### Question:
EventFilter implements EventSink { public synchronized void lock(Object l) { Ensure.not_null(l); Ensure.is_null(m_lock); m_lock = l; } EventFilter(); synchronized void connect(EventSink s); synchronized EventSink connected(); EventFilterInfo info(); synchronized void lock(Object l); synchronized void unlock(Object l); synchronized boolean locked();  }### Answer:
@Test(expected = AssertionError.class) public void cannot_lock_with_null_lock() throws Exception { m_filter.lock(null); }