Armstrong Number in Java – Definition, Logic, Program & Dry Run (With Examples)

An Armstrong number (also called a Narcissistic number) is a number in which the sum of each digit raised to the power of the total number of digits is equal to the original number.

Simple Definition

A number is an Armstrong number if
Sum of (each digit ^ number of digits) = original number

---

Examples of Armstrong Numbers

✔ 3-Digit Armstrong Number

153

1³ + 5³ + 3³ = 1 + 125 + 27 = 153

✔ More Examples

• 370 → 3³ + 7³ + 0³ = 370

• 371 → 3³ + 7³ + 1³ = 371

• 9474 → 9⁴ + 4⁴ + 7⁴ + 4⁴ = 9474

---

Logic to Check Armstrong Number in Java

To check whether a number is Armstrong or not, follow these steps:

1. Input the number

2. Count the number of digits

3. Extract each digit using % operator

4. Raise each digit to the power of total digits

5. Add the result

6. Compare sum with original number

---

Java Program to Check Armstrong Number

import java.util.Scanner;

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

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

        int original = num;
        int sum = 0;
        int digits = 0;
        int temp = num;

        // Step 1: Count digits
        while (temp != 0) {
            digits++;
            temp /= 10;
        }

        temp = num;

        // Step 2: Calculate Armstrong sum
        while (temp != 0) {
            int digit = temp % 10;
            sum += Math.pow(digit, digits);
            temp /= 10;
        }

        // Step 3: Compare
        if (sum == original) {
            System.out.println(original + " is an Armstrong number");
        } else {
            System.out.println(original + " is not an Armstrong number");
        }
    }
}

---

Dry Run of Program (Example: 153)

Step	Value
Input number	153
Number of digits	3
Digits extracted	3, 5, 1
Calculation	3³ + 5³ + 1³
Sum	27 + 125 + 1 = 153
Output	Armstrong Number

---

Armstrong Number for 4-Digit Numbers

This logic also works for 4-digit, 5-digit, or any length numbers.

Example:

9474 → 9⁴ + 4⁴ + 7⁴ + 4⁴ = 9474 ✔

---

Why Armstrong Numbers Are Important?

• Frequently asked in BCA / MCA / ICSE / CBSE exams

• Helps understand loops, conditions, math logic

• Improves problem-solving skills in Java

• Common interview and coding test question

---

Time and Space Complexity

• Time Complexity: O(n)

• Space Complexity: O(1)

(where n = number of digits)

---

Key Points to Remember

✔ Uses modulus (%) to extract digits
✔ Uses Math.pow() for power calculation
✔ Works for any number of digits
✔ Logic-based and exam-friendly program