* one to preserve compatibility.
 */
@SuppressWarnings("JdkObsolete")
public abstract class ForwardingSortedSet<E extends @Nullable Object> extends ForwardingSet<E>
    implements SortedSet<E> {

  /** Constructor for use by subclasses. */
  protected ForwardingSortedSet() {}

  @Override
  protected abstract SortedSet<E> delegate();

  @Override

import org.jspecify.annotations.NullUnmarked;
import org.jspecify.annotations.Nullable;

/**
 * Tests for {@code ForwardingSortedSet}.
 *
 * @author Louis Wasserman
 */
@NullUnmarked
public class ForwardingSortedSetTest extends TestCase {
  static class StandardImplForwardingSortedSet<T> extends ForwardingSortedSet<T> {
    private final SortedSet<T> backingSortedSet;

    StandardImplForwardingSortedSet(SortedSet<T> backingSortedSet) {

  @Override
  SortedSet<Integer> createUnfiltered(Iterable<Integer> contents) {
    TreeSet<Integer> result = Sets.newTreeSet(contents);
    // we have to make the result not Navigable
    return new ForwardingSortedSet<Integer>() {
      @Override
      protected SortedSet<Integer> delegate() {
        return result;
      }
    };
  }

  @Override

 *
 * @author Louis Wasserman
 * @since 12.0
 */
@GwtIncompatible
public abstract class ForwardingNavigableSet<E extends @Nullable Object>
    extends ForwardingSortedSet<E> implements NavigableSet<E> {

  /** Constructor for use by subclasses. */
  protected ForwardingNavigableSet() {}

  @Override
  protected abstract NavigableSet<E> delegate();

  @Override

        public boolean apply(Entry<String, String> entry) {
          return entry.getKey() != null && entry.getValue() != null;
        }
      };

  private static class NonNavigableSortedSet extends ForwardingSortedSet<String> {

    private final SortedSet<String> delegate = Sets.newTreeSet(Ordering.natural());

    @Override
    protected SortedSet<String> delegate() {
      return delegate;
    }
  }

        public boolean apply(Entry<String, String> entry) {
          return entry.getKey() != null && entry.getValue() != null;
        }
      };

  private static class NonNavigableSortedSet extends ForwardingSortedSet<String> {

    private final SortedSet<String> delegate = Sets.newTreeSet(Ordering.natural());

    @Override
    protected SortedSet<String> delegate() {
      return delegate;
    }
  }

    assertThat(map.entrySet()).isEmpty();
    assertTrue(map.isEmpty());
    assertThat(map.get("five")).isNull();
  }

  private static class NonNavigableSortedSet extends ForwardingSortedSet<String> {
    private final SortedSet<String> delegate = Sets.newTreeSet();

    @Override
    protected SortedSet<String> delegate() {
      return delegate;
    }
  }

      return set;
    }
    return new UnmodifiableNavigableSet<>(set);
  }

  static final class UnmodifiableNavigableSet<E extends @Nullable Object>
      extends ForwardingSortedSet<E> implements NavigableSet<E>, Serializable {
    private final NavigableSet<E> delegate;
    private final SortedSet<E> unmodifiableDelegate;

    UnmodifiableNavigableSet(NavigableSet<E> delegate) {

    assertThat(map.entrySet()).isEmpty();
    assertTrue(map.isEmpty());
    assertThat(map.get("five")).isNull();
  }

  private static class NonNavigableSortedSet extends ForwardingSortedSet<String> {
    private final SortedSet<String> delegate = Sets.newTreeSet();

    @Override
    protected SortedSet<String> delegate() {
      return delegate;
    }
  }

        throw new UnsupportedOperationException();
      }
    };
  }

  private static <E extends @Nullable Object> SortedSet<E> removeOnlySortedSet(SortedSet<E> set) {
    return new ForwardingSortedSet<E>() {
      @Override
      protected SortedSet<E> delegate() {
        return set;
      }

      @Override
      public boolean add(@ParametricNullness E element) {