}
  }

  abstract boolean isPartialView();

  public final @Nullable V put(K k, V v) {
    throw new UnsupportedOperationException();
  }

  public final @Nullable V remove(Object o) {
    throw new UnsupportedOperationException();
  }

  public final void putAll(Map<? extends K, ? extends V> map) {
    throw new UnsupportedOperationException();
  }

   * <p>If the mapped keys contain duplicates (according to {@link Object#equals(Object)}), the
   * values are merged using the specified merging function. If the merging function returns {@code
   * null}, then the collector removes the value that has been computed for the key thus far (though
   * future occurrences of the key would reinsert it).
   *
   * <p>Entries will appear in the encounter order of the first occurrence of the key.
   *

   *
   * <p>If the mapped keys contain duplicates (according to {@link Object#equals(Object)}), the
   * values are merged using the specified merging function. If the merging function returns {@code
   * null}, then the collector removes the value that has been computed for the key thus far (though
   * future occurrences of the key would reinsert it).
   *
   * <p>Entries will appear in the encounter order of the first occurrence of the key.
   *