尝试停止应用程序的 Do-While 循环

Question

I'm creating an application for a homework, the problem is that I am trying to create a do-while loop to exit the application (Using the question "Do you want to exit (Y/N)"). To work with the do-while loop, I created a method to store the app and then called the method in the do-while loop, so that when I try to stop the loop, the method loops once more. I want when I type "Y" to the console the whole program stops and doesn't loop one more time. Thank you for reading.

I created a simple example to explain my problem.

Here's the method:

    public static void App(){
    Scanner sc = new Scanner(System.in);
    
    System.out.print("Write a number: ");
    int num1 = sc.nextInt();
    System.out.print("Write another number: ");
    int num2 = sc.nextInt();
    
    System.out.println("\nResult: "+(num1+num2));
}  

And here I'm trying to create the loop in the main method:

    public static void main(String[] args) {
    
    Scanner sc2 = new Scanner(System.in);      
    App();
  
    String answer;
    do {
     System.out.println("Do you want to exit (Y/N)?");
     answer = sc2.next();
     App();
    } while (answer.equalsIgnoreCase("N")) ;
}

Answer 1

The reason why your program is running again after you type n is because the App() method is ran after the question is asked within the do part of the loop.

This code below is the simplest fix I could think of.

public static void main(String[] args) {
    Scanner sc2 = new Scanner(System.in);
    // I removed the line 'App();' as the App method will always run at least one time. Therefore putting that method within the 'do' part of the loop allows us to ask the user if they wish to exit or not after they have received their answer.
    String answer;
    do {
        App();
        System.out.print("Do you want to exit (Y/N)?"); //I changed the 'println' to 'print' here
        answer = sc2.next();
    } while (answer.equalsIgnoreCase("N")) ;
}

As a side note, methods in java should be lower-case when following typical Java naming conventions. While this will not affect how your code runs, I would suggest renaming the method from App() to app() .

Answer 2

the problem is that I am trying to create a do-while loop to exit the application

You already have that in your program.

so that when I try to stop the loop, the method loops once more...

That doesn't fit the goal of your program.

I want when I type "Y" to the console the whole program stops and doesn't loop one more time

A lot of context that doesn't fit right in.

But anyway, you just have to reorganize your program.

In other words, just move your App() method.

public static void main(String[] args) {
    
    Scanner sc2 = new Scanner(System.in);      
  
    String answer;
    do {
     App();
     System.out.println("Do you want to exit (Y/N)?");
     answer = sc2.next();
    } while (answer.equalsIgnoreCase("N")) ;
}

Also, I spotted a lot of bad practices, so I kind of fixed them:

public static void main(String[] args) throws Exception {
    try(Scanner sc2 = new Scanner(System.in)){
        String answer;
        do {
            App();
            System.out.print("Do you want to exit (Y/N)?");
            answer = sc2.nextLine();
        } while (answer.equalsIgnoreCase("N")) ;
    }
}

Lastly, maybe (just maybe) try to solve your problem first before seeking help for your homework.

Answer 3

Everything looks good in your code, Just change the execution logic as shown in code blocks.

public static void main(String[] args) {
Scanner sc2 = new Scanner(System.in);      
App();   //remove this line from here

String answer;
do {
 App();  //call App function here so that it got executed at least one time 
 System.out.println("Do you want to exit (Y/N)?");
 answer = sc2.next();
 App();   //remove this as well
} while (answer.equalsIgnoreCase("N")) ;

}

Answer 4

Here is yet another approach except it uses a while loops instead of do/while loops. Two different approaches are provided and both provide User entry validation:

Approach #1:

public static void appMethod() {
    Scanner sc = new Scanner(System.in);
    
    int num1 = Integer.MIN_VALUE;    // Initialize with some obscure value.
    int num2 = Integer.MIN_VALUE;    // Initialize with some obscure value.
    
    while (num1 == Integer.MIN_VALUE) {
        System.out.print("Write a number: ");
        try {
            num1 = sc.nextInt();
        } catch ( java.util.InputMismatchException ex) { 
            System.out.println("Invalid Entry! Try again..." 
                               + System.lineSeparator());
            sc.nextLine(); // consume the ENTER key hit otherwise this error will keep cycling.
            num1 = Integer.MIN_VALUE;
        }
    }
    while (num2 == Integer.MIN_VALUE) {
        System.out.print("Now, write yet another number: ");
        try {
            num2 = sc.nextInt();
        } catch ( java.util.InputMismatchException ex) { 
            System.out.println("Invalid Entry! Try again..." 
                               + System.lineSeparator());
            sc.nextLine(); // consume the ENTER key hit otherwise this error will keep cycling.
            num2 = Integer.MIN_VALUE;
        }
    }
    
    System.out.println("\nResult: " + num1 +" + " + num2 + " = " + (num1 + num2));
}

Approach #2:

This next approach makes use of the Scanner#nextLine() method. The thing to remember about nextLine() is that, if you use it in your console application then basically recommend you use it for everything (all prompts). A 'quit' mechanism is also available in this version. Read the comments in code:

public static void appMethod() {
    Scanner sc = new Scanner(System.in);
    
    // Retrieve first number...
    String num1 = "";
    while (num1.isEmpty()) {
        System.out.print("Write a number (q to quit): ");
        // Making use of the Scanner#nextLine() method
        num1 = sc.nextLine();
        // Has 'q' been supplied to Quit?
        if (num1.equalsIgnoreCase("q")) {
            return;
        }
        /* Validate the fact that a signed or unsigned Integer or 
           Floating Point value has been entered. If not show Msg. */
        if (!num1.matches("-?\\d+(\\.\\d+)?")) { 
            System.out.println("Invalid Entry! (" + num1 + ") Try again..." 
                               + System.lineSeparator());
            num1 = "";  // empty num1 so as to re-loop.
        }
    }
    
    // Retrieve second number...
    String num2 = "";
    while (num2.isEmpty()) {
        System.out.print("Now, write yet another number (q to quit): ");
        num2 = sc.nextLine();
        if (num2.equalsIgnoreCase("q")) {
            return;
        }
        if (!num2.matches("-?\\d+(\\.\\d+)?")) { 
            System.out.println("Invalid Entry! (" + num2 + ") Try again..." 
                               + System.lineSeparator());
            num2 = "";
        }
    }
    
    // Convert the numerical strings to double data type values.
    double number1 = Double.parseDouble(num1);
    double number2 = Double.parseDouble(num2);
    
    // Display the result.
    System.out.println("\nResult: " + num1 +" + " + num2 + " = " + (number1 + number2));
}