import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

/**
 * Tests for {@link Functions}.
 *
 * @author Mike Bostock
 * @author Vlad Patryshev
 */
@GwtCompatible
@NullMarked
public class FunctionsTest extends TestCase {

  public void testIdentity_same() {
    Function<@Nullable String, @Nullable String> identity = Functions.identity();
    assertNull(identity.apply(null));

 * @author Ben Yu
 * @since 12.0
 */
/*
 * A nullable bound would let users create a TypeParameter instance for a parameter with a nullable
 * bound. However, it would also let them create `new TypeParameter<@Nullable T>() {}`, which
 * wouldn't behave as users might expect. Additionally, it's not clear how the TypeToken API could
 * support even a "normal" `TypeParameter<T>` when `<T>` has a nullable bound. (See the discussion

import java.util.logging.Level;
import org.jspecify.annotations.Nullable;

/** Implementation of {@link Futures#immediateFuture}. */
@GwtCompatible
// TODO(cpovirk): Make this final (but that may break Mockito spy calls).
class ImmediateFuture<V extends @Nullable Object> implements ListenableFuture<V> {
  static final ListenableFuture<?> NULL = new ImmediateFuture<@Nullable Object>(null);

  private static final Iterable<@Nullable Integer> iterableNull = Arrays.asList((Integer) null);
  private static final Iterable<@Nullable Integer> iterableNullNull =
      Arrays.asList((Integer) null, null);
  private static final Iterable<@Nullable Integer> iterableNull1 = Arrays.asList(null, 1);
  private static final Iterable<@Nullable Integer> iterable1Null = Arrays.asList(1, null);

    return (Collector) TO_OPTIONAL;
  }

  private static final Object NULL_PLACEHOLDER = new Object();

  private static final Collector<@Nullable Object, ?, @Nullable Object> ONLY_ELEMENT =
      Collector.<@Nullable Object, ToOptionalState, @Nullable Object>of(
          ToOptionalState::new,
          (state, o) -> state.add((o == null) ? NULL_PLACEHOLDER : o),
          ToOptionalState::combine,
          state -> {

    }
  }

  @Override
  public @Nullable Entry<E> firstEntry() {
    return delegate().firstEntry();
  }

  /**
   * A sensible definition of {@link #firstEntry()} in terms of {@code entrySet().iterator()}.
   *
   * <p>If you override {@link #entrySet()}, you may wish to override {@link #firstEntry()} to
   * forward to this implementation.
   */
  protected @Nullable Entry<E> standardFirstEntry() {

  protected abstract ListeningExecutorService delegate();

  @Override
  public <T extends @Nullable Object> ListenableFuture<T> submit(Callable<T> task) {
    return delegate().submit(task);
  }

  @Override
  public ListenableFuture<?> submit(Runnable task) {
    return delegate().submit(task);
  }

  @Override
  public <T extends @Nullable Object> ListenableFuture<T> submit(
      Runnable task, @ParametricNullness T result) {

import java.util.Map.Entry;
import java.util.Set;
import org.jspecify.annotations.Nullable;

/**
 * Implementation of {@link Multimaps#filterKeys(SetMultimap, Predicate)}.
 *
 * @author Louis Wasserman
 */
@GwtCompatible
final class FilteredKeySetMultimap<K extends @Nullable Object, V extends @Nullable Object>
    extends FilteredKeyMultimap<K, V> implements FilteredSetMultimap<K, V> {

   */
  @CanIgnoreReturnValue // TODO(kak): consider removing this
  @Override
  protected final <T extends @Nullable Object> RunnableFuture<T> newTaskFor(Callable<T> callable) {
    return TrustedListenableFutureTask.create(callable);
  }

  @CanIgnoreReturnValue // TODO(kak): consider removing this
  @Override
  public ListenableFuture<?> submit(Runnable task) {
    return (ListenableFuture<?>) super.submit(task);

import java.util.Map;
import org.jspecify.annotations.Nullable;

/**
 * An object representing the differences between two maps.
 *
 * @author Kevin Bourrillion
 * @since 2.0
 */
@DoNotMock("Use Maps.difference")
@GwtCompatible
public interface MapDifference<K extends @Nullable Object, V extends @Nullable Object> {
  /**