* @param <V> the most general type of values this listener can listen for; for example {@code
 *     Object} if any key is acceptable
 * @author Charles Fry
 * @since 10.0
 */
@GwtCompatible
public interface RemovalListener<K, V> {
  /**
   * Notifies the listener that a removal occurred at some point in the past.
   *
   * <p>This does not always signify that the key is now absent from the cache, as it may have
   * already been re-added.

    List<RemovalNotification<Integer, Integer>> notifications = new ArrayList<>();
    Cache<Integer, Integer> cache =
        CacheBuilder.newBuilder()
            .removalListener(
                new RemovalListener<Integer, Integer>() {
                  @Override
                  public void onRemoval(RemovalNotification<Integer, Integer> notification) {
                    notifications.add(notification);

import org.jspecify.annotations.NullUnmarked;

/**
 * Utility {@link RemovalListener} implementations intended for use in testing.
 *
 * @author mike nonemacher
 */
@GwtCompatible
@NullUnmarked
final class TestingRemovalListeners {

  /** Returns a new no-op {@code RemovalListener}. */
  static <K, V> NullRemovalListener<K, V> nullRemovalListener() {
    return new NullRemovalListener<>();
  }

  }

  public void testGet() {
    Object computed = new Object();
    LoadingCache<Object, Object> cache =
        CacheBuilder.newBuilder()
            .maximumSize(0)
            .removalListener(listener)
            .build(constantLoader(computed));

    Object key = new Object();
    assertThat(cache.getUnchecked(key)).isSameInstanceAs(computed);

  }

  public void testGet() {
    Object computed = new Object();
    LoadingCache<Object, Object> cache =
        CacheBuilder.newBuilder()
            .maximumSize(0)
            .removalListener(listener)
            .build(constantLoader(computed));

    Object key = new Object();
    assertThat(cache.getUnchecked(key)).isSameInstanceAs(computed);

  private RemovalListeners() {}

  /**
   * Returns a {@code RemovalListener} which processes all eviction notifications using {@code
   * executor}.
   *
   * @param listener the backing listener
   * @param executor the executor with which removal notifications are asynchronously executed
   */
  public static <K, V> RemovalListener<K, V> asynchronous(
      RemovalListener<K, V> listener, Executor executor) {
    checkNotNull(listener);

import org.jspecify.annotations.NullUnmarked;

/**
 * Utility {@link RemovalListener} implementations intended for use in testing.
 *
 * @author mike nonemacher
 */
@GwtCompatible
@NullUnmarked
final class TestingRemovalListeners {

  /** Returns a new no-op {@code RemovalListener}. */
  static <K, V> NullRemovalListener<K, V> nullRemovalListener() {
    return new NullRemovalListener<>();
  }