How to Reverse a Number in Java

In this section, we will learn how to reverse a number in Java using while loop, for loop and recursion.

To reverse a number, follow the steps given below:

• First, we find the remainder of the given number by using the modulo (%) operator.
• Multiply the variable reverse by 10 and add the remainder into it.
• Divide the number by 10.

Repeat the above steps until the number becomes 0.

There are three ways to reverse a number in Java:

• Reverse a number using while loop
• Reverse a number using for loop
• Reverse a number using recursion

Let's apply the above steps in an example.

Example

Suppose, we want to reverse the number 1234.

In this example, we have taken three variables named number (the number to be reversed), remainder (stores the remainder), reverse (stores the reverse number) initialized 0.

Iteration 1:

Now the value of the number and reverse variable is 123 and 4, respectively.

Iteration 2:

Now the value of the number and reverse variable is 12 and 43, respectively.

Iteration 3:

Now the value of the number and reverse variable is 1 and 432, respectively.

Iteration 4:

Now the variable number become 0. Hence, we get the reverse number 4321.

Let's implement the above logic in a Java program.

Reverse a number using while loop

ReverseNumberExample1.java

Output

The reverse of the given number is: 456789

Reverse a number using for loop

In the following program, we have replaced the while loop by a for loop. It also removes the last digit of the number, after each iteration. When the condition, number!=0 becomes false, the loop exits and we get the reversed number.

ReverseNumberExample2.java

Output

The reverse of the given number is: 654321

In the above program, we can also write the for loop as follows:

Reverse a number using recursion

ReverseNumberExample3.java

Output 1:

Enter the number that you want to reverse: 9
The reverse of the given number is: 9

Output 2:

Enter the number that you want to reverse: 7654123
The reverse of the given number is: 3214567

The following program reverses both numbers, positive and negative. When we enter a number, it first checks the number is positive or negative. If the number is negative, it converts the number into positive by multiplying -1. After that, it performs the same steps (as we have performed in the above programs) to reverse a number. At last, again it checks the number is negative or positive. To make the number negative, it again multiplies the reverse number by -1.

ReverseNumberExample4.java

Output 1:

Enter the number that you want to reverse: -98765
The reverse of the given number is: -56789

Output 2:

Enter the number that you want to reverse: 321987
The reverse of the given number is: 789123