package com.kickstart;
public class CreditCardChecker {
    /**
     * Variables
     */
    public String longNumber;
    public int checkDigit;
    public int checkSum;
    public int evenNumber;
    /**
     * Constructor for initializing objects of class CreditCardChecker
     */
    public CreditCardChecker(String longNumber, int checkDigit, int checkSum, int evenNumber) {
        this.longNumber = longNumber;
        this.checkDigit = checkDigit;
        this.checkSum = checkSum;
        this.evenNumber = evenNumber;
    }
    /**
     * Getters
     */
    public int getCheckDigit() {
        return checkDigit;
    }
    public int getCheckSum() {
        return checkSum;
    }
    public int getEvenNumber() {
        return evenNumber;
    }
    public String getLongNumber() {
        return longNumber;
    }
    /**
     * Setters
     */
    public void setEvenNumber(int evenNumber) {
        this.evenNumber = evenNumber;
    }
    public void setCheckSum(int checkSum) {
        this.checkSum = checkSum;
    }
    public void setCheckDigit(int checkDigit) {
        this.checkDigit = checkDigit;
    }
    public void setLongNumber(String longNumber) {
        this.longNumber = longNumber;
    }
    /**
     * Method to check that long number has exactly 16 digits and if character are only digits.
     * @return true if lenght of string is 16 and contains only digits, false otherwise
     */
    public boolean isCorrectLength() {
        return getLongNumber().length() == 16 && getLongNumber().matches("[0-9]+");
    }
    /**
     * Method multiplies every even number by two and checks if $result is more than 10, if true $result - 9
     * otherwise just append number to StringBuilder
     * @return String of length 16
     */
    public String everyEvenNumberIsMultipliedBy2(){
        // You can use any variable here you want to append results {String, Integer, etc.}
        StringBuilder result = new StringBuilder();
        // Method described on line n. 56
        if(isCorrectLength()){
            // For loop through String "longNumber"
            for(int i = 0; i < getLongNumber().length(); i++){
                // When character on index of "i" from for loop is an Odd number
                if(Character.getNumericValue(getLongNumber().charAt(i)) % 2 == 0){
                    // We multiplicity number by two and check if number is more than 10
                    if((Character.getNumericValue(getLongNumber().charAt(i)) * 2) > 10)
                        // If so we do on number minus nine and write it to our "result" variable
                        result.append(Character.getNumericValue(getLongNumber().charAt(i)) * 2 - 9);
                    else
                        // Otherwise we just append it to result without modifying
                        result.append(getLongNumber().charAt(i));
                } else
                    // So we do here. I could write this better (we do repetition on this line and line n. 85), but I wanted you to understand what is happening here
                    result.append(getLongNumber().charAt(i));
            }
        }
        // We return "result" which is of type StringBuilder so we parse it to String
        return result.toString();
    }
}

这是主课。在这里创建类“creditcardchecker”的对象，并使用类中实现的方法。
注意，包名是我的。你在创建的项目上有你自己的回应。您必须编辑包路径。

package com.kickstart;
public class Main {
    public static void main(String[] args) {
        CreditCardChecker creditCardChecker = new CreditCardChecker("1234567891234567", 2, 10, 2);
    // Note that all did all you asked in questing in comment. You will have to implement other methods as checkSum, etc. Or create new question.
            System.out.println(creditCardChecker.getLongNumber().length());
            System.out.println(creditCardChecker.getLongNumber());
        System.out.println(creditCardChecker.everyEvenNumberIsMultipliedBy2().length());
        System.out.println(creditCardChecker.everyEvenNumberIsMultipliedBy2());
    }
}