// NPE on null listener, so we propagate that contract up into the add method as well.
    checkNotNull(runnable, "Runnable was null.");
    checkNotNull(executor, "Executor was null.");

    // Lock while we check state. We must maintain the lock while adding the new pair so that
    // another thread can't run the list out from under us. We only add to the list if we have not
    // yet started execution.
    synchronized (this) {

import java.util.List;
import java.util.NoSuchElementException;
import junit.framework.TestCase;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Unit test for {@link PeekingIterator}.
 *
 * @author Mick Killianey
 */
@SuppressWarnings("serial") // No serialization is used in this test
@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
public class PeekingIteratorTest extends TestCase {

  /**

import java.util.List;
import java.util.NoSuchElementException;
import junit.framework.TestCase;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Unit test for {@link PeekingIterator}.
 *
 * @author Mick Killianey
 */
@SuppressWarnings("serial") // No serialization is used in this test
@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
public class PeekingIteratorTest extends TestCase {

  /**