How to Reverse a Number in Java

The challenge

Given a number, write a function to output its reverse digits. (e.g. given 123 the answer is 321)

Numbers should preserve their sign; i.e. a negative number should still be negative when reversed.

Examples

 123 ->  321
-456 -> -654
1000 ->    1

The solution in Java code

Option 1:

public class ReverseNumber {

  public static int reverse(int number) {
    int result = 0;
    while(number != 0) {
      result = result * 10 + number % 10;
      number = number / 10;
    }
    return result;
  }

}

Option 2:

public class ReverseNumber {

  public static int reverse(int n) {
    return Integer.valueOf(new StringBuilder(""+n)
                              .reverse()
                              .toString()
                              .replace("-","")
            ) * Integer.signum(n);
  }

}

Option 3:

public class ReverseNumber {

  public static int reverse(int number) {
    
    String temp = "";
    
    if(number < 0){
      temp += "-";
      number = Math.abs(number);
    }
    String num = "" + number;
    
    for(int i = num.length()-1; i >= 0; i--){
      temp += num.charAt(i);
    }
    
    return Integer.parseInt(temp);
  }

}

Test cases to validate our solution

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class ReverseNumberTest {

	@Test
	public void reverse() throws Exception {
		assertEquals( 321, ReverseNumber.reverse( 123 ) );
		assertEquals( -321, ReverseNumber.reverse( -123 ) );
		assertEquals( 1, ReverseNumber.reverse( 1000 ) );
		assertEquals( 4321234, ReverseNumber.reverse( 4321234 ) );
		assertEquals( 5, ReverseNumber.reverse( 5 ) );
		assertEquals( 89898989, ReverseNumber.reverse( 98989898 ) );
	}

}