Square Every Digit in Java

The challenge

You need to square every digit of a number and concatenate them.

For example, if we run 9119 through the function, 811181 will come out, because 9² is 81 and 1² is 1.

Note: The function accepts an integer and returns an integer.

The solution in Java code

Option 1:

public class SquareDigit {

  public int squareDigits(int n) {
    String result = ""; 
    
    while (n != 0) {
      int digit = n % 10 ;
      result = digit*digit + result ;
      n /= 10 ;
    }
    
    return Integer.parseInt(result) ;
  }

}

Option 2:

import java.util.stream.Collectors;

public class SquareDigit {

    public int squareDigits(int n) {
        return Integer.parseInt(String.valueOf(n)
                  .chars()
                  .map(i -> Integer.parseInt(String.valueOf((char) i)))
                  .map(i -> i * i)
                  .mapToObj(String::valueOf)
                  .collect(Collectors.joining("")));
    }

}

Option 3:

public class SquareDigit {
  private static final int BASE = 10;
  
  public int squareDigits(int n) {
    if (n < BASE) {
      return n * n;
    }
    int digit = n % BASE;
    int squaredDigit = digit * digit;
    return squaredDigit + (squaredDigit < BASE ? BASE : BASE * BASE) * squareDigits(n / BASE);
  }
}

Test cases to validate our solution

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class SquareDigitTest {
    @Test
    public void test() {
      assertEquals(811181, new SquareDigit().squareDigits(9119));
    }
    
}