* completing the returned {@code Future}.)
   *
   * <p>The returned {@code Future} attempts to keep its cancellation state in sync with that of the
   * input future and that of the future returned by the chain function. That is, if the returned
   * {@code Future} is cancelled, it will attempt to cancel the other two, and if either of the
   * other two is cancelled, the returned {@code Future} will receive a callback in which it will

own children.

  `How should I know?' said Alice, surprised at her own courage.
`It's no business of MINE.'

  The Queen turned crimson with fury, and, after glaring at her
for a moment like a wild beast, screamed `Off with her head!
Off--'

  `Nonsense!' said Alice, very loudly and decidedly, and the
Queen was silent.

   * already been determined.
   */
  public abstract <T extends @Nullable Object> ComparisonChain compare(
      @ParametricNullness T left, @ParametricNullness T right, Comparator<T> comparator);

  /**
   * Compares two {@code int} values as specified by {@link Ints#compare}, <i>if</i> the result of
   * this comparison chain has not already been determined.
   */

  public void testFutureGetThrowsRuntimeException() throws Exception {
    BadFuture badInput = new BadFuture(Futures.immediateFuture(20));
    ListenableFuture<String> chain = buildChainingFuture(badInput);
    ExecutionException e = assertThrows(ExecutionException.class, () -> chain.get());
    assertSame(RuntimeException.class, e.getCause().getClass());
  }

  /** Proxy to throw a {@link RuntimeException} out of the {@link #get()} method. */

   *
   * <p>The default implementation of this method in {@code AbstractFuture} does nothing. This is
   * intended for very lightweight cleanup work, for example, timing statistics or clearing fields.
   * If your task does anything heavier consider, just using a listener with an executor.
   *
   * @since 20.0
   */
  @ForOverride
  protected void afterDone() {}

 * actions taken by the java garbage collection system:
 *
 * <ul>
 *   <li>invoking the {@code finalize} methods of unreachable objects
 *   <li>clearing weak references to unreachable referents
 *   <li>enqueuing weak references to unreachable referents in their reference queue
 * </ul>
 *

 * ListenableFuture}</a>.
 *
 * <p>This class is GWT-compatible.
 *
 * <h3>Purpose</h3>
 *
 * <p>The main purpose of {@code ListenableFuture} is to help you chain together a graph of
 * asynchronous operations. You can chain them together manually with calls to methods like {@link
 * Futures#transform(ListenableFuture, com.google.common.base.Function, Executor) Futures.transform}

  @CheckForNull
  ValueReference<K, V> getValueReference();

  /** Sets the value reference for this entry. */
  void setValueReference(ValueReference<K, V> valueReference);

  /** Returns the next entry in the chain. */
  @CheckForNull
  ReferenceEntry<K, V> getNext();

  /** Returns the entry's hash. */
  int getHash();

  /** Returns the key for this entry. */
  @CheckForNull
  K getKey();

  /*

      Contribution(s) alone or by combination of their Contribution(s)
      with the Work to which such Contribution(s) was submitted. If You
      institute patent litigation against any entity (including a
      cross-claim or counterclaim in a lawsuit) alleging that the Work
      or a Contribution incorporated within the Work constitutes direct
      or contributory patent infringement, then any patent licenses

 * reference. It allows you to represent "a {@code T} that must be present" and a "a {@code T} that
 * might be absent" as two distinct types in your program, which can aid clarity.
 *
 * <p>Some uses of this class include
 *
 * <ul>
 *   <li>As a method return type, as an alternative to returning {@code null} to indicate that no
 *       value was available