* appropriate primitive specialization such as {@code ToIntFunction}) instead whenever possible.
 * Otherwise, at least reduce <i>explicit</i> dependencies on this type by using lambda expressions
 * or method references instead of classes, leaving your code easier to migrate in the future.
 *
 * <p>See the Guava User Guide article on <a
 * href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code Function}</a>.
 *

[guava-snapshot-api-diffs]: https://guava.dev/releases/snapshot-jre/api/diffs/
[Guava Explained]: https://github.com/google/guava/wiki/Home
[Guava Beta Checker]: https://github.com/google/guava-beta-checker

<!-- References -->

[using Guava in your build]: https://github.com/google/guava/wiki/UseGuavaInYourBuild

/**
 * An immutable object that may contain a non-null reference to another object. Each instance of
 * this type either contains a non-null reference, or contains nothing (in which case we say that
 * the reference is "absent"); it is never said to "contain {@code null}".
 *
 * <p>A non-null {@code Optional<T>} reference can be used as a replacement for a nullable {@code T}

    return Platform.emptyToNull(string);
  }

  /**
   * Returns {@code true} if the given string is null or is the empty string.
   *
   * <p>Consider normalizing your string references with {@link #nullToEmpty}. If you do, you can
   * use {@link String#isEmpty()} instead of this method, and you won't need special null-safe forms

   * SoftReference} (by default, strong references are used). Softly-referenced objects will be
   * garbage-collected in a <i>globally</i> least-recently-used manner, in response to memory
   * demand.
   *
   * <p><b>Warning:</b> in most circumstances it is better to set a per-cache {@linkplain
   * #maximumSize(long) maximum size} instead of using soft references. You should only use this

        break;
      }
      try {
        Thread.sleep(10);
      } catch (InterruptedException e) {
        /* ignore */
      }
      try {
        // Fill up heap so soft references get cleared.
        filler = new byte[Math.max(filler.length, filler.length * 2)];
      } catch (OutOfMemoryError e) {
      }
    }

    CacheTesting.processPendingNotifications(cache);

        void executeAndCompare(ListIterator<E> reference, Iterator<E> target) {
          internalExecuteAndCompare(reference, target, NEXT_METHOD);
        }
      };
  Stimulus<E, Iterator<E>> remove =
      new Stimulus<E, Iterator<E>>("remove") {
        @Override
        void executeAndCompare(ListIterator<E> reference, Iterator<E> target) {
          internalExecuteAndCompare(reference, target, REMOVE_METHOD);
        }
      };

    if (threadStarted) {
      return;
    }

    Reference<?> reference;
    while ((reference = queue.poll()) != null) {
      /*
       * This is for the benefit of phantom references. Weak and soft references will have already
       * been cleared by this point.
       */
      reference.clear();
      try {
        ((FinalizableReference) reference).finalizeReferent();
      } catch (Throwable t) {

    if (threadStarted) {
      return;
    }

    Reference<?> reference;
    while ((reference = queue.poll()) != null) {
      /*
       * This is for the benefit of phantom references. Weak and soft references will have already
       * been cleared by this point.
       */
      reference.clear();
      try {
        ((FinalizableReference) reference).finalizeReferent();
      } catch (Throwable t) {

 * <i>similar</i>.
 *
 * <h3>Nullability</h3>
 *
 * <p>A converter always converts {@code null} to {@code null} and non-null references to non-null
 * references. It would not make sense to consider {@code null} and a non-null reference to be
 * "different representations of the same information", since one is distinguishable from