Fascinating Number in Java with Explanation, Definition, Examples, and Program

Fascinating Number in Java

A Fascinating Number is a special number that, when multiplied by 2 and 3, and all three results are concatenated together, produces a 9-digit string that contains all digits from 1 to 9 exactly once. Zero (0) must not appear in the final combined string.

Example

Example 1: 192

Multiply the number by 1, 2, and 3:

• 192 × 1 = 192
• 192 × 2 = 384
• 192 × 3 = 576

Now concatenate them:

"192" + "384" + "576" = "192384576"

The result 192384576 contains all digits from 1 to 9 exactly once and is a valid Fascinating Number.

Example 2: 853

• 853 × 1 = 853
• 853 × 2 = 1706
• 853 × 3 = 2559

Concatenate:

"853" + "1706" + "2559" = "85317062559"

This contains more than 9 digits and does not contain digits 1–9 exactly once. So, 853 is NOT a Fascinating Number.

Java Program to Check Fascinating Number

public class FascinatingNumber {

    // Function to check if a number is fascinating
    static boolean isFascinating(int num) {
        String concat = num + "" + (num * 2) + "" + (num * 3);

        // If length is not 9, it cannot be fascinating
        if (concat.length() != 9) return false;

        // Check if digits 1 to 9 appear exactly once
        for (char digit = '1'; digit <= '9'; digit++) {
            if (concat.indexOf(digit) == -1 || concat.indexOf(digit) != concat.lastIndexOf(digit)) {
                return false;
            }
        }

        return true;
    }

    public static void main(String[] args) {
        int num = 192;

        if (isFascinating(num)) {
            System.out.println(num + " is a Fascinating Number");
        } else {
            System.out.println(num + " is NOT a Fascinating Number");
        }
    }
}

Practice Challenges

1. Print all Fascinating Numbers between 100 and 10,000.
2. Modify the program to take user input and check the number.
3. Update the program to show the concatenated result along with validation steps.