Java Program to Reverse a Number

Example 1: Reverse a Number using a while loop in Java

class Main {
  public static void main(String[] args) {

    int num = 1234, reversed = 0;
    
    System.out.println("Original Number: " + num);

    // run loop until num becomes 0
    while(num != 0) {
    
      // get last digit from num
      int digit = num % 10;
      reversed = reversed * 10 + digit;

      // remove the last digit from num
      num /= 10;
    }

    System.out.println("Reversed Number: " + reversed);
  }
}

Output

Reversed Number: 4321

In this program, while loop is used to reverse a number as given in the following steps:

• First, the remainder of the num divided by 10 is stored in the variable digit. Now, the digit contains the last digit of num, i.e. 4.
  digit is then added to the variable reversed after multiplying it by 10. Multiplication by 10 adds a new place in the reversed number. One-th place multiplied by 10 gives you tenth place, tenth gives you hundredth, and so on. In this case, reversed contains 0 * 10 + 4 = 4.
  num is then divided by 10 so that now it only contains the first three digits: 123.
• After second iteration, digit equals 3, reversed equals 4 * 10 + 3 = 43 and num = 12
• After third iteration, digit equals 2, reversed equals 43 * 10 + 2 = 432 and num = 1
• After fourth iteration, digit equals 1, reversed equals 432 * 10 + 1 = 4321 and num = 0
• Now num = 0, so the test expression num != 0 fails and while loop exits. reversed already contains the reversed number 4321.

Example 2: Reverse a number using a for loop in Java

class Main {
  public static void main(String[] args) {
    
    int num = 1234567, reversed = 0;

    for(;num != 0; num /= 10) {
      int digit = num % 10;
      reversed = reversed * 10 + digit;
    }

    System.out.println("Reversed Number: " + reversed);
  }
}

Output

Reversed Number: 7654321

In the above program, the while loop is replaced by a for loop where:

• no initialization expression is used
• test expression remains the same (num != 0)
• update/increment expression contains num /= 10.

So, after each iteration, the update expression runs which removes the last digit of num.

When the for loop exits, reversed will contain the reversed number.