this.nextOrNull = firstOrNull;
  }

  /**
   * Returns the element that follows {@code previous}, or returns {@code null} if no elements
   * remain. This method is invoked during each call to {@link #next()} in order to compute the
   * result of a <i>future</i> call to {@code next()}.
   */
  protected abstract @Nullable T computeNext(T previous);

  @Override

import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code poll()} operations on a queue. Can't be invoked directly;
 * please see {@link com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Jared Levy
 */
@GwtCompatible

import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests removeAll operations on a list. Can't be invoked directly;
 * please see {@link com.google.common.collect.testing.ListTestSuiteBuilder}.
 *
 * @author George van den Driessche
 */
@GwtCompatible

import com.google.errorprone.annotations.CanIgnoreReturnValue;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests conditional {@code setCount()} operations on a multiset. Can't
 * be invoked directly; please see {@link MultisetTestSuiteBuilder}.
 *
 * @author Chris Povirk
 */
@GwtCompatible
@Ignore("test runners must not instantiate and run this directly, only via suites we build")

import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code remove(int)} operations on a list. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.ListTestSuiteBuilder}.
 *
 * @author Chris Povirk
 */
@GwtCompatible

import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code toString()} operations on a collection. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.CollectionTestSuiteBuilder}.
 *
 * @author Kevin Bourrillion
 */
@GwtCompatible

import com.google.common.collect.testing.features.MapFeature;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests {@code containsValue()} operations on a map. Can't be invoked
 * directly; please see {@link com.google.common.collect.testing.MapTestSuiteBuilder}.
 *
 * @author George van den Driessche
 * @author Chris Povirk
 */
@GwtCompatible

import java.util.List;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests creation (typically through a constructor or static factory
 * method) of a set. Can't be invoked directly; please see {@link
 * com.google.common.collect.testing.SetTestSuiteBuilder}.
 *
 * @author Chris Povirk
 */
@GwtCompatible

import com.google.common.collect.Multisets;
import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests multiset-specific read operations. Can't be invoked directly;
 * please see {@link com.google.common.collect.testing.SetTestSuiteBuilder}.
 *
 * @author Jared Levy
 */
@GwtCompatible

import com.google.common.collect.Multisets;
import com.google.common.collect.testing.features.CollectionSize;
import org.junit.Ignore;

/**
 * A generic JUnit test which tests multiset-specific read operations. Can't be invoked directly;
 * please see {@link com.google.common.collect.testing.SetTestSuiteBuilder}.
 *
 * @author Jared Levy
 */
@GwtCompatible