title: Java Program to Reverse a Number

description: Learn different methods to reverse a number in Java with complete code examples and explanations.

Reversing a number is a fundamental programming concept that involves extracting digits from right to left and reconstructing them in reverse order. For example, reversing 1234 gives us 4321.

This program demonstrates multiple approaches to reverse a number in Java, from basic iterative methods to advanced techniques.

This is the most straightforward approach using digit extraction and mathematical operations.

import java.util.Scanner;

public class ReverseNumber {

public static void main(String[] args) {

Scanner scanner = new Scanner(System.in);

System.out.print("Enter a number to reverse: ");

int number = scanner.nextInt();

int originalNumber = number;

int reversedNumber = 0;

// Handle negative numbers

boolean isNegative = number < 0;

number = Math.abs(number);

while (number != 0) {

int digit = number % 10; // Extract last digit

reversedNumber = reversedNumber * 10 + digit; // Build reversed number

number = number / 10; // Remove last digit

}

// Apply sign if original was negative

if (isNegative) {

reversedNumber = -reversedNumber;

}

System.out.println("Original number: " + originalNumber);

System.out.println("Reversed number: " + reversedNumber);

scanner.close();

}

}

**Output:**

The same logic can be implemented using a for loop, which is more compact but requires knowing the number of digits.

import java.util.Scanner;

public class ReverseNumberForLoop {

// Method to count digits in a number

public static int countDigits(int number) {

if (number == 0) return 1;

int count = 0;

number = Math.abs(number);

while (number > 0) {

count++;

number /= 10;

}

return count;

}

public static void main(String[] args) {

Scanner scanner = new Scanner(System.in);

System.out.print("Enter a number to reverse: ");

int number = scanner.nextInt();

int originalNumber = number;

int reversedNumber = 0;

boolean isNegative = number < 0;

number = Math.abs(number);

int digitCount = countDigits(number);

for (int i = 0; i < digitCount; i++) {

int digit = number % 10;

reversedNumber = reversedNumber * 10 + digit;

number = number / 10;

}

if (isNegative) {

reversedNumber = -reversedNumber;

}

System.out.println("Original number: " + originalNumber);

System.out.println("Reversed number: " + reversedNumber);

scanner.close();

}

}

This approach converts the number to a string and uses built-in string methods.

import java.util.Scanner;

public class ReverseNumberString {

public static void main(String[] args) {

Scanner scanner = new Scanner(System.in);

System.out.print("Enter a number to reverse: ");

int number = scanner.nextInt();

// Convert to string and handle negative sign

String numberStr = String.valueOf(Math.abs(number));

// Reverse the string

String reversedStr = new StringBuilder(numberStr).reverse().toString();

// Convert back to integer

int reversedNumber = Integer.parseInt(reversedStr);

// Apply negative sign if original was negative

if (number < 0) {

reversedNumber = -reversedNumber;

}

System.out.println("Original number: " + number);

System.out.println("Reversed number: " + reversedNumber);

scanner.close();

}

}

A recursive approach that demonstrates functional programming concepts.

import java.util.Scanner;

public class ReverseNumberRecursive {

// Recursive method to reverse a number

public static int reverseNumber(int number, int reversedNumber) {

if (number == 0) {

return reversedNumber;

}

int digit = number % 10;

reversedNumber = reversedNumber * 10 + digit;

return reverseNumber(number / 10, reversedNumber);

}

public static void main(String[] args) {

Scanner scanner = new Scanner(System.in);

System.out.print("Enter a number to reverse: ");

int number = scanner.nextInt();

boolean isNegative = number < 0;

int absoluteNumber = Math.abs(number);

int reversedNumber = reverseNumber(absoluteNumber, 0);

if (isNegative) {

reversedNumber = -reversedNumber;

}

System.out.println("Original number: " + number);

System.out.println("Reversed number: " + reversedNumber);

scanner.close();

}

}

This enhanced version also checks if the number is a palindrome.

import java.util.Scanner;

public class CompleteReverseNumber {

public static int reverseNumber(int number) {

int reversedNumber = 0;

number = Math.abs(number);

while (number != 0) {

int digit = number % 10;

reversedNumber = reversedNumber * 10 + digit;

number = number / 10;

}

return reversedNumber;

}

public static boolean isPalindrome(int number) {

int absNumber = Math.abs(number);

return absNumber == reverseNumber(absNumber);

}

public static void main(String[] args) {

Scanner scanner = new Scanner(System.in);

System.out.print("Enter a number to reverse: ");

int number = scanner.nextInt();

int reversedNumber = reverseNumber(number);

// Apply negative sign if original was negative

if (number < 0) {

reversedNumber = -reversedNumber;

}

System.out.println("Original number: " + number);

System.out.println("Reversed number: " + reversedNumber);

// Check if palindrome

if (isPalindrome(number)) {

System.out.println(Math.abs(number) + " is a palindrome!");

} else {

System.out.println(Math.abs(number) + " is not a palindrome.");

}

scanner.close();

}

}

**Sample Output:**

- `number % 10` extracts the last digit

- `number / 10` removes the last digit

- `reversedNumber * 10 + digit` adds the extracted digit to the left

- Store the sign separately and apply it at the end

- Work with absolute value during reversal

- Single digit numbers (0-9)

- Negative numbers

- Numbers ending with zeros (e.g., 1200 becomes 21)

- **Time Complexity**: O(log n) where n is the input number (number of digits)

- **Space Complexity**: O(1) for iterative methods, O(log n) for recursive method

1. **Palindrome Detection**: Checking if a number reads the same forwards and backwards

2. **Mathematical Operations**: Various number theory problems

3. **Data Processing**: Reversing sequences or patterns

4. **Algorithm Practice**: Understanding digit manipulation techniques

1. Modify the program to reverse only the digits, keeping the sign in the original position

2. Create a version that reverses numbers in different bases (binary, octal, hexadecimal)

3. Implement a method that reverses alternating digits (1st, 3rd, 5th, etc.)

4. Write a program that finds all palindromic numbers in a given range

This program provides a solid foundation for understanding digit manipulation in Java programming!