Reverse a Number in Java: Loop Logic and Code Example

Reverse Integer in Java

Learning to reverse a number in Java is a common beginner interview problem because it tests loops, modulo arithmetic, and basic number manipulation. This example shows how to reverse a number in Java without using helper APIs, using simple while-loop logic.

Interview Problem Statement

• Reverse an integer in Java without using any API.
• Reverse a number in Java.

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Code Example

package com.adevguide.java.problems;

public class ReverseInteger {

public static void main(String\[\] args) {
    // Variable to hold input value
    Integer input = 123;
    // Variable to hold reversed value
    Integer reverse = 0;
    // loop till input value becomes zero
    while (input != 0) {
        // multiple previous reverse value by 10 , add the remainder to it and save it back
        // in reverse variable
        reverse = reverse \* 10 + input % 10;
        // Divide input by 10 and store quotient value in input
        input = input / 10;
    }
    // Print reverse Value
    System.out.println("Reverse Integer ::" + reverse);
}

}

Code Explanation

To reverse an integer in Java, we will follow the following steps:

1. Find one’s place of the input and add it to 10 x previously calculated reverse value.
2. Change input value to the quotient of division of input by 10.

Example:

Input = 123

Iteration 1:

reverse = 0 * 10 + 123 % 10

reverse = 3

Input = 12

Iteration 2:

reverse = 3 * 10 + 12 % 10

reverse = 32

Input = 1

Iteration 3:

reverse = 32 * 10 + 1 % 10

reverse = 321

Input = 0

Source Code

As always, you can find all our source code at GitHub.

GitHub Link

YouTube Videos

1. “Reverse Number Program in Java - [updated 2025] Interview Prep | Java Programming Tutorial” https://www.youtube.com/watch?v=I-saO_f6NEU

2. “Java Program to Reverse a Number with Explanation” https://www.youtube.com/watch?v=pZ3HeOwglwQ

3. “LeetCode #7: Reverse Integer (Java) | Don’t Fall for the Overflow Trap!” https://www.youtube.com/watch?v=WV90AnTVSY0