🔢 Reverse a Number in Java (With Explanation, Algorithm & Dry Run)

Introduction

Reversing a number is one of the most common and important programs in Java, especially for BCA students and beginners. This question is frequently asked in college exams, viva, and interviews.

In this article, you will learn how to reverse a number in Java, along with a step-by-step explanation, algorithm, dry run, and output.

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What Does “Reverse a Number” Mean?

Reversing a number means changing the order of digits from last to first.

Example

• Input: 1234

• Output: 4321

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Logic to Reverse a Number in Java

We use:

• Modulus operator (%) → to get the last digit

• Division operator (/) → to remove the last digit

Formula Used

digit = number % 10
reverse = reverse * 10 + digit
number = number / 10

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Algorithm to Reverse a Number

1. Start

2. Read a number

3. Initialize reverse = 0

4. Repeat while number ≠ 0

   • Get last digit using % 10

   • Add digit to reverse

   • Remove last digit using / 10

5. Print reversed number

6. End

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Java Program to Reverse a Number

import java.util.Scanner;

public class ReverseNumber {
    public static void main(String[] args) {
        int number, reverse = 0;

        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        number = sc.nextInt();

        while (number != 0) {
            int digit = number % 10;
            reverse = reverse * 10 + digit;
            number = number / 10;
        }

        System.out.println("Reversed Number: " + reverse);
    }
}

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Dry Run of the Program

Input

number = 123
reverse = 0

Step	number	digit = number % 10	reverse = reverse * 10 + digit
1	123	3	0 × 10 + 3 = 3
2	12	2	3 × 10 + 2 = 32
3	1	1	32 × 10 + 1 = 321

Final Output

Reversed Number: 321

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Output Screen

Enter a number: 123
Reversed Number: 321

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Important Points for Exams

• % 10 → extracts last digit

• / 10 → removes last digit

• Loop continues until number becomes 0

• Works for any positive integer

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Time and Space Complexity

• Time Complexity: O(n)

• Space Complexity: O(1)