如何使我的while（）循环返回到if（）语句？

Question

package somePackage;

import java.util.Scanner;

public class SomeClass {
    private static Scanner input;

    public static void main(String[] args) {

        input = new Scanner(System.in);
        System.out.print("Please enter a command (start or stop) : ");
        String scanner = input.nextLine();

        if ("start".equals(scanner)) {
            System.out.println("System is starting");
        } else if ("stop".equals(scanner)) {
            System.out.println("System is closing");
        }

        while (!"start".equals(scanner) && (!"stop".equals(scanner))) {
            System.out.print("Please try again : ");
            scanner = input.nextLine();
        }
    }
}

when a user doesn't input "start" or "stop". The program will ask the user to "Try again : ". Let's say the user inputs "start" after that, the output will be blank. How can I make my loop go back to the original System.out.print() in the if() or else if() statement?

PS , I'm new to Java so any feedback would help :) Thanks!

Answer 1

If the if statement just need to be shown one time, is enough to put that after the while loop, because if type start or stop break to the while loop and it will print the correct message, for example:

public class SomeClass {
    private static Scanner input;

    public static void main(String[] args) {

        input = new Scanner(System.in);
        System.out.print("Please enter a command (start or stop) : ");
        String scanner = input.nextLine();

        while (!"start".equals(scanner) && (!"stop".equals(scanner))) {
            System.out.print("Please try again : ");
            scanner = input.nextLine();
        }
        if ("start".equals(scanner)) {
            System.out.println("System is starting");
        } else if ("stop".equals(scanner)) {
            System.out.println("System is closing");
        }
    }
}

Answer 2

A while loop can't "go back to" a statement outside its body.

You need everything you want to loop back to inside the loop's body. For example:

System.out.print("Please enter a command (start or stop) : ");
while (true) {
  scanner = input.nextLine();

  if ("start".equals(scanner)) {
    System.out.println("System is starting");
    break;  // Exits the loop, so it doesn't run again.
  } else if ("stop".equals(scanner)) {
    System.out.println("System is closing");
    break;
  }

  // No need for conditional, we know it's neither "start" nor "stop".

  System.out.print("Please try again : ");
  // After this statement, the loop will run again from the start.
}

Answer 3

You could simply loop until you get the desired output; an example using a do-while :

input = new Scanner(System.in);

String scanner;

do {
    System.out.print("Please enter a command (start or stop) : ");
    scanner = input.nextLine();
} while (!"start".equals(scanner) && !"stop".equals(scanner));

if ("start".equals(scanner)) {
    System.out.println("System is starting");
}
else if ("stop".equals(scanner)) {
    System.out.println("System is closing");
}