*     class under test.
   */
  protected abstract Map<K, V> makePopulatedMap() throws UnsupportedOperationException;

  /**
   * Creates a new key that is not expected to be found in {@link #makePopulatedMap()}.
   *
   * @return a key.
   * @throws UnsupportedOperationException if it's not possible to make a key that will not be found
   *     in the map.

   *     class under test.
   */
  protected abstract Map<K, V> makePopulatedMap() throws UnsupportedOperationException;

  /**
   * Creates a new key that is not expected to be found in {@link #makePopulatedMap()}.
   *
   * @return a key.
   * @throws UnsupportedOperationException if it's not possible to make a key that will not be found
   *     in the map.

    return new SimpleForwardingSortedMap<>(
        new TreeMap<String, Integer>(Ordering.<String>natural().nullsFirst()));
  }

  @Override
  protected SortedMap<String, Integer> makePopulatedMap() {
    SortedMap<String, Integer> sortedMap = makeEmptyMap();
    sortedMap.put("one", 1);
    sortedMap.put("two", 2);
    sortedMap.put("three", 3);
    return sortedMap;
  }

  @Override

    @Override
    protected Map<String, Integer> makeEmptyMap() {
      return makeTable().row('a');
    }

    @Override
    protected Map<String, Integer> makePopulatedMap() {
      Table<Character, String, Integer> table = makeTable();
      table.put('a', "one", 1);
      table.put('a', "two", 2);
      table.put('a', "three", 3);
      table.put('b', "four", 4);

    @Override
    protected Map<String, Integer> makeEmptyMap() {
      return makeTable().row('a');
    }

    @Override
    protected Map<String, Integer> makePopulatedMap() {
      Table<Character, String, Integer> table = makeTable();
      table.put('a', "one", 1);
      table.put('a', "two", 2);
      table.put('a', "three", 3);
      table.put('b', "four", 4);

  @Override
  protected Map<String, String> makeEmptyMap() {
    return transformValues(new HashMap<String, String>(), Functions.<String>identity());
  }

  @Override
  protected Map<String, String> makePopulatedMap() {
    Map<String, Integer> underlying = new HashMap<>();
    underlying.put("a", 1);
    underlying.put("b", 2);
    underlying.put("c", 3);
    return transformValues(underlying, Functions.toStringFunction());
  }

  protected Map<String, Collection<Integer>> makeEmptyMap() {
    Multimap<String, Integer> multimap = createMultimap();
    return multimap.asMap();
  }

  @Override
  protected Map<String, Collection<Integer>> makePopulatedMap() {
    Multimap<String, Integer> multimap = createMultimap();
    populate(multimap);
    return multimap.asMap();
  }

  protected Map<String, Collection<Integer>> makeEmptyMap() {
    Map<String, Integer> map = new HashMap<>();
    return Multimaps.forMap(map).asMap();
  }

  @Override
  protected Map<String, Collection<Integer>> makePopulatedMap() {
    Map<String, Integer> map = new HashMap<>();
    map.put("foo", 1);
    map.put("bar", 2);
    map.put("cow", 3);
    return Multimaps.forMap(map).asMap();
  }

  @Override

  @Override
  protected Map<String, String> makeEmptyMap() {
    return transformValues(new HashMap<String, String>(), Functions.<String>identity());
  }

  @Override
  protected Map<String, String> makePopulatedMap() {
    Map<String, Integer> underlying = new HashMap<>();
    underlying.put("a", 1);
    underlying.put("b", 2);
    underlying.put("c", 3);
    return transformValues(underlying, Functions.toStringFunction());
  }

  protected Map<String, Collection<Integer>> makeEmptyMap() {
    Multimap<String, Integer> multimap = createMultimap();
    return multimap.asMap();
  }

  @Override
  protected Map<String, Collection<Integer>> makePopulatedMap() {
    Multimap<String, Integer> multimap = createMultimap();
    populate(multimap);
    return multimap.asMap();
  }