Flutter/Dart HTTP请求类型转换异常：如何修复该报错？

问题根源 #

报错type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'List<dynamic>' in type cast的核心原因：接口返回的是单个JSON对象，但你的代码错误地将其强制转换为List类型，导致类型不匹配。

修复步骤 #

1. 确认接口返回格式 #

先明确接口返回的JSON结构（示例）：

{
  "year": 2024,
  "totalAmount": 3200.5,
  "bills": [
    {"id": 1, "title": "餐饮", "amount": 1800},
    {"id": 2, "title": "交通", "amount": 600}
  ]
}

该结构是单个对象（外层为{}），而非数组（外层为[]），这是关键前提。

2. 修正请求解析代码 #

把原来强制转List的逻辑，改为解析为Map<String, dynamic>，再传入实体类的fromJson方法：

import 'dart:convert';
import 'package:http/http.dart' as http;

final response = await http.get(Uri.parse('你的接口地址'));
if (response.statusCode == 200) {
  // 正确：将响应解析为Map对象，而非List
  Map<String, dynamic> responseData = jsonDecode(response.body) as Map<String, dynamic>;
  YearBill yearBill = YearBill.fromJson(responseData);
  // 后续处理yearBill实例
} else {
  throw Exception('请求失败，状态码：${response.statusCode}');
}

3. 适配实体类的fromJson方法 #

确保YearBill的fromJson方法接收Map<String, dynamic>参数，内部正确处理嵌套数组：

class YearBill {
  final int year;
  final double totalAmount;
  final List<Bill> bills;

  YearBill({
    required this.year,
    required this.totalAmount,
    required this.bills,
  });

  factory YearBill.fromJson(Map<String, dynamic> json) {
    return YearBill(
      year: json['year'] as int,
      totalAmount: (json['totalAmount'] as num).toDouble(),
      // 处理嵌套的bills数组：先转List再映射为Bill对象
      bills: (json['bills'] as List<dynamic>)
          .map((billJson) => Bill.fromJson(billJson as Map<String, dynamic>))
          .toList(),
    );
  }
}

class Bill {
  final int id;
  final String title;
  final double amount;

  Bill({
    required this.id,
    required this.title,
    required this.amount,
  });

  factory Bill.fromJson(Map<String, dynamic> json) {
    return Bill(
      id: json['id'] as int,
      title: json['title'] as String,
      amount: (json['amount'] as num).toDouble(),
    );
  }
}

额外优化建议 #

为避免类型转换异常扩散，可使用可选类型转换并设置默认值：

year: json['year'] as? int ?? 0,
totalAmount: (json['totalAmount'] as? num ?? 0).toDouble(),

内容的提问来源于stack exchange，提问作者Bill Tertis