assertTrue("An Object should be equal to itself.", collection.equals(collection));
  }

  // Comparisons to null *are* defined.
  @SuppressWarnings("UndefinedEquals")
  public void testEquals_null() {
    // noinspection ObjectEqualsNull
    assertFalse("An object should not be equal to null.", collection.equals(null));
  }

  // A collection should essentially never be equal to a non-collection.
  @SuppressWarnings("UndefinedEquals")

   * given collection in the order returned by the collection's iterator.
   *
   * @param collection the elements that the set should contain
   * @return a new {@code CompactLinkedHashSet} containing those elements (minus duplicates)
   */
  public static <E extends @Nullable Object> CompactLinkedHashSet<E> create(
      Collection<? extends E> collection) {

                  }

                  @Override
                  public SampleElements<Entry<String, Collection<String>>> samples() {
                    return new SampleElements<>(
                        mapEntry("a", (Collection<String>) ImmutableSortedSet.of("alex")),
                        mapEntry("b", (Collection<String>) ImmutableSortedSet.of("bob", "bagel")),

 * collection instance can only be one size at once.
 *
 * @author George van den Driessche
 */
@SuppressWarnings("rawtypes") // maybe avoidable if we rework the whole package?
@GwtCompatible
public enum CollectionSize implements Feature<Collection>, Comparable<CollectionSize> {
  /** Test an empty collection. */
  ZERO(0),
  /** Test a one-element collection. */
  ONE(1),
  /** Test a three-element collection. */
  SEVERAL(3),

 * collection instance can only be one size at once.
 *
 * @author George van den Driessche
 */
@SuppressWarnings("rawtypes") // maybe avoidable if we rework the whole package?
@GwtCompatible
public enum CollectionSize implements Feature<Collection>, Comparable<CollectionSize> {
  /** Test an empty collection. */
  ZERO(0),
  /** Test a one-element collection. */
  ONE(1),
  /** Test a three-element collection. */
  SEVERAL(3),

        UnsupportedOperationException.class,
        () -> collection.removeAll(MinimalCollection.of(e0())));
    expectUnchanged();
    assertTrue(collection.contains(e0()));
  }

  /*
   * AbstractCollection fails the removeAll(null) test when the subject
   * collection is empty, but we'd still like to test removeAll(null) when we
   * can. We split the test into empty and non-empty cases. This allows us to
   * suppress only the former.

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.testing.AbstractCollectionTester;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code size()} operations on a collection. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Kevin Bourrillion
 */
@GwtCompatible

  public interface CollectionsImplEnum {
    <E extends Comparable<E>> Collection<E> create(Collection<E> contents);

    String name();
  }

  public interface MapsImplEnum {
    <K extends Comparable<K>, V> Map<K, V> create(Map<K, V> contents);

    String name();
  }

  public interface InternerImplEnum {
    <E> Interner<E> create(Collection<E> contents);

    String name();
  }

  public void testRemove_nullPresent() {
    collection = getSubjectGenerator().create(createArrayWithNullElement());

    int initialSize = collection.size();
    assertTrue("remove(null) should return true", collection.remove(null));
    assertEquals(
        "remove(present) should decrease a collection's size by one.",
        initialSize - 1,
        collection.size());
    expectMissing((E) null);
  }

  public void testAddAll_supportedSomePresent() {
    assertTrue(
        "addAll(somePresent) should return true",
        collection.addAll(MinimalCollection.of(e3(), e0())));
    assertTrue("should contain " + e3(), collection.contains(e3()));
    assertTrue("should contain " + e0(), collection.contains(e0()));
  }

  @CollectionFeature.Require(absent = SUPPORTS_ADD)
  @CollectionSize.Require(absent = ZERO)