Frequency of Each Digit

/*
 * Problem : Count how many times each digit appears.
 * Input   : One integer.
 * Output  : One line per digit that appears.
 * Approach: Use an array of 10 counters, one for each digit.
 * Time    : O(d)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        long number = Math.abs(new Scanner(System.in).nextLong());

        // Step 1: Make an array with 10 counters. counts[3] will count the digit 3.
        int[] counts = new int[10];

        // Step 2: A do-while loop runs at least once, so the number 0 is counted too.
        do {
            // Step 3: Take the last digit. Example: 112233 % 10 = 3.
            int digit = (int) (number % 10);

            // Step 4: Add 1 to the counter of this digit. The digit is the index of the array.
            counts[digit]++;

            // Step 5: Remove the last digit. Example: 112233 / 10 = 11223.
            number = number / 10;
        } while (number > 0);

        // Step 6: Print every digit that appeared at least once. Index 0 to 9 gives the smallest digit first.
        for (int digit = 0; digit <= 9; digit++) {
            if (counts[digit] > 0) {
                System.out.println(digit + ": " + counts[digit]);
            }
        }
    }
}