monitor.enterWhen(notEmpty);
    try {
      return extract();
    } finally {
      monitor.leave();
    }
  }

  @CanIgnoreReturnValue
  @Override
  public @Nullable E peek() {
    Monitor monitor = this.monitor;
    if (monitor.enterIf(notEmpty)) {
      try {
        return items[takeIndex];
      } finally {
        monitor.leave();
      }
    } else {

import org.jspecify.annotations.NullUnmarked;

/**
 * Inputs, expected outputs, and helper methods for tests of {@link StatsAccumulator}, {@link
 * Stats}, {@link PairedStatsAccumulator}, and {@link PairedStats}.
 *
 * @author Pete Gillin
 */
@NullUnmarked
class StatsTesting {
  // TODO(cpovirk): Convince myself that this larger error makes sense.
  static final double ALLOWED_ERROR = isAndroid() ? .25 : 1e-10;

  // Inputs and their statistics:

/**
 * A mutable object which accumulates double values and tracks some basic statistics over all the
 * values added so far. The values may be added singly or in groups. This class is not thread safe.
 *
 * @author Pete Gillin
 * @author Kevin Bourrillion
 * @since 20.0
 */
@J2ktIncompatible
@GwtIncompatible
public final class StatsAccumulator {
  /** Creates a new accumulator. */
  public StatsAccumulator() {}

      if (source.isEmpty()) {
        throw PermittedMetaException.NSEE;
      }

      destination.push(source.pop());
      stackWithLastReturnedElementAtTop = destination;
      return destination.peek();
    }

    private void throwIfInvalid(IteratorFeature methodFeature) {
      if (!features.contains(methodFeature)) {
        if (stackWithLastReturnedElementAtTop == null) {

		}
	}
}

func Test_metacacheReader_peek(t *testing.T) {
	r := loadMetacacheSample(t)
	defer r.Close()
	for i, want := range loadMetacacheSampleNames {
		got, err := r.peek()
		if err == io.EOF {
			break
		}
		if err != nil {
			t.Fatal(err)
		}
		if got.name != want {
			t.Errorf("entry %d, want %q, got %q", i, want, got.name)
		}
		gotObj, err := r.next()

 * StatsAccumulator#add} and {@link StatsAccumulator#addAll} methods. For tests of the {@link
 * StatsAccumulator#snapshot} method which returns {@link Stats} instances, see {@link StatsTest}.
 *
 * @author Pete Gillin
 */
@NullUnmarked
public class StatsAccumulatorTest extends TestCase {

  private StatsAccumulator emptyAccumulator;
  private StatsAccumulator emptyAccumulatorByAddAllEmptyIterable;

      while (peekingItr.hasNext()) {
        Range<C> range = peekingItr.next();
        while (peekingItr.hasNext()) {
          Range<C> nextRange = peekingItr.peek();
          if (range.isConnected(nextRange)) {
            checkArgument(
                range.intersection(nextRange).isEmpty(),
                "Overlapping ranges not permitted but found %s overlapping %s",

    public boolean offer(E e) {
      synchronized (mutex) {
        return delegate().offer(e);
      }
    }

    @Override
    public @Nullable E peek() {
      synchronized (mutex) {
        return delegate().peek();
      }
    }

    @Override
    public @Nullable E poll() {
      synchronized (mutex) {
        return delegate().poll();
      }
    }

    public boolean offer(E e) {
      synchronized (mutex) {
        return delegate().offer(e);
      }
    }

    @Override
    public @Nullable E peek() {
      synchronized (mutex) {
        return delegate().peek();
      }
    }

    @Override
    public @Nullable E poll() {
      synchronized (mutex) {
        return delegate().poll();
      }
    }

            context.pushedUserProperties = new HashSet<>();
            protoSession.getUserProperties().entrySet().stream()
                    .filter(k -> !sys.contains(k.getKey()))
                    .peek(k -> context.pushedUserProperties.add(k.getKey()))
                    .forEach(k -> System.setProperty(k.getKey(), k.getValue()));
        } else {
            protoSession.getUserProperties().entrySet().stream()