* @return the non-null reference that was validated
   * @throws NullPointerException if {@code reference} is null
   * @see Verify#verifyNotNull Verify.verifyNotNull()
   */
  @CanIgnoreReturnValue
  public static <T> T checkNotNull(@Nullable T reference) {
    if (reference == null) {
      throw new NullPointerException();
    }
    return reference;
  }

  /**

   * set is serializable.
   *
   * @param elements the elements that the set should contain
   * @return a new thread-safe set containing those elements (minus duplicates)
   * @throws NullPointerException if {@code elements} or any of its contents is null
   * @since 15.0
   */
  public static <E> Set<E> newConcurrentHashSet(Iterable<? extends E> elements) {
    Set<E> set = newConcurrentHashSet();

## Version 3.14.4

_2019-09-29_

 *  Fix: Cancel calls that fail due to unexpected exceptions. We had a bug where an enqueued call
    would never call back if it crashed with an unchecked throwable, such as a
    `NullPointerException` or `OutOfMemoryError`. We now call `Callback.onFailure()` with an
    `IOException` that reports the call as canceled. The triggering exception is still delivered to
    the thread's `UncaughtExceptionHandler`.

    assertEquals(2, (int) result.next());
    assertThrows(NullPointerException.class, () -> result.hasNext());
    assertThrows(NullPointerException.class, () -> result.next());
    // There is no way to get "through" to the 3.  Buh-bye
  }

  public void testConcatVarArgsContainingNull() {
    assertThrows(
        NullPointerException.class,
        () ->
            Iterators.concat(

    assertEquals(2, (int) result.next());
    assertThrows(NullPointerException.class, () -> result.hasNext());
    assertThrows(NullPointerException.class, () -> result.next());
    // There is no way to get "through" to the 3.  Buh-bye
  }

  public void testConcatVarArgsContainingNull() {
    assertThrows(
        NullPointerException.class,
        () ->
            Iterators.concat(

   * necessary.
   *
   * <p>The returned iterator supports {@code remove()} when the corresponding input iterator
   * supports it.
   *
   * @throws NullPointerException if any of the provided iterators is null
   */
  @SafeVarargs
  public static <T extends @Nullable Object> Iterator<T> concat(Iterator<? extends T>... inputs) {

   * @return {@code ImmutableListMultimap} mapping the result of evaluating the function {@code
   *     keyFunction} on each value in the input collection to that value
   * @throws NullPointerException if any element of {@code values} is {@code null}, or if {@code
   *     keyFunction} produces {@code null} for any key
   */
  public static <K, V> ImmutableListMultimap<K, V> index(

   * @return {@code ImmutableListMultimap} mapping the result of evaluating the function {@code
   *     keyFunction} on each value in the input collection to that value
   * @throws NullPointerException if any element of {@code values} is {@code null}, or if {@code
   *     keyFunction} produces {@code null} for any key
   */
  public static <K, V> ImmutableListMultimap<K, V> index(