@NotNull
  public Collection<Integer> get(int key) {
    final int[] asArray = getAsArray(key);
    return new AbstractList<>() {
      @NotNull
      @Override
      public Integer get(int index) {
        return asArray[index];
      }

      @Override
      public int size() {
        return asArray.length;
      }
    };
  }
@NotNull
  public Collection<Integer> get(int key)