@ElementTypesAreNonnullByDefault
public abstract class AbstractCollectionTester<E extends @Nullable Object>
    extends AbstractContainerTester<Collection<E>, E> {

  // TODO: replace this with an accessor.
  protected Collection<E> collection;

  @Override
  protected Collection<E> actualContents() {
    return collection;
  }

  // TODO: dispose of this once collection is encapsulated.

@ElementTypesAreNonnullByDefault
public abstract class AbstractCollectionTester<E extends @Nullable Object>
    extends AbstractContainerTester<Collection<E>, E> {

  // TODO: replace this with an accessor.
  protected Collection<E> collection;

  @Override
  protected Collection<E> actualContents() {
    return collection;
  }

  // TODO: dispose of this once collection is encapsulated.