Exiting a program with System.exit(0) in java

Question

I am a little confused about how to implement System.exit(0); in my program. What I want to have happen is for the user to type the word "exit" and for the program to terminate. I would also like the program to return "0.0" when "clear" is entered, but I think I am having trouble working with my variables because num1 and num2 are doubles but I want "exit" and "clear" to be acceptable inputs, as well.

If anyone has any insight, that would be greatly appreciated. Here is my code:

package projectapcs;

/**
 *
 * @author Apple
 */

import java.util.Scanner;

public class ProjectAPCS {

    public static void exit(int x){

    }

    public static void clear(int y){

    }

    public static void main(String[] args) {



    double num1;
    double num2;
    char char1;
    String operation;


    Scanner input = new Scanner(System.in);

    System.out.println 
    ("Enter 'exit' to quit the calculator, 'help' for more options or 'continue' to use the   calculator");


    System.out.println
    ("\nEnter the first number:");
    num1 = input.nextInt();



     System.out.println 
     ("Display:" +  num1);

     System.out.println("Please enter operation:");
    operation = input.next();



    System.out.println("Enter the second number:");
    num2 = input.nextInt();
    System.out.println ("Display:" + num2);





    if ("+".equals(operation)){

        System.out.println((num1 + num2));
    } 
    else if ("-".equals(operation)){ 

        System.out.println((num1 - num2));
    }
    else if ("/".equals(operation)){ 

        System.out.println((num1 / num2));
    }
    else if ("*".equals(operation)){ 

        System.out.println((num1 * num2));
    }
    else if ("-x".equals(operation)){

        System.out.println (-num1);
    }
    else if ("x^2".equals(operation)){

        System.out.println (num1 * num1);
    }
    else if ("sqrt".equals(operation)){

        System.out.println (Math.sqrt(num1));

    }


    else if ("H".equals(num1)){

       System.out.println("Type + for the addition of values");
       System.out.println("Type - for the substraction of values");
       System.out.println("Type * for the multiplcation of values");
       System.out.println("Type / for the division of values");
       System.out.println("Type negate for the negation of values");
       System.out.println("Type ^2 for squaring of values");
    } 

    else if("E".equals(num1)){

       System.out.println ("Calculator program terminated...");
       System.exit(0);
    }

  }

}

Answer 1

If you want to allow arbitrary inputs, read input as a String, then convert to double as needed:

String input = scanner.next();
if (input.equals("exit")) {
    exit();
else if (input.equals("clear")) {
    clear();
} else {
    try {
        double number = Double.parseDouble(input);
        /* do something with `number` */
    } catch (NumberFormatException e) {
        System.err.println("Not a double.");
    }
}