Java：如果用户输入不是单个字母小写字母，如何将用户返回主菜单？

Question

I'm trying to run a program that will allow the user to input both a char keyCharacter and a String theString. Then, using these inputs, I will mask the keyCharacter if it occurs in theString with a "$", remove the keyCharacter from the theString, and finally, count the number of times the keyCharacter occurs in theString altogether.

Every method is working fine, except the method getKeyCharacter where the user has to input a char: The user can only enter a single letter (eg q, or z).

If the user enters anything other than that single letter (which can be anything from a word, phrase, sentence, special character like # or $, blank space or tabs, or just pressing enter), then the program returns the user to the original question that asks for the keyCharacter from the user. This should continue looping back to that original question until the user enters a valid input.

Since I'm still a beginner to java and loops are my weakness so far, this part is causing me a lot of trouble. I know I should be using a while loop, it is the logic behind the nested loops that is really confusing me.

From searching for possible solutions, I know there are these things called regex and try-catch exception that could help with my issue, but since we haven't gone over that explicitly in class, I'd prefer not to deal with that for now. Thank you.

Here's a paste of my code:

import java.util.*;

public class Foothill {

    // main method
    public static void main (String[] args) {
        char keyCharacter = getKeyCharacter();
        String theString = getString();    
        maskCharacter(theString, keyCharacter);
        countKey(theString, keyCharacter);
        removeCharacter(theString, keyCharacter);
    }

    // get keyCharacter
    public static char getKeyCharacter() {
        Scanner inputStream = new Scanner(System.in);
        boolean stop = false;
        String firstPrompt, strKeyCharacter;
        char keyCharacter = ' ';

        while (stop != true) {
            firstPrompt = "Please enter a SINGLE character to act as key: ";
            System.out.print(firstPrompt);
            strKeyCharacter = inputStream.nextLine(); 

            while (strKeyCharacter.length() != 1) {
                firstPrompt = "Please enter a SINGLE character to act as key: ";
                System.out.print(firstPrompt);
                strKeyCharacter = inputStream.nextLine();
            }

            keyCharacter = strKeyCharacter.charAt(0);

            while (strKeyCharacter.length() == 1) {
                firstPrompt = "Please enter a SINGLE character to act as key: ";
                System.out.print(firstPrompt);
                strKeyCharacter = inputStream.nextLine();
                if (keyCharacter == 'a' || keyCharacter == 'b' || keyCharacter == 'c' || keyCharacter == 'd' 
                  || keyCharacter == 'e' || keyCharacter == 'f' || keyCharacter == 'g' || keyCharacter == 'h'
                  || keyCharacter == 'i' || keyCharacter == 'j' || keyCharacter == 'k' || keyCharacter == 'l'
                  || keyCharacter == 'm' || keyCharacter == 'n' || keyCharacter == 'o' || keyCharacter == 'p'
                  || keyCharacter == 'q' || keyCharacter == 'r' || keyCharacter == 's' || keyCharacter == 't'
                  || keyCharacter == 'u' || keyCharacter == 'v' || keyCharacter == 'w' || keyCharacter == 'x'
                  || keyCharacter == 'y' || keyCharacter == 'z') {
                    System.out.println("You entered: " + keyCharacter + '\n');
                    stop = true;
                } else {
                    break;
                }
            }
        }
        return keyCharacter;
    }

    // declare final = 4 to be constant
    public static final int minimumLength = 4;

    // get theString
    public static String getString() {
        Scanner inputStream = new Scanner(System.in);
        String secondPrompt, theString;
        do {
            secondPrompt = "Please enter a phrase or sentence >= 4: ";
            System.out.print(secondPrompt);
            theString = inputStream.nextLine();
            System.out.print('\n');
        } while (theString.length() < minimumLength || theString == null || theString.length() == 0);
        inputStream.close();
        return theString;
    }

    // mask keyCharacter with $
    public static String maskCharacter(String theString, char keyCharacter) {
        theString = theString.replace(keyCharacter, '$');
        System.out.println("String with " + " '" + keyCharacter + "' " + " masked.");
        System.out.println(theString + '\n');
        return theString;
    }

    // count number of times keyCharacter occurs in theString
    public static void countKey(String theString, char keyCharacter) {
        int countChar = 0;
        for (int charTimes = 0; charTimes < theString.length(); charTimes++) {
            if (theString.charAt(charTimes) == keyCharacter) {
                countChar++;
            }
        }
        System.out.println( "The key character occurs " + countChar + " times. \n");
        return;
    }

    // remove keyCharacter from theString
    public static void removeCharacter(String theString, char keyCharacter) {
        theString = theString.replace(String.valueOf(keyCharacter), "");
        System.out.println("String with " + "'" + keyCharacter + "' removed: ");
        System.out.println(theString);
        return;
    }
}

And here's a paste of my run (as you can see, there is some serious debugging to be done in my program):

Please enter a SINGLE character to act as key: f
Please enter a SINGLE character to act as key: f
You entered: f

Please enter a SINGLE character to act as key: f
You entered: f

Please enter a SINGLE character to act as key: f
You entered: f

Please enter a SINGLE character to act as key: f
You entered: f

Please enter a SINGLE character to act as key: 

// which then continues so on so forth...

Answer 1

public static char getKeyCharacter(){

    Scanner inputStream = new Scanner(System.in);
    boolean stop = false;
    String firstPrompt, strKeyCharacter;
    char keyCharacter = ' ';

    while(!stop){

        firstPrompt = "Please enter a SINGLE character to act as key: ";
        System.out.println(firstPrompt);
        strKeyCharacter = inputStream.nextLine();

        //check if the input contains only 1 character
        boolean isSingleChar = (strKeyCharacter.length() == 1);
        //check if the input character is within the ASCII code of 97 (a) to 122 (z)
        boolean isValidChar = 
                strKeyCharacter.charAt(0) >= 97 &&
                strKeyCharacter.charAt(0) <= 122;

        if(isSingleChar && isValidChar){
            keyCharacter = strKeyCharacter.charAt(0);
            stop = true;
        }


    }

    return keyCharacter;
}