}
  }

  public void testNullFriendlyTransform() {
    Iterator<@Nullable Integer> input = Arrays.<@Nullable Integer>asList(1, 2, null, 3).iterator();
    Iterator<String> result =
        Iterators.transform(
            input,
            new Function<@Nullable Integer, String>() {
              @Override
              public String apply(@Nullable Integer from) {
                return String.valueOf(from);
              }

   *
   * @since 2.0
   */
  public static class Builder<K, V> extends ImmutableMap.Builder<K, V> {
    private transient @Nullable Object[] keys;
    private transient @Nullable Object[] values;
    private final Comparator<? super K> comparator;

    /**
     * Creates a new builder. The returned builder is equivalent to the builder generated by {@link

import java.util.stream.Collector;
import java.util.stream.Stream;
import junit.framework.Test;
import junit.framework.TestCase;
import junit.framework.TestSuite;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Unit test for {@code Sets}.
 *
 * @author Kevin Bourrillion
 * @author Jared Levy
 */
@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
public class SetsTest extends TestCase {

import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Static convenience methods that help a method or constructor check whether it was invoked
 * correctly (that is, whether its <i>preconditions</i> were met).
 *

  }

  static <R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>
      Table<R, C, V> table(Table<R, C, V> table, @CheckForNull Object mutex) {
    return new SynchronizedTable<>(table, mutex);
  }

  static final class SynchronizedTable<
          R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>

  public void testToStringImplWithNullKeys() throws Exception {
    Map<@Nullable String, String> hashmap = Maps.newHashMap();
    hashmap.put("foo", "bar");
    hashmap.put(null, "baz");

    assertEquals(hashmap.toString(), Maps.toStringImpl(hashmap));
  }

  public void testToStringImplWithNullValues() throws Exception {
    Map<String, @Nullable String> hashmap = Maps.newHashMap();
    hashmap.put("foo", "bar");

  public void testToStringImplWithNullKeys() throws Exception {
    Map<@Nullable String, String> hashmap = Maps.newHashMap();
    hashmap.put("foo", "bar");
    hashmap.put(null, "baz");

    assertEquals(hashmap.toString(), Maps.toStringImpl(hashmap));
  }

  public void testToStringImplWithNullValues() throws Exception {
    Map<String, @Nullable String> hashmap = Maps.newHashMap();
    hashmap.put("foo", "bar");

   */
  interface Trusted<V extends @Nullable Object> extends ListenableFuture<V> {}

  /**
   * A less abstract subclass of AbstractFuture. This can be used to optimize setFuture by ensuring
   * that {@link #get} calls exactly the implementation of {@link AbstractFuture#get}.
   */
  abstract static class TrustedFuture<V extends @Nullable Object> extends AbstractFuture<V>
      implements Trusted<V> {

import java.util.function.Consumer;
import java.util.function.Function;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A {@link NavigableMap} whose contents will never change, with many other important properties
 * detailed at {@link ImmutableCollection}.
 *

   * @since 12.0
   */
  public static <E extends @Nullable Object> NavigableSet<E> unmodifiableNavigableSet(
      NavigableSet<E> set) {
    if (set instanceof ImmutableCollection || set instanceof UnmodifiableNavigableSet) {
      return set;
    }
    return new UnmodifiableNavigableSet<>(set);
  }

  static final class UnmodifiableNavigableSet<E extends @Nullable Object>