### EventListener.Factory

In the preceding example we used a field, `callStartNanos`, to track the elapsed time of each event. This is handy, but it won’t work if multiple calls are executing concurrently. To accommodate this, use a `Factory` to create a new `EventListener` instance for each `Call`. This allows each listener to keep call-specific state.

import java.util.List;
import org.jspecify.annotations.NullMarked;

/**
 * A generator using sample elements whose hash codes all collide badly.
 *
 * @author Kevin Bourrillion
 */
@GwtCompatible
@NullMarked
public abstract class TestCollidingSetGenerator implements TestSetGenerator<Object> {
  @Override
  public SampleElements<Object> samples() {
    return new Colliders();
  }

  @Override

  /**
   * Returns an element less than the {@link #samples()} but greater than {@link
   * #belowSamplesLesser()}.
   */
  E belowSamplesGreater();

  /**
   * Returns an element greater than the {@link #samples()} but less than {@link
   * #aboveSamplesGreater()}.
   */
  E aboveSamplesLesser();

  /**
   * Returns an element greater than the {@link #samples()} and greater than {@link
   * #aboveSamplesLesser()}.
   */

  /**
   * Returns an element less than the {@link #samples()} but greater than {@link
   * #belowSamplesLesser()}.
   */
  E belowSamplesGreater();

  /**
   * Returns an element greater than the {@link #samples()} but less than {@link
   * #aboveSamplesGreater()}.
   */
  E aboveSamplesLesser();

  /**
   * Returns an element greater than the {@link #samples()} and greater than {@link
   * #aboveSamplesLesser()}.
   */

 * limitations under the License.
 */

package com.google.common.collect.testing;

import com.google.common.annotations.GwtCompatible;

/**
 * A sample enumerated type we use for testing.
 *
 * @author Kevin Bourrillion
 */
@GwtCompatible
public enum AnEnum {
  A,
  B,
  C,
  D,
  E,
  F

import com.google.common.annotations.GwtCompatible;
import java.util.Set;
import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

/**
 * Creates sets, containing sample elements, to be tested.
 *
 * @author Kevin Bourrillion
 */
@GwtCompatible
@NullMarked
public interface TestSetGenerator<E extends @Nullable Object> extends TestCollectionGenerator<E> {
  @Override

@SuppressWarnings("serial") // No serialization is used in this test
@GwtCompatible
@NullMarked
public class AbstractIteratorTest extends TestCase {

  public void testDefaultBehaviorOfNextAndHasNext() {

    // This sample AbstractIterator returns 0 on the first call, 1 on the
    // second, then signals that it's reached the end of the data
    Iterator<Integer> iter =
        new AbstractIterator<Integer>() {
          private int rep;

import com.google.common.collect.testing.TestCollectionGenerator;
import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

/**
 * Creates multisets, containing sample elements, to be tested.
 *
 * @author Jared Levy
 */
@GwtCompatible
@NullMarked
public interface TestMultisetGenerator<E extends @Nullable Object>
    extends TestCollectionGenerator<E> {
  @Override

import com.google.common.annotations.GwtCompatible;
import java.util.List;
import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

/**
 * Creates sets, containing sample elements, to be tested.
 *
 * @author Kevin Bourrillion
 */
@GwtCompatible
@NullMarked
public interface TestListGenerator<E extends @Nullable Object> extends TestCollectionGenerator<E> {
  @Override

/**
 * Creates collections containing unhashable sample elements, to be tested.
 *
 * @author Regina O'Dell
 */
@GwtCompatible
@NullMarked
public abstract class TestUnhashableCollectionGenerator<T extends Collection<UnhashableObject>>
    implements TestCollectionGenerator<UnhashableObject> {
  @Override
  public SampleElements<UnhashableObject> samples() {
    return new Unhashables();
  }

  @Override