### Question:
LinkedTreeSet extends AbstractSet<E> { @Override public String toString() { return orderedElements.toString(); } LinkedTreeSet();  LinkedTreeSet(Collection<? extends E> other); @Override boolean add(E e); @Override boolean remove(Object o); @Override boolean removeAll(Collection<?> c); @Override boolean retainAll(Collection<?> c); @Override void clear(); @Override boolean contains(Object o); @Override Iterator<E> iterator(); @Override int hashCode(); @Override boolean equals(Object o); @Override int size(); @Override String toString();  }### Answer:
@Test void testToString() throws Exception { Set<Object> set = new LinkedTreeSet<>(); assertThat(set).hasToString("[]"); set.add("abc"); set.add("def"); assertThat(set).hasToString("[abc, def]"); }