Java Program to reverse a number using for, while and Recursion

In the following examples of three Java programs to reverse a number using for loop, while loop, and recursion.

Using for loop :

Program :

public class ReverseNumber {

    public static void main(String[] args) {
        int num = 12345;
        int reversed = 0;
        for (; num != 0; num /= 10) {
            int digit = num % 10;
            reversed = reversed * 10 + digit;
        }
        System.out.println("Reversed number is " + reversed);
    }
}

Output :

Reversed number is 54321

In this program, we first declare an integer variable num and initialize it with the value 12345. We then declare another integer variable reversed and initialize it with 0. We then use a for loop to reverse the number.

In each iteration of the loop, we extract the last digit of the number using the modulus operator %, and add it to the reversed number multiplied by 10. We then divide the original number by 10 to remove the last digit. We repeat this process until the original number becomes 0. Finally, we print the reversed number.

Using while loop :

Program :

public class ReverseNumber {

    public static void main(String[] args) {
        int num = 12345;
        int reversed = 0;
        while (num != 0) {
            int digit = num % 10;
            reversed = reversed * 10 + digit;
            num /= 10;
        }
        System.out.println("Reversed number is " + reversed);
    }
}

Output :

Reversed number is 54321

Using recursion :

public class ReverseNumber {

    public static void main(String[] args) {
        int num = 12345;
        int reversed = reverse(num);
        System.out.println("Reversed number is " + reversed);
    }

    public static int reverse(int num) {
        if (num < 10) {
            return num;
        } else {
            int lastDigit = num % 10;
            int remainingDigits = num / 10;
            int reversed = reverse(remainingDigits);
            return lastDigit * (int)Math.pow(10, (int)Math.log10(remainingDigits) + 1) + reversed;
        }
    }
}

Output :

Reversed number is 54321

In this program, we first declare an integer variable num and initialize it with the value 12345. We then call the reverse method, passing in num, and store the returned value in the reversed variable. Finally, we print the reversed number.

The reverse method is defined as a static method that takes an integer parameter num and returns an integer. The method uses recursion to reverse the number. If the number is less than 10, it simply returns the number, since a single-digit number is already reversed. Otherwise, it extracts the last digit of the number using the modulus operator %, and the remaining digits using integer division /.

It then recursively calls itself with the remaining digits and stores the returned value in the reversed variable. Finally, it computes the final reversed number by multiplying the last digit with 10 raised to the power of the number of digits in the remaining digits plus 1, and adding it to the reversed number.