Java Program to Reverse a Number

Summary: In this programming example, we will learn to reverse a number in Java.

Example:

Input:  123
Output: 321

Input:  5821
Output: 1285

There are multiple ways to reverse a number in Java. Let’s discuss each method one by one.

Method 1: Using While Loop

1. Input a number (num).
2. Loop while num!=0:
   1. Extract the last digit from num using num%10 and concatenate it to the reverse (rev).
   2. Remove the last digit from num using num/10.
3. Output the reverse.

import java.util.Scanner;
 
public class Main{
    public static void main(String args[]){
        Scanner in = new Scanner(System.in);
        int num, rev =0, digit;
        
        //Input a number
        System.out.print("Enter a number: ");
        num = in.nextInt();
 
        //loop until num!=0
        while(num!=0){
            digit = num%10;          //extract last digit
            rev = 10*rev + num%10;   //append the digit to the reverse
            num = num/10;            //remove the last digit 
        }
        
        //output the reverse
        System.out.println("Reversed Number: "+rev);
    }
}

Output:

Enter a number: 123
Reversed Number: 321

Method 2: Without Using Another Variable

Unlike the above method, instead of appending the digits to the reverse vairiable, we can output them directly.

import java.util.Scanner;
 
public class Main {
    public static void main(String args[]){
        Scanner in = new Scanner(System.in);
        
        //Input a number
        System.out.print("Enter a number: ");
        int num = in.nextInt();
        
        System.out.print("Reversed Number: ");
        
        //loop until num!=0
        while(num!=0){
            System.out.print(num%10);    //extract last digit and output
            num = num/10;                //remove the last digit from num
        }
    }
}

Output:

Enter a number: 1009
Reversed Number: 9001

The concept of reversing a number in java is useful in checking palindrome numbers.