Reversing a Number using Java

FAQ's on Java Series by Hemant Yadav - 01

*Hello everyone, I am Starting a practice question series which can help you to enhance the logical thinking as well as to get hands on java *

Question 01- Reverse a Number

import java.util.Scanner;

public class ReverseNumber {

    public static void main(String[] args) {

        int a; //variable declaration

        a=123;  //variable initialization  hard code value

        Scanner sc=new Scanner(System.in);            
        System.out.println("Please enter a number");  //123     3
        int num=sc.nextInt();
        int rev=0;
        int rem;
        while(num>0)
        {
            rem=num%10; 
            rev=rev*10+rem;
            num=num/10;
        }

        System.out.println(rev);
    }

}

HOW IT WORKS ?

1. Input a number → User enters a number (e.g., 123).

2. Extract last digit → num % 10 gives the remainder (last digit). Example: 123 % 10 = 3.

3. Build reversed number → rev = rev * 10 + rem. At first,
   rev = 0 * 10 + 3 = 3.

4. Remove last digit → num = num / 10. Example: 123 / 10 = 12.

5. Repeat until num becomes 0.

6. Iteration steps for input 123:

Step 1 → rem = 3, rev = 3, num = 12

Step 2 → rem = 2, rev = 32, num = 1

Step 3 → rem = 1, rev = 321, num = 0

Output result → Print reversed number (321).