okay. Thanks for using Calculator. Goodbye!");
                    System.exit(0);
                }
                else {
                    System.out.println("Invalid input. Please try again...");   
                    System.out.println("*************************************************************************");
                    youDecide = false;
                }
        }
    
    }

}

Answer

After adding the edits suggested by J.F. and Javaman along with some research, I was able to solve the problem by adding the following lines to my code:

import java.util.InputMismatchException;
import java.util.Scanner;


class Calculator {
    public static void main(String[] args) {
        System.out.println("Welcome To Calculator!");
        System.out.println("*************************************************************************");
        Scanner userinput = new Scanner(System.in);
        char operator;
        double num1, num2;
        String choice;
        while(true) {
            System.out.println("Please enter a number: ");
            num1 = userinput.nextDouble();
            System.out.println("Please enter an available operator (+, -, *, /): ");
            while(true) {
                operator = userinput.next().charAt(0);  
                if(operator == '+' || operator == '-' || operator == '*' || operator == '/') {
                    break;
                }else {
                    System.out.println("Invalid input. Please enter an available operator, i.e (+, -, *, /): ");
                }
            }
            System.out.println("Please enter another number: ");
            num2 = userinput.nextDouble();
            double output;
    
            switch(operator) {
            case '+':
                output = num1 + num2;
                break;      
            case '-':
                output = num1 - num2;
                break;  
            case '*':
                output = num1 * num2;
                break;      
            case '/':
                output = num1 / num2;
                break;
            default:
                System.out.println("Invalid input. Please enter an available operator, i.e (+, -, *, /): ");
                continue;
            }
            System.out.println("*************************************************************************");                
            if(num2 == 0) {
                System.out.println("Math error! A number cannot be divided by zero.");
            }else {
                System.out.println("The answer is: " + "n" + output);
            } 
            System.out.println("*************************************************************************");
            System.out.println("Would you like to calculate again?");
            System.out.println("Please enter Y for yes, or N for no");
            choice = userinput.next();
     
            if(choice.equalsIgnoreCase("Y")) {
                System.out.println("Okay. Let's continue!");
                System.out.println("*************************************************************************");
            }else if(choice.equalsIgnoreCase("N")) {
                System.out.println("*************************************************************************");
                System.out.println("Okay. Thanks for using Calculator. Goodbye!");
                System.exit(0);
            }else {
                System.out.println("Invalid input. Please try again...");   
                System.out.println("*************************************************************************");        
                **while (!("Y").equalsIgnoreCase(choice) && !("N").equalsIgnoreCase(choice)) {
                    System.out.println("Invalid input. Please try again...");   
                    System.out.println("*************************************************************************");        
                    System.out.println("Please enter Y for yes, or N for no");
                    choice = userinput.next();
                    if(choice.equalsIgnoreCase("Y")) {
                        System.out.println("Okay. Let's continue!");
                        System.out.println("*************************************************************************");
                    }else if(choice.equalsIgnoreCase("N")) {
                        System.out.println("*************************************************************************");
                        System.out.println("Okay. Thanks for using Calculator. Goodbye!");
                        System.exit(0);**
                    }
                }
            }
        }
   }

}


Source: stackoverflow