### Question:
LinkedTreeSet extends AbstractSet<E> { @Override public int size() { return elements.size(); } 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 testSize() throws Exception { Set<Object> set = new LinkedTreeSet<>(); assertThat(set).isEmpty(); assertThat(set).hasSize(0); set.add("abc"); set.add("xyz"); assertThat(set).isNotEmpty(); assertThat(set).hasSize(2); }