@Test
@DisplayName("Doubling of characters")
void doubleChar()
{

    assertNull(StringUtils.doubleChar(null, 'a'));
    assertEquals("", StringUtils.doubleChar("", 'a'));
    assertEquals("b", StringUtils.doubleChar("b", 'a'));
    assertEquals("aa", StringUtils.doubleChar("a", 'a'));
    assertEquals("aaaa", StringUtils.doubleChar("aa", 'a'));
    assertEquals("baabaa", StringUtils.doubleChar("baba", 'a'));
    assertEquals("aabaab", StringUtils.doubleChar("abab", 'a'));
}