Java:如果用户输入不是单个字母小写字母，我如何将用户返回到主菜单？

Question

我正在尝试运行一个允许用户输入 char keyCharacter 和 String theString 的程序。然后，使用这些输入，如果 keyCharacter 在 theString 中出现，我将用“$”屏蔽它，从 theString 中删除 keyCharacter，最后，计算 keyCharacter 在 theString 中出现的总次数。

除了用户必须输入字符的 getKeyCharacter 方法外，每个方法都工作正常:用户只能输入一个字母(例如 q 或 z)。

如果用户输入的不是单个字母(可以是单词、短语、句子、# 或 $ 等特殊字符、空格或制表符，或者只是按回车)以外的任何内容，则程序会返回用户到要求用户提供 keyCharacter 的原始问题。这应该继续循环回到原来的问题，直到用户输入有效的输入。

由于我还是java的初学者，到目前为止循环是我的弱点，这部分给我带来了很多麻烦。我知道我应该使用 while 循环，嵌套循环背后的逻辑让我很困惑。

通过搜索可能的解决方案，我知道这些称为正则表达式和 try-catch 异常的东西可以帮助解决我的问题，但由于我们没有在类里面明确地讨论过这些，我不想处理现在。谢谢。

这是我的代码粘贴:

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;
    }
}

这是我运行的粘贴(如您所见，我的程序中需要进行一些认真的调试):

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;
}