问题是您使用的变量名为selection，它是在循环内部定义的，因此无法在循环外部访问。要解决此问题，您应该在循环之前声明变量selection，然后检查循环条件内部selection的值。

public static void main(String[] args) {
    String menu = ("Please choose one option from the following menu: \n"
    + "1. Calculate the sum of integers 1 to m\n"
    + "2. Calculate the factorial of a given number\n"
    + "3. Calculate the amount of odd integers in a given sequence\n"
    + "4. Display the leftmost digit of a given number\n"
    + "5. Calculate the greatest common divisor of two given integers\n"
    + "6. Quit\n");

    Scanner console = new Scanner(System.in);
    int selection = 0;
    do {
        System.out.println(menu);
        selection = console.nextInt();

        switch (selection) {
            case 1:
                sumOfIntegers();
                break;
            case 2:
                factorialOfNumber();
                break;
            case 3:
                calcOddIntegers();
                break;
            case 4:
                displayLeftDigit();
                break;
            case 5:
                greatestCommonDivisor();
                break;
            case 6:
                System.out.println("Bye");
                break;
        }
    } while (selection != 6);
    console.close();
}

通过此修改，程序将执行从用例中选择的方法，然后在完成后返回菜单。

这听起来好像可以用递归来解决。
我个人会实现这个类，（或集成一个新的方法）：

public class Cmd {

    private static final String menu = ("Please choose one option from the following menu: \n"
            + "1. Calculate the sum of integers 1 to m\n"
            + "2. Calculate the factorial of a given number\n"
            + "3. Calculate the amount of odd integers in a given sequence\n"
            + "4. Display the leftmost digit of a given number\n"
            + "5. Calculate the greatest common divisor of two given integers\n"
            + "6. Quit\n");

    private static final Scanner console = new Scanner(System.in);

    public static void loop() {

        System.out.println(menu);
        
        int selection = console.nextInt();


        switch (selection) {
            case 1:
                sumOfIntegers();
                break;
            case 2:
                factorialOfNumber();
                break;
            case 3:
                calcOddIntegers();
                break;
            case 4:
                displayLeftDigit();
                break;
            case 5:
                greatestCommonDivisor();
                break;
            case 6:
                System.out.println("Bye");
                console.close();
                return;
        }
        loop();
    }
}

这样可以确保每次选择后都返回到相同的状态，并将6作为退出条件。Main方法如下所示：

public static void main(String[] args) {

    Cmd.loop();

}

请记住，如果您不想实现额外的类，您可以简单地将loop方法添加到同一个类main is中。

public class wopa {

    private static final String menu = ("Please choose one option from the following menu: \n"
            + "1. Calculate the sum of integers 1 to m\n"
            + "2. Calculate the factorial of a given number\n"
            + "3. Calculate the amount of odd integers in a given sequence\n"
            + "4. Display the leftmost digit of a given number\n"
            + "5. Calculate the greatest common divisor of two given integers\n"
            + "6. Quit\n");

    private static final Scanner console = new Scanner(System.in);

    public static void main(String[] args) {
        loop();
    }

    public static void loop() {

        System.out.println(menu);

        int selection = console.nextInt();

        switch (selection) {
            case 1:
                sumOfIntegers();
                break;
            case 2:
                factorialOfNumber();
                break;
            case 3:
                calcOddIntegers();
                break;
            case 4:
                displayLeftDigit();
                break;
            case 5:
                greatestCommonDivisor();
                break;
            case 6:
                System.out.println("Bye");
                console.close();
                return;
        }
        loop();
    }
}

如果你想保持原有的模式不变，你可以在main方法中添加loop方法，而不是使用递归，将它包含在while（true）中（不太喜欢这个选项）