Flutter解析JSON遇_InternalLinkedHashMap类型转换错误求解决

示例JSON #

{    
    "statusCode": 200,    
    "message": "",    
    "result": [        
        {             
            "userName": "RAJ",
            "hierarchyLevel": 3,
            "sequence": 1,
            "isSuccess": 1
        },
        {
            "userName": "SAM",
            "hierarchyLevel": 3,
            "sequence": 1,
            "isSuccess": 1         
        }
    ]
}

原代码 #

int jsonStatus = response.statusCode;                  
if (jsonStatus == 200) {                    
    var jsonData = json.decode(response.body);                    
    if (jsonData["statusCode"] == 200) {                      
        List<dynamic> listValJson = jsonData["result"];                      
        List<String> userList = new List<String>.from(listValJson);                    
    }          
}

报错信息 #

Error: type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'String'

问题原因与解决方案 #

错误原因 #

listValJson中的每一项是_InternalLinkedHashMap<String, dynamic>类型（对应JSON里的对象），而List<String>.from(listValJson)试图直接把这些Map转换成String，导致类型不匹配报错。

正确写法 #

遍历listValJson，逐个取出每个对象里的userName字段，再收集成字符串列表：

int jsonStatus = response.statusCode;                  
if (jsonStatus == 200) {                    
    var jsonData = json.decode(response.body);                    
    if (jsonData["statusCode"] == 200) {                      
        List<dynamic> listValJson = jsonData["result"];                      
        // 提取所有userName并转为String列表
        List<String> userList = listValJson
            .map((item) => item["userName"] as String)
            .toList();                    
    }          
}

进阶优化（可选） #

如果JSON结构复杂或需要频繁解析，建议创建实体类配合json_serializable包实现类型安全的解析：

1. 添加依赖到pubspec.yaml：

dependencies:
  json_annotation: ^4.8.1

dev_dependencies:
  build_runner: ^2.4.4
  json_serializable: ^6.7.0

2. 创建User实体类：

import 'package:json_annotation/json_annotation.dart';

part 'user.g.dart';

@JsonSerializable()
class User {
  final String userName;
  final int hierarchyLevel;
  final int sequence;
  final int isSuccess;

  User({
    required this.userName,
    required this.hierarchyLevel,
    required this.sequence,
    required this.isSuccess,
  });

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
  Map<String, dynamic> toJson() => _$UserToJson(this);
}

3. 运行命令生成序列化代码：

flutter pub run build_runner build

4. 解析JSON：

int jsonStatus = response.statusCode;                  
if (jsonStatus == 200) {                    
    var jsonData = json.decode(response.body);                    
    if (jsonData["statusCode"] == 200) {                      
        List<dynamic> listValJson = jsonData["result"];                      
        List<User> users = listValJson.map((item) => User.fromJson(item)).toList();
        List<String> userList = users.map((user) => user.userName).toList();                    
    }          
}

内容的提问来源于stack exchange，提问作者Kumar