import junit.framework.AssertionFailedError;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A helper for concurrency testing. One or more {@code TestThread} instances are instantiated in a
 * test with reference to the same "lock-like object", and then their interactions with that object
 * are choreographed via the various methods on this class.
 *

    assertThat(fooReference1).isNotSameInstanceAs(fooReference2);

    assertThat(mapCache.put(fooReference1, "bar")).isNull();
    assertThat(mapCache.get(fooReference1)).isEqualTo("bar"); // ensure first reference is cached
    assertThat(mapCache.remove(fooReference2)).isEqualTo("bar");
    assertThat(mapCache.get(fooReference1)).isNull();
  }

  @Override
  @CheckForNull
  V getIfCached(@CheckForNull Object key) {
    V value = super.getIfCached(key);
    if (value != null) {
      return value;
    }

    // Store a local reference to the cache entry. If the backing map is immutable, this,
    // in combination with immutable cache entries, will ensure a thread-safe cache.
    CacheEntry<K, V> entry;

     * been cleared by this point.
     */
    reference.clear();

    if (reference == frqReference) {
      /*
       * The client no longer has a reference to the FinalizableReferenceQueue. We can stop.
       */
      return false;
    }

    try {
      finalizeReferentMethod.invoke(reference);
    } catch (Throwable t) {

public final class ExecutionSequencer {

  private ExecutionSequencer() {}

  /** Creates a new instance. */
  public static ExecutionSequencer create() {
    return new ExecutionSequencer();
  }

  /** This reference acts as a pointer tracking the head of a linked list of ListenableFutures. */
  private final AtomicReference<ListenableFuture<@Nullable Void>> ref =
      new AtomicReference<>(immediateVoidFuture());

      try {
        return items[takeIndex];
      } finally {
        monitor.leave();
      }
    } else {
      return null;
    }
  }

  // this doc comment is overridden to remove the reference to collections
  // greater in size than Integer.MAX_VALUE
  /**
   * Returns the number of elements in this queue.
   *
   * @return the number of elements in this queue
   */
  @CanIgnoreReturnValue

 * apps use <a href="https://dagger.dev">Dagger</a>. For server code, common options include <a
 * href="https://github.com/google/guice/wiki/Motivation">Guice</a> and <a
 * href="https://docs.spring.io/spring-framework/docs/current/reference/html/core.html#beans-introduction">Spring</a>.
 * Frameworks typically offer a way to register multiple listeners independently and then request

   * immutable list is returned.)
   *
   * <p><b>Note:</b> in almost all circumstances, the returned {@link ImmutableList} retains a
   * strong reference to {@code this}, which may prevent the original list from being garbage
   * collected. If you want the original list to be eligible for garbage collection, you should
   * create and use a copy of the sub list (e.g., {@code

    public V getValue() {
      updateIndex();
      /*
       * If the entry has been removed from the map, we return null, even though that might not be a
       * valid value. That's the best we can do, short of holding a reference to the most recently
       * seen value. And while we *could* do that, we aren't required to: Map.Entry explicitly says
       * that behavior is undefined when the backing map is modified through another API. (It even

    assertEquals(1, removalListener.getCount());
  }

  // A simple type whose .toString() will return the same value each time, but without maintaining
  // a strong reference to that value.
  static class Key {
    private final int value;
    private WeakReference<String> toString;

    Key(int value) {
      this.value = value;
    }

    @Override
    public synchronized String toString() {