* elements of each iterator in {@code inputs}. The input iterators are not polled until
   * necessary.
   *
   * <p>The returned iterator supports {@code remove()} when the corresponding input iterator
   * supports it. The methods of the returned iterator may throw {@code NullPointerException} if any
   * of the input iterators is null.
   */

The [bytes] package adds several functions that work with iterators:
- [Lines] returns an iterator over the
  newline-terminated lines in the byte slice s.
- [SplitSeq] returns an iterator over
  all substrings of s separated by sep.
- [SplitAfterSeq] returns an iterator
  over substrings of s split after each instance of sep.
- [FieldsSeq] returns an iterator over
  substrings of s split around runs of whitespace characters,
  as defined by unicode.IsSpace.

  }

  public void testSize1() {
    Iterator<Integer> iterator = singleton(0).iterator();
    assertEquals(1, Iterators.size(iterator));
  }

  public void testSize_partiallyConsumed() {
    Iterator<Integer> iterator = asList(1, 2, 3, 4, 5).iterator();
    iterator.next();
    iterator.next();
    assertEquals(3, Iterators.size(iterator));
  }

  }

  public void testSize1() {
    Iterator<Integer> iterator = singleton(0).iterator();
    assertEquals(1, Iterators.size(iterator));
  }

  public void testSize_partiallyConsumed() {
    Iterator<Integer> iterator = asList(1, 2, 3, 4, 5).iterator();
    iterator.next();
    iterator.next();
    assertEquals(3, Iterators.size(iterator));
  }

    List<?> list = emptyList();
    Iterator<?> iterator = list.iterator();
    PeekingIterator<?> peekingIterator = Iterators.peekingIterator(iterator);

    assertThrows(NoSuchElementException.class, () -> peekingIterator.peek());
  }

  public void testPeekDoesntChangeIteration() {
    List<?> list = Lists.newArrayList("A", "B", "C");
    Iterator<?> iterator = list.iterator();

        new Iterable<V>() {
          private boolean calledIteratorAlready = false;

          @Override
          public Iterator<V> iterator() {
            checkState(!calledIteratorAlready);
            calledIteratorAlready = true;
            return Iterators.forArray(v3());
          }
        };

    multimap().putAll(k3(), iterable);
  }

  @MapFeature.Require(SUPPORTS_PUT)

All `go/types` data structures that expose sequences using a pair of
methods such as `Len() int` and `At(int) T` now also methods that
return iterators, allowing you to simplify code such as this:

```go
params := fn.Type.(*types.Signature).Params()
for i := 0; i < params.Len(); i++ {
   use(params.At(i))
}
```

to this:

```go
for param := range fn.Signature().Params().Variables() {
   use(param)
}
```

        @Override
        protected Set<K> delegate() {
          return delegate.keySet();
        }

        @Override
        public Iterator<K> iterator() {
          return Iterators.unmodifiableIterator(delegate.keySet().iterator());
        }

        @Override
        public boolean removeAll(Collection<?> c) {
          return delegate.keySet().removeAll(c);
        }

        @Override

    List<?> list = emptyList();
    Iterator<?> iterator = list.iterator();
    PeekingIterator<?> peekingIterator = Iterators.peekingIterator(iterator);

    assertThrows(NoSuchElementException.class, () -> peekingIterator.peek());
  }

  public void testPeekDoesntChangeIteration() {
    List<?> list = Lists.newArrayList("A", "B", "C");
    Iterator<?> iterator = list.iterator();

      @Override
      public Iterator<T> iterator() {
        return Iterators.concat(
            /* lazily generate the iterators on each input only as needed */
            new AbstractIndexedListIterator<Iterator<? extends T>>(inputs.length) {
              @Override
              public Iterator<? extends T> get(int i) {
                return inputs[i].iterator();
              }
            });
      }