## 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));
}

}