public K firstKey() {
      // correctly throws NoSuchElementException when filtered map is empty.
      return keySet().iterator().next();
    }

    @Override
    @ParametricNullness
    public K lastKey() {
      SortedMap<K, V> headMap = sortedMap();
      while (true) {
        // correctly throws NoSuchElementException when filtered map is empty.
        K key = headMap.lastKey();

    public K firstKey() {
      // correctly throws NoSuchElementException when filtered map is empty.
      return keySet().iterator().next();
    }

    @Override
    @ParametricNullness
    public K lastKey() {
      SortedMap<K, V> headMap = sortedMap();
      while (true) {
        // correctly throws NoSuchElementException when filtered map is empty.
        K key = headMap.lastKey();

import java.util.AbstractMap;
import java.util.AbstractQueue;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.Set;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ConcurrentMap;

    iterator.next().use {
      assertThat(it.key()).isEqualTo("c")
      it.assertValue(0, "c1")
      it.assertValue(1, "c2")
    }
    assertThat(iterator.hasNext()).isFalse()
    assertFailsWith<NoSuchElementException> {
      iterator.next()
    }
  }

  @ParameterizedTest
  @ArgumentsSource(FileSystemParamProvider::class)