SimilarNumberCounter.java

import java.util.HashMap;
import java.util.Map;

public class SimilarNumberCounter {

    private static Map<Integer, Integer> digitCounts = new HashMap<>();

    private static void initializeDigitCounts() {
        for (int i = 0; i <= 9; i++) {
            digitCounts.put(i, 0);
        }
    }

    /**
     * Because the input number range from 0 to 99,999, only need to return int
     *
     * @param number the input number
     * @return the count of the similar numbers
     */
    private static int countSimilarNumbers(int number) {
        countApparenceForDigits(number);

        int length = countNumberLength(number);
        int result = 0;
        result = calcFactorial(length);
        for (int i = 0; i <= 9; i++) {
            result /= calcFactorial(digitCounts.get(i));
        }
        return result;
    }


    private static void countApparenceForDigits(int number) {
        int currentValue = number;
        while (currentValue > 0) {
            int digit = currentValue % 10;
            int count = digitCounts.get(digit);
            digitCounts.put(digit, count + 1);
            currentValue /= 10;
        }
    }
    
    private static int countNumberLength(int number) {
        return String.valueOf(number).length();
    }


    private static int calcFactorial(int number) {
        int result = 1;
        for (int i = 1; i <=number; i++) {
            result = result * i;
        }

        return result;
    }

    public static void main(String[] args) {
        int number = Integer.parseInt(args[0]);
        initializeDigitCounts();

        System.out.println("The count of similar numbers of " + number + " : "
                            + countSimilarNumbers(number));
    }
}