java - 如何连续循环菜单直到选择退出选项，以及如果选择无效操作则重复菜单

Question

抱歉，如果标题有点令人困惑。我一整天都在做这个工作，所以我的大脑有点烧焦了。我想做的是允许用户继续接收菜单选项，直到选择“退出”，但只输入一个字符串；如果做出了无效的菜单选择，则程序将向用户报告并再次提供菜单选项。我只是非常不知道该做什么，非常感谢任何帮助。

    public class McLemorePProg5 {

    public static void main(String[] args) {

        //Create scanner
        java.util.Scanner mango = new java.util.Scanner(System.in);

    //Print Statements

        //User is requested to enter a string
        System.out.printf("Please enter a string:\n");

        //User enters a string
                String s = mango.nextLine();

        //Operation menu is displayed       
        System.out.println("(1) Report the number of vowels in the string (a, e, i, o, u, y)");

        System.out.println("(2) Report the number of consonants in the string");

        System.out.println("(3) Report the number of lower case letters in the string");

        System.out.println("(4) Print out the characters of the strings that are in even positions in the string");

        System.out.println("(5) Exit program ");

        System.out.printf("\nPlease enter an operation number: (1), (2), (3), (4) or (5): \n");

        //User enters an operation
        int op = mango.nextInt();

                switch (op) {

                case 1:
                    System.out.println("The number of vowels in the string is " + countVowels(s));
                    break;
                case 2:
                    System.out.println("The number of consonants is " + countConsonants(s));
                    break;
                case 3: 
                    System.out.println("The number of lowercase letters in the string is " + countLowercase(s));
                    break;
                case 4:
                    System.out.println("the characters in even positions are:"); printEvenChars(s);
                    break;
                case 5:
                    System.out.println("Program ended"); System.exit(1);
                default:    
                    System.out.println("Invalid operation choice. Please choose again");

                }//end switch

            //Close scanner
            mango.close();  

}//end main

    //Methods

    public static int countVowels(String s) { //Calculate the number of vowels
        int vowelCount = 0;

        for ( int i = 0; i < s.length(); i++) {

            if (s.charAt(i) == 'A' || s.charAt(i) == 'a' || s.charAt(i) == 'E' || s.charAt(i) == 'e' ||
                s.charAt(i) == 'I' || s.charAt(i) == 'i' || s.charAt(i) == 'O' || s.charAt(i) == 'o' ||
                s.charAt(i) == 'U' || s.charAt(i) == 'u' || s.charAt(i) == 'Y' || s.charAt(i) == 'y') {
                    vowelCount++; } //end if    
          }//end for loop

        return vowelCount;

    }//end countVowels

    public static int countConsonants(String s) { //Calculate the number of consonants
        int consonantCount = 0;

        for (int i = 0; i < s.length(); i++) {

            if (s.charAt(i) == 'A' || s.charAt(i) == 'a' || s.charAt(i) == 'E' || s.charAt(i) == 'e' ||
                s.charAt(i) == 'I' || s.charAt(i) == 'i' || s.charAt(i) == 'O' || s.charAt(i) == 'o' ||
                s.charAt(i) == 'U' || s.charAt(i) == 'u' || s.charAt(i) == 'Y' || s.charAt(i) == 'y') {
                 }  //end if 

            else {
                consonantCount++;
            }//end else

         }//end for loop

        return consonantCount;

    }//end countConsonants

    public static int countLowercase(String s) { //Calculate the number of lower case letters
        int lowercaseCount = 0;

        for (int i = 0; i < s.length(); i++) {

            if (s.charAt(i) >= 'a' && s.charAt(i) <= 'z') {
                lowercaseCount++; }//end if statement

         }//end for loop

        return lowercaseCount;

    }//end countLowercase

    public static void printEvenChars(String s) { //Prints the characters in even positions in the string
        String evenChars = "";

        for (int i = 0; i < s.length(); i++) {

            if(i % 2 == 0) {
                evenChars += s.charAt(i); }//end if statement

        }//end for loop

        System.out.println(evenChars);

    }//end printEvenChars
    }//end class

Answer 1

除非绝对必要，否则不要诉诸System.exit()。以下是您应该采取的行动的概述:

int op = mango.nextInt();

while (op != 5) {
  switch(op) {
    // your switch cases
    case 5:
      // just do nothing
      break;
  }

  // once you've processed that command, read in the next one.
  // If it's 5, the loop will exit
  // you may want to also print the menu here, so it appears after each command
  op = mango.nextInt();
}

mango.close();