如何使用用户定义的索引值在dart中创建列表？

class UserDefinedIndexList<T> {
  List<T> _list = [];

  void addAtIndex(int index, T item) {
    if (index >= 0 && index <= _list.length) {
      _list.insert(index, item);
    } else {
      throw RangeError.range(index, 0, _list.length);
    }
  }

  T getByIndex(int index) {
    if (index >= 0 && index < _list.length) {
      return _list[index];
    } else {
      throw RangeError.range(index, 0, _list.length - 1);
    }
  }

  int get length => _list.length;
}

void main() {
  UserDefinedIndexList<String> myList = UserDefinedIndexList<String>();
  
  // 添加元素到特定的索引位置
  myList.addAtIndex(0, "First");
  myList.addAtIndex(1, "Second");
  myList.addAtIndex(2, "Third");
  
  // 获取特定索引位置的元素
  print(myList.getByIndex(0));  // 输出: First
  print(myList.getByIndex(1));  // 输出: Second
  print(myList.getByIndex(2));  // 输出: Third
  
  // 打印列表长度
  print(myList.length);  // 输出: 3
}