*
   * <p>The returned object is serializable if this object is serializable.
   *
   * @since 10.0
   */
  public final <S extends @Nullable T> Equivalence<Iterable<S>> pairwise() {
    // Ideally, the returned equivalence would support Iterable<? extends T>. However,
    // the need for this is so rare that it's not worth making callers deal with the ugly wildcard.
    return new PairwiseEquivalence<>(this);
  }

        .inOrder();
  }

  public void testToMapWithNullKeys() {
    Iterable<@Nullable String> strings = asList("one", null, "three");
    assertThrows(
        NullPointerException.class,
        () -> Maps.toMap((Iterable<String>) strings, Functions.constant("foo")));
  }

  public void testToMapWithNullValues() {
    Iterable<String> strings = ImmutableList.of("one", "two", "three");

      checkEmpty(cache);
    }
  }

  /* ---------------- Local utilities -------------- */

  /** Most of the tests in this class run against every one of these caches. */
  private Iterable<LoadingCache<Object, Object>> caches() {
    // lots of different ways to configure a LoadingCache
    CacheBuilderFactory factory = cacheFactory();
    return Iterables.transform(
        factory.buildAllPermutations(),

      checkEmpty(cache);
    }
  }

  /* ---------------- Local utilities -------------- */

  /** Most of the tests in this class run against every one of these caches. */
  private Iterable<LoadingCache<Object, Object>> caches() {
    // lots of different ways to configure a LoadingCache
    CacheBuilderFactory factory = cacheFactory();
    return Iterables.transform(
        factory.buildAllPermutations(),

    return fromArrayWithExpectedSize(array, expectedSize);
  }

  /**
   * Returns an immutable set containing each of {@code elements}, minus duplicates, in the order
   * each appears first in the source iterable. This method iterates over {@code elements} only
   * once.
   *
   * <p><b>Performance note:</b> This method will sometimes recognize that the actual copy operation

      synchronized (mutex) {
        return delegate().put(key, value);
      }
    }

    @Override
    public boolean putAll(@ParametricNullness K key, Iterable<? extends V> values) {
      synchronized (mutex) {
        return delegate().putAll(key, values);
      }
    }

    @Override
    public boolean putAll(Multimap<? extends K, ? extends V> multimap) {

      synchronized (mutex) {
        return delegate().put(key, value);
      }
    }

    @Override
    public boolean putAll(@ParametricNullness K key, Iterable<? extends V> values) {
      synchronized (mutex) {
        return delegate().putAll(key, values);
      }
    }

    @Override
    public boolean putAll(Multimap<? extends K, ? extends V> multimap) {

          @Override
          public Object load(Object key) throws Exception {
            return new Object();
          }

          @Override
          public Map<Object, Object> loadAll(Iterable<?> keys) throws Exception {
            Map<Object, Object> result = new HashMap<>();
            for (Object key : keys) {
              Object value = new Object();
              result.put(key, value);

    assertTrue(emptyConcat.isEmpty());
  }

  public void testConcat_infiniteIterable() throws IOException {
    CharSource source = CharSource.wrap("abcd");
    Iterable<CharSource> cycle = Iterables.cycle(ImmutableList.of(source));
    CharSource concatenated = CharSource.concat(cycle);

    String expected = "abcdabcd";

          @Override
          public Object load(Object key) throws Exception {
            return new Object();
          }

          @Override
          public Map<Object, Object> loadAll(Iterable<?> keys) throws Exception {
            Map<Object, Object> result = new HashMap<>();
            for (Object key : keys) {
              Object value = new Object();
              result.put(key, value);