Java Intermediate: 07/11/2021

Main.java

package com.javafx.test;

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        scoopIceCream("chocolate", true);
        scoopIceCream("vanilla", false);
        scoopIceCream("strawberry", true);
        System.out.println(average(5, 4)); // prints 4.5
        System.out.println(average(0, 10)); // prints 5.0
        System.out.println(average(1.5, 3.75)); // prints 2.625
        System.out.print("Enter a credit card number: ");
        long number = input.nextLong();

        // print whether the number is valid
        if (isValid(number)) {
            String cardType;

            // determine card type based on prefix
            if (prefixMatched(number, 4)) {
                cardType = "Visa";
            } else if (prefixMatched(number, 5)) {
                cardType = "Master";
            } else if (prefixMatched(number, 6)) {
                cardType = "Discover";
            } else {
                cardType = "American Express";
            }

            // display the card type
            System.out.println("That is a valid " + cardType + " card number.");
        } else {
            System.out.println("That is not a valid credit card number.");
        }

        input.close();
    }
    public static void scoopIceCream(String flavor, boolean wantCone) {
        if (wantCone) {
            System.out.println(
                    "Here's a scoop of " + flavor + " ice cream on a cone!"
            );
        } else {
            System.out.println("Here's a scoop of " + flavor + " ice cream!");
        }
    }
    public static double average(double a, double b) {
        return (a + b) / 2;
    }


    /**
     * Returns true if the card number is valid
     * @param number  credit card number
     * @return true if the card number is valid
     */
    public static boolean isValid(long number) {
        // number has a valid prefix if it begins with 4, 5, 6, or 37
        boolean prefixMatches =
                prefixMatched(number, 4) ||
                        prefixMatched(number, 5) ||
                        prefixMatched(number, 6) ||
                        prefixMatched(number, 37);

        if (number < 0) {
            // if number is negative, it's invalid
            return false;
        } else if (getSize(number) < 13 || getSize(number) > 16) {
            // if number is NOT between 13 and 16 digits (inclusive), it's invalid
            return false;
        } else if (!prefixMatches) {
            // if the number does not have a valid prefix, it's invalid
            return false;
        } else {
            // if the number passes all of the other tests,
            // do the Luhn check (aka Mod 10 check)
            int sum = sumOfDoubleEvenPlace(number) + sumOfOddPlace(number);
            return sum % 10 == 0;
        }
    }

    /**
     * Returns the sum of the doubled even-place digits,
     * from right to left
     * @param number  credit card number
     * @return the sum of the doubled even-place digits,
     * from right to left
     */
    public static int sumOfDoubleEvenPlace(long number) {
        int sum = 0;

        // start at the second-to-last digit
        number /= 10;

        int result;

        // while there are still digits remaining
        while (number != 0) {
            // get the rightmost digit
            int rightmostDigit = (int) (number % 10);

            // double it
            rightmostDigit *= 2;

            // the result is either the doubled value itself
            // or the sum of the two digits of doubled value
            result = getDigit(rightmostDigit);

            // add that result to the running sum
            sum += result;

            // move 2 digits to the left (the next even place)
            number /= 100;
        }

        return sum;
    }

    /**
     * Returns the given number if it is a single digit,
     * otherwise return the sum of the two digits
     * @param n  a doubled even-place digit of a credit card number
     * @return the given number if it is a single digit,
     * otherwise the sum of the two digits
     */
    public static int getDigit(int n) {
        // if n is 2 digits
        if (n > 9) {
            // get the first and second digits
            int firstDigit = 1; // if n is 2 digits, n can only be from 10-18
            int secondDigit = n % 10;

            // result is the sum of the 2 digits
            return firstDigit + secondDigit;
        } else {
            // if n is 1 digit, result is n itself
            return n;
        }
    }

    /**
     * Returns the sum of the odd-place digits,
     * from right to left
     * @param number  credit card number
     * @return the sum of the odd-place digits,
     * from right to left
     */
    public static int sumOfOddPlace(long number) {
        int sum = 0;

        // while there are still digits remaining in number
        while (number != 0) {
            // get the rightmost digit
            int rightmostDigit = (int) (number % 10);

            // add the rightmost digit to the sum
            sum += rightmostDigit;

            // move 2 digits to the left (the next odd place)
            number /= 100;
        }

        return sum;
    }

    /**
     * Returns true if d is a prefix for a number
     * @param number  credit card number
     * @param d  credit card number prefix
     * @return true if d is a prefix for a number
     */
    public static boolean prefixMatched(long number, int d) {
        return getPrefix(number, getSize(d)) == d;
    }

    /**
     * Return the number of digits in the given number
     * @param number  a long integer
     * @return the number of digits in the given number
     */
    public static int getSize(long number) {
        int numberOfDigits = 0;

        while (number != 0) {
            number /= 10;
            numberOfDigits++;
        }

        return numberOfDigits;
    }

    /**
     * Return the first k number of digits from number.
     * If the number of digits in number is less than k,
     * return number.
     * @param number  credit card number
     * @param k  number of first digits to retrieve from number
     * @return the first k number of digits from number.
     * If the number of digits in number is less than k,
     * returns number.
     */
    public static long getPrefix(long number, int k) {
        // If the number of digits in number is less than k,
        // return number
        if (getSize(number) < k) {
            return number;
        } else {
            // keep removing rightmost digits
            // until the length of the number is equal to k
            while (getSize(number) > k) {
                number /= 10;
            }

            // once loop ends, number is the prefix
            return number;
        }
    }

}