Java Program to Reverse a Number

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

Introduction

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.

Method 1: Using While Loop (Most Common)

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:

Enter a number to reverse: 1234
Original number: 1234
Reversed number: 4321

Method 2: Using For Loop

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();
    }
}

Method 3: Using String Manipulation

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();
    }
}

Method 4: Using Recursion

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();
    }
}

Method 5: Complete Program with Palindrome Check

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:

Enter a number to reverse: 12321
Original number: 12321
Reversed number: 12321
12321 is a palindrome!

Key Concepts Explained

1. Digit Extraction

number % 10 extracts the last digit
number / 10 removes the last digit

2. Building Reversed Number

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

3. Handling Negative Numbers

Store the sign separately and apply it at the end
Work with absolute value during reversal

4. Edge Cases

Single digit numbers (0-9)
Negative numbers
Numbers ending with zeros (e.g., 1200 becomes 21)

Time and Space Complexity

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

Common Applications

Palindrome Detection: Checking if a number reads the same forwards and backwards
Mathematical Operations: Various number theory problems
Data Processing: Reversing sequences or patterns
Algorithm Practice: Understanding digit manipulation techniques

Practice Exercises

Modify the program to reverse only the digits, keeping the sign in the original position
Create a version that reverses numbers in different bases (binary, octal, hexadecimal)
Implement a method that reverses alternating digits (1st, 3rd, 5th, etc.)
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!

Java Program to Reverse a Number

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