public class DecimalToBinary {

    public static StringBuilder decimalToBinary(int number) {
        int num = 11;
        StringBuilder result = new StringBuilder();

        int i = 0;
        while (number > 0) {

            int remainder = number % 2;
            result.append(String.valueOf(remainder));
            number = number / 2;
            i++;
        }
        return result.reverse();
    }
}


import org.junit.Assert;
import org.junit.Test;

import static org.junit.Assert.*;

public class DecimalToBinaryTest {

    @Test // adnotacja
    public void shouldDecimalToBinary() {

        // given
        int number = 11;

        // when
       StringBuilder result = DecimalToBinary.decimalToBinary(number);

        // then
        Assert.assertEquals(1011, result);

    }
}