Neon Number in Java

A Neon Number is a special type of number in which the sum of the digits of its square is equal to the original number. It is a commonly asked question in Java programming assignments and coding interviews as it helps test loop logic, digit extraction, and mathematical thinking.

What Is a Neon Number?

A number n is called a Neon Number if:

Step 1 → Find square of n  
Step 2 → Sum the digits of the square  
If Sum == n → It is a Neon Number  
  

Example

Consider the number 9:

• Square = 9 × 9 = 81
• Digit sum = 8 + 1 = 9

Since the digit sum equals the number, 9 is a Neon Number.

How Do We Check a Neon Number in Java?

Steps to check a Neon Number:

1. Accept the number from the user
2. Find its square
3. Extract digits of the square one by one
4. Add all the digits
5. Compare the sum with the original number

Java Program to Check Neon Number

import java.util.*;

public class NeonNumber {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter a number: ");
        int num = sc.nextInt();

        int square = num * num;
        int sum = 0;

        while (square > 0) {
            int digit = square % 10;
            sum += digit;
            square /= 10;
        }

        if (sum == num) {
            System.out.println(num + " is a Neon Number.");
        } else {
            System.out.println(num + " is NOT a Neon Number.");
        }
    }
}

  

Example Walkthrough

Let's verify 9 step-by-step:

• Square = 81
• Digit extraction: 8, 1
• Sum = 8 + 1 = 9

Since the sum equals the original number, it is a Neon Number.

Known Neon Numbers

Only a few Neon Numbers exist:

• 0 → Square = 0, Sum = 0
• 1 → Square = 1, Sum = 1
• 9 → Square = 81, Sum = 9

These are the only Neon numbers in the positive integer range.

Practice Challenge

Write a Java program to find all Neon Numbers between 1 and 10,000. Loop through each number, apply the Neon logic, and print only the matching numbers.