return null;
        }

        @Override
        public int size() {
          return 0;
        }

        @Override
        public Iterator<Object> iterator() {
          return ImmutableSet.of().iterator();
        }
      };

  /** Queue that discards all elements. */
  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value

    assertFalse(CharSource.concat(c1, c2, c3).isEmpty());

    CharSource emptyConcat = CharSource.concat(CharSource.empty(), CharSource.empty());
    assertTrue(emptyConcat.isEmpty());
  }

  public void testConcat_infiniteIterable() throws IOException {
    CharSource source = CharSource.wrap("abcd");

  }

  @Override
  ImmutableSet<Entry<K, V>> createEntrySet() {
    class EntrySet extends ImmutableMapEntrySet<K, V> {
      @Override
      public UnmodifiableIterator<Entry<K, V>> iterator() {
        return asList().iterator();
      }

      @Override
      public Spliterator<Entry<K, V>> spliterator() {
        return asList().spliterator();
      }

      @Override

     *            初期容量
     */
    public CaseInsensitiveSet(final int initialCapacity) {
        map = new CaseInsensitiveMap<>(initialCapacity);
    }

    @Override
    public Iterator<String> iterator() {
        return map.keySet().iterator();
    }

    @Override
    public int size() {
        return map.size();
    }

    @Override
    public boolean isEmpty() {
        return map.isEmpty();
    }

    multimap.put("foo", 2);
    multimap.put("bar", 3);
    Collection<Entry<String, Integer>> entries = multimap.entries();
    Iterator<Entry<String, Integer>> iterator = entries.iterator();
    Entry<String, Integer> entrya = iterator.next();
    Entry<String, Integer> entryb = iterator.next();

    assertEquals(2, (int) multimap.get("foo").set(0, 4));
    assertFalse(multimap.containsEntry("foo", 2));

    }

    @Override
    public SortedMultiset<E> descendingMultiset() {
      return new StandardDescendingMultiset() {

        @Override
        Iterator<Entry<E>> entryIterator() {
          return backingMultiset.descendingMultiset().entrySet().iterator();
        }
      };
    }

    @Override
    public NavigableSet<E> elementSet() {
      return new StandardElementSet();
    }

    @Override

  private Dispatcher dispatcher;

  public void testPerThreadQueuedDispatcher() {
    dispatcher = Dispatcher.perThreadDispatchQueue();
    dispatcher.dispatch(1, integerSubscribers.iterator());

    assertThat(dispatchedSubscribers)
        .containsExactly(
            i1,
            i2,
            i3, // Integer subscribers are dispatched to first.
            s1,

        start = System.currentTimeMillis();
        for (Iterator<Map.Entry<String, Object>> i = hmap.entrySet().iterator(); i.hasNext();) {
            i.next();
        }
        System.out.println("HashMap iteration:" + (System.currentTimeMillis() - start));

        start = System.currentTimeMillis();
        for (Iterator<Map.Entry<String, Object>> i = amap.entrySet().iterator(); i.hasNext();) {
            i.next();
        }

  }

  /**
   * A sensible definition of {@link #pollFirst} in terms of the {@code iterator} method. If you
   * override {@link #iterator} you may wish to override {@link #pollFirst} to forward to this
   * implementation.
   */
  @CheckForNull
  protected E standardPollFirst() {
    return Iterators.pollNext(iterator());
  }

  @Override
  @CheckForNull
  public E pollLast() {

                @Override
                public Iterator<Entry<String, String>> iterator() {
                  return new Iterator<Entry<String, String>>() {

                    final Iterator<Entry<String, String>> iterator = map.entrySet().iterator();

                    @Override
                    public void remove() {
                      iterator.remove();
                    }