### Question:
Slf4jTraceeBackend extends BackendBase { @Override public String get(String key) { if ((key != null) && traceeKeys.get().contains(key)) return MDC.get(key); else return null; } Slf4jTraceeBackend(ThreadLocal<Set<String>> traceeKeys); @Override boolean containsKey(String key); @Override int size(); @Override boolean isEmpty(); @Override String get(String key); @Override void put(String key, String value); @Override void remove(String key); @Override void clear(); @Override void putAll(Map<? extends String, ? extends String> entries); @Override Map<String, String> copyToMap();  }### Answer:
@Test public void returnNullIfKeyIsNull() { assertThat(OUT.get(null), is(nullValue())); }