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3 Ways of Reverse a Number in Java

In this Post, we will learn different ways to reverse a number in java.

This is frequently asked question in automation testing  interviews.

Logic 1: Reverse a Number in java using while loop

package demopkg;

import java.util.Scanner;

public class RevNumberLogic1 {
    
    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter a number: ");
        int num = sc.nextInt();
        
        int rev =0;
        
        while(num!=0) {
            
            rev = rev*10 + num%10;
            num = num/10;
            
        }
        
        System.out.println("Reverse of number: " + rev);
        
    }

}

Code explanation:  


Logic 2: Using StringBuffer class

package demopkg;

import java.util.Scanner;

public class RevNumberLogic2 {
    
    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter a number: ");
        int num = sc.nextInt();
        
        //Using StringBuffer
        StringBuffer rev;
        StringBuffer sb = new StringBuffer(String.valueOf(num));
        rev=sb.reverse();
        
        System.out.println(rev);
        
        
    }

}

Code explanation:  


Logic 3: Using StringBuilder class

package demopkg;

import java.util.Scanner;

public class RevNumberLogic3 {
    
    public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.println("Enter a number: ");
    int num = sc.nextInt();
    
    //Using StringBuilder
    
    StringBuilder sb = new StringBuilder();
    sb.append(num);
    StringBuilder rev = sb.reverse();
    System.out.println(rev);
    
    }
}

Code explanation:  


Reverse A Number in Java Using Collections:

The reverse() method of the collections class we can reverse the order of specified list.

import java.util.*;  
public class CollectionsReverseExm2 {  
    public static void main(String[] args) {          
        //Create a list of Integer  
            List<Integer>  m = new ArrayList<Integer>();  
            m.add(10);  
            m.add(20);  
            m.add(30);  
            m.add(40);  
            System.out.println("Original List:- " + m);        
            Collections.reverse(m);  
            System.out.println("Modified List:- " + m);  
          }  
}

Output:

Original List:- [1, 2, 3, 4] Modified List:- [4, 3, 2, 1]

 

Reverse a Number in Java using recursion:

// Java program to reverse
// an integer recursively
class RevNumberUsingRecusion {

    // Recursive function to print
    // the number in reversed form
    public static void Reverse(int num)
    {

        // base condition to end recursive calls
        if (num < 10) {
            System.out.println(num);
            return;
        }

        else {

            // print the unit digit of the given number
            System.out.print(num % 10);

            // calling function for remaining number other
            // than unit digit
            Reverse(num / 10);
        }
    }

    // Main Method code
    public static void main(String args[])
    {
        // number to be reversed
        int num = 7654;

        System.out.print("Reversed Number: ");

        // calling recursive function
        // to print the number in
        // reversed form
        Reverse(num);
    }
}

Output:

Reversed Number: 4567


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I hope you have enjoyed this post. Happy Learning 🙂

Must Read:

Swap Two Numbers in Java

Reverse String in Java

 

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