### Question:
SimpleList implements List<E> { @Override public boolean retainAll(Collection<?> c) { return retainImpl(this, c); } SimpleList(); private  SimpleList(Object[] data); static List<T> asList(T... arr); @Override int size(); @Override boolean isEmpty(); @Override boolean contains(Object o); @Override Iterator<E> iterator(); @Override Object[] toArray(); @Override T[] toArray(T[] a); @Override boolean add(E e); @Override boolean remove(Object o); @Override boolean containsAll(Collection<?> c); @Override boolean addAll(Collection<? extends E> c); @Override boolean addAll(int index, Collection<? extends E> c); @Override boolean removeAll(Collection<?> c); @Override boolean retainAll(Collection<?> c); void sort(Comparator<? super E> c); @Override void clear(); @Override E get(int index); @Override E set(int index, E element); @Override void add(int index, E element); @Override E remove(int index); @Override int indexOf(Object o); @Override int lastIndexOf(Object o); int lastIndexOfImpl(Object o, int from, int to); @Override ListIterator<E> listIterator(); @Override ListIterator<E> listIterator(int index); @Override List<E> subList(int fromIndex, int toIndex); @Override boolean equals(Object obj); @Override int hashCode(); @Override String toString(); static void ensureSize(List<?> list, int size);  }### Answer:
@Test(dataProvider = "lists") public void retainAll(List<Number> list) { list.add(3); list.add(3.3f); list.add(4L); list.add(4.4); list.retainAll(Collections.singleton(4L)); assertEquals(list.size(), 1); assertEquals(list.get(0), 4L); }