assertNull(navigableMap.lowerEntry(k0()));
    assertNull(navigableMap.lowerKey(k0()));
    assertNull(navigableMap.floorEntry(k0()));
    assertNull(navigableMap.floorKey(k0()));
    assertNull(navigableMap.ceilingEntry(k0()));
    assertNull(navigableMap.ceilingKey(k0()));
    assertNull(navigableMap.higherEntry(k0()));
    assertNull(navigableMap.higherKey(k0()));
  }

  @CollectionSize.Require(ZERO)

    assertNull(navigableMap.lowerEntry(k0()));
    assertNull(navigableMap.lowerKey(k0()));
    assertNull(navigableMap.floorEntry(k0()));
    assertNull(navigableMap.floorKey(k0()));
    assertNull(navigableMap.ceilingEntry(k0()));
    assertNull(navigableMap.ceilingKey(k0()));
    assertNull(navigableMap.higherEntry(k0()));
    assertNull(navigableMap.higherKey(k0()));
  }

  @CollectionSize.Require(ZERO)

    try {
      assertNull(navigableMap.lastKey());
      fail();
    } catch (NoSuchElementException e) {
    }
  }

  @CollectionSize.Require(ONE)
  public void testSingletonMapFirst() {
    assertEquals(a.getKey(), navigableMap.firstKey());
  }

  @CollectionSize.Require(ONE)
  public void testSingletonMapLast() {
    assertEquals(a.getKey(), navigableMap.lastKey());
  }

  @Override
  protected <K, V> NavigableMap<K, V> create() {
    @SuppressWarnings("unchecked")
    NavigableMap<K, V> innermost =
        new SafeTreeMap<>((Comparator<? super K>) Ordering.natural().nullsFirst());
    TestMap<K, V> inner = new TestMap<>(innermost, mutex);
    NavigableMap<K, V> outer = Synchronized.navigableMap(inner, mutex);
    return outer;
  }

     * restriction.
     */
    private final Range<C> restriction;

    private final NavigableMap<Cut<C>, Range<C>> rangesByLowerBound;
    private final NavigableMap<Cut<C>, Range<C>> rangesByUpperBound;

    private SubRangeSetRangesByLowerBound(
        Range<Cut<C>> lowerBoundWindow,
        Range<C> restriction,
        NavigableMap<Cut<C>, Range<C>> rangesByLowerBound) {
      this.lowerBoundWindow = checkNotNull(lowerBoundWindow);

    try {
      assertNull(navigableMap.lastKey());
      fail();
    } catch (NoSuchElementException e) {
    }
  }

  @CollectionSize.Require(ONE)
  public void testSingletonMapFirst() {
    assertEquals(a.getKey(), navigableMap.firstKey());
  }

  @CollectionSize.Require(ONE)
  public void testSingletonMapLast() {
    assertEquals(a.getKey(), navigableMap.lastKey());
  }

        TestSortedMapGenerator<K, V> delegate, Bound to, Bound from) {
      super(delegate, to, from);
    }

    @Override
    NavigableMap<K, V> createSubMap(SortedMap<K, V> sortedMap, K firstExclusive, K lastExclusive) {
      NavigableMap<K, V> map = (NavigableMap<K, V>) sortedMap;
      if (from == Bound.NO_BOUND && to == Bound.INCLUSIVE) {
        return map.headMap(lastInclusive, true);

  }

  <K, V> void testNavigationAgainstExpected(
      NavigableMap<K, V> expected, NavigableMap<K, V> navigableMap, Iterable<K> keysToTest) {
    for (K key : keysToTest) {
      assertEquals(expected.lowerEntry(key), navigableMap.lowerEntry(key));
      assertEquals(expected.floorEntry(key), navigableMap.floorEntry(key));
      assertEquals(expected.ceilingEntry(key), navigableMap.ceilingEntry(key));

     * restriction.
     */
    private final Range<C> restriction;

    private final NavigableMap<Cut<C>, Range<C>> rangesByLowerBound;
    private final NavigableMap<Cut<C>, Range<C>> rangesByUpperBound;

    private SubRangeSetRangesByLowerBound(
        Range<Cut<C>> lowerBoundWindow,
        Range<C> restriction,
        NavigableMap<Cut<C>, Range<C>> rangesByLowerBound) {
      this.lowerBoundWindow = checkNotNull(lowerBoundWindow);

import java.util.Iterator;
import java.util.NavigableMap;
import java.util.NavigableSet;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.SortedMap;
import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Skeletal implementation of {@link NavigableMap}.
 *
 * @author Louis Wasserman
 */
@GwtIncompatible