### Question:
DigitsOnlyCleaner implements Cleaner { public String clean(String value) { char[] tmp = new char[value.length()]; int pos = 0; for (int ix = 0; ix < tmp.length; ix++) { char ch = value.charAt(ix); if (ch >= '0' && ch <= '9') tmp[pos++] = ch; } if (pos == 0) return null; return new String(tmp, 0, pos); }  String clean(String value);  }### Answer:
@Test public void testEmpty() { assertTrue(cleaner.clean("") == null); }