return Collections.unmodifiableCollection(collection);
  }

  @Override
  public void clear() {
    // Clear each collection, to make previously returned collections empty.
    for (Collection<V> collection : map.values()) {
      collection.clear();
    }
    map.clear();
    totalSize = 0;
  }

  // Views

  /**
   * {@inheritDoc}
   *

import java.util.Collection;
import java.util.Map;

/**
 * Test {@link Multimap#asMap()} for an arbitrary multimap with {@link MapInterfaceTest}.
 *
 * @author George van den Driessche
 * @author Jared Levy
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class AbstractMultimapAsMapImplementsMapTest
    extends MapInterfaceTest<String, Collection<Integer>> {

      checkNotNull(elements);
      if (elements instanceof Collection) {
        Collection<?> collection = (Collection<?>) elements;
        getReadyToExpandTo(size + collection.size());
        if (collection instanceof ImmutableCollection) {
          ImmutableCollection<?> immutableCollection = (ImmutableCollection<?>) collection;
          size = immutableCollection.copyIntoArray(contents, size);

    assertTrue(copy instanceof ImmutableMultiset);
    return copy;
  }

  @CanIgnoreReturnValue
  @GwtIncompatible // SerializableTester
  static <E> Collection<E> reserializeAndAssertElementsEqual(Collection<E> original) {
    Collection<E> copy = reserialize(original);
    assertTrue(Iterables.elementsEqual(original, copy));
    assertTrue(copy instanceof ImmutableCollection);
    return copy;
  }

  /**
   * For documenting collections that support no optional features, such as {@link
   * java.util.Collections#emptySet}
   */
  NONE;

  private final Set<Feature<? super Collection>> implied;

  CollectionFeature(Feature<? super Collection>... implied) {
    this.implied = Helpers.copyToSet(implied);
  }

  @Override
  public Set<Feature<? super Collection>> getImpliedFeatures() {
    return implied;

import org.junit.Ignore;

/**
 * Base class for multiset collection tests.
 *
 * @author Jared Levy
 */
@GwtCompatible
@Ignore // Affects only Android test runner, which respects JUnit 4 annotations on JUnit 3 tests.
public class AbstractMultisetTester<E> extends AbstractCollectionTester<E> {
  protected final Multiset<E> getMultiset() {
    return (Multiset<E>) collection;
  }

  protected void initThreeCopies() {

  public ImmutableMultimapAsMapImplementsMapTest() {
    super(false, false, false);
  }

  @Override
  protected Map<String, Collection<Integer>> makeEmptyMap() {
    return ImmutableMultimap.<String, Integer>of().asMap();
  }

  @Override
  protected Map<String, Collection<Integer>> makePopulatedMap() {
    Multimap<String, Integer> delegate = HashMultimap.create();
    populate(delegate);

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import java.util.Collection;
import java.util.Collections;
import java.util.Set;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * GWT implementation of {@link ImmutableSet} that forwards to another {@code Set} implementation.
 *

    }

    @Override
    public Collection<V> values() {
      assertTrue(Thread.holdsLock(mutex));
      /* TODO: verify that the Collection is also synchronized? */
      return super.values();
    }

    @Override
    public Set<Entry<K, V>> entries() {
      assertTrue(Thread.holdsLock(mutex));
      /* TODO: verify that the Collection is also synchronized? */
      return super.entries();
    }

  @CollectionSize.Require(ZERO)
  public void testIsEmpty_yes() {
    assertTrue("isEmpty() should return true", collection.isEmpty());
  }

  @CollectionSize.Require(absent = ZERO)
  public void testIsEmpty_no() {
    assertFalse("isEmpty() should return false", collection.isEmpty());
  }