How to reverse a number in Java

Carvia Tech | September 07, 2019 | 1 min read | 1,108 views | Java Coding Challenges


Approach

  1. run a loop till num is greater than zero.

  2. extract last digit using modulus operator (digit = num % 10)

  3. add last digit with sum * 10 to make number reverse (sum = (sum * 10) + digit)

  4. divide num by to 10 so that we can get new last digit.

Reverse a number

Utils.java
public class Utils {

    public static long reverse(long num) {
        long sum = 0;
        while (num != 0) {
            long lastDigit = (num % 10);
            sum = (sum * 10) + lastDigit;
            num = num / 10;
        }
        return sum;
    }

}

Junit test for program

We can quickly write Junit Testcase to test our method for various scenarios, here we will cover only basic ones in one testcase.

UtilsTest,java
import org.junit.Test;

import static org.hamcrest.Matchers.equalTo;
import static org.junit.Assert.*;

public class UtilsTest {

    @Test
    public void reverse() {
        assertThat(Utils.reverse(201), equalTo(102L));
        assertThat(Utils.reverse(100), equalTo(1L));
        assertThat(Utils.reverse(1), equalTo(1L));
        assertThat(Utils.reverse(1564654), equalTo(4564651L));
    }
}

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