如何在字符串的奇数和偶数之间切换？

Question

我正在为程序编写代码，我必须在奇数和偶数之间切换。当我尝试执行所有操作时，只重复了一个数字几次。我在做什么错了？

这就是我所拥有的： 套餐creditCard；

导入java.util.Scanner;

公共类Program2CCValidation {

public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    boolean validCreditCard = false;
    int creditCard;
    while (!validCreditCard) {
        System.out.print("Please enter a credit card number: ");
        String potentialCCN = scan.nextLine();
        int lastDigit = Integer.parseInt(potentialCCN.substring(potentialCCN.length()-1));
        System.out.println("Check credit card number.");
        String reversedCCN = "";
        for (int i = potentialCCN.length()-1; i >= 0; i--) { 
            reversedCCN = reversedCCN + potentialCCN.charAt(i);
        }
        
        System.out.println(reversedCCN);
        boolean isOddDigit = false;
        int currentDigit = 0;
        int sum = 0;
        for (int i = 0; i < reversedCCN.length(); i += 2) {
            currentDigit = Integer.parseInt(potentialCCN.substring(potentialCCN.length()-1));
        System.out.println(currentDigit);
            if (currentDigit + 1 < potentialCCN.length()) {
                }
            if (isOddDigit) {
                currentDigit= currentDigit*2;
            }
        }
    }
}

}

这就是我想要做的：

//set up a Scanner
//set up a boolean named validCreditCard and set it to false       
//loop while not a valid credit card  
//prompt the user for a potential credit card number                     
//Get the credit card number as a String - store in potentialCCN (use scanner's nextLine)
//use Luhn to validate credit card using the following steps:
//store the digit AS AN INT for later use in lastDigit (probably requires Integer.parseInt(String)
//TEST then comment out! - check the last digit - System.out.println(lastDigit);
//remove the last digit from potentialCCN and store in potentialCCN using String's substring
//TEST then comment out! - check potentialCCN - System.out.println(potentialCCN);
//create reversedCCN as a empty String
//reverse the digits using a for loop by adding characters to reversedCCN
//TEST then comment out! - check reversedCCN - System.out.println(reversedCCN);
//set boolean isOddDigit to false
//set up an integer called current digit and set it to 0
//create an integer named sum and set it to 0
//multiply the digits in odd positions by 2, then subtract 9 from any number higher than 9 using:
//(for loop running 0 to less than length of reversed CCN)
//set currentDigit equal to the integer version of the current character
//Test then comment out! - currentDigit -System.out.print(currentDigit);
//toggle isOddDigit
//if isOddDigit
//multiply currentDigit by 2 and store in currentDigit
//if currentDigit > 9, subtract 9 from currentDigit and store in currentDigit
//TEST then comment out - Check currentDigit - System.out.println(currentDigit);
}//end if isOddDigit