import java.util.AbstractSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
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;

    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();