| type '_InternalLinkedHashMap' is not a subtype of type 'List' in Flutter: Causes and How to Fix

type '_InternalLinkedHashMap' is not a subtype of type 'List' in Flutter: Causes and How to Fix

February 10, 2025

Discover how to resolve the error 'type '_InternalLinkedHashMap' is not a subtype of type 'List' in Flutter with our comprehensive guide.

What is type '_InternalLinkedHashMap' is not a subtype of type 'List' Error in Flutter

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

When developing in Flutter, particularly when working with JSON data, you might encounter a common runtime error: '_InternalLinkedHashMap' is not a subtype of type 'List'.' This error signifies a type mismatch in the data being handled and often involves misunderstandings between Map and List.

Key Concepts Involved

\_InternalLinkedHashMap: This is a private implementation of a Map in Dart, which is a data structure used to store key-value pairs. Here, it signifies a dynamic structure where keys are strings and values can be of any type.

List: Represents a list (array) in Dart that can hold elements of any type. Lists are ordered collections, unlike maps.

Scenario Explanation

When working with JSON data in Flutter, you often need to decode JSON using the `jsonDecode()` function. This function returns a Dart object based on the structure of the input JSON.

Sometimes, JSON data might be structured as a dictionary (object), which corresponds to a Map in Dart. However, developers may unintentionally expect a structure as a List.

This specific error occurs when the code expects a `List` type but receives a `_InternalLinkedHashMap` instead. This often happens when accessing JSON as a list without recognizing its true structure as an object.

Practical Code Illustrations

Consider a backend returning the following JSON response:

{
  "name": "Flutter",
  "version": "2.0"
}

Attempting to parse this JSON string expecting a list may result in the error:

import 'dart:convert';

void main() {
  String jsonString = '{"name": "Flutter", "version": "2.0"}';
  var parsedJson = jsonDecode(jsonString);

  // Attempting to treat parsed JSON as a List
  List<dynamic> data = parsedJson; // Causes error
}

Strategies to Approach the Problem

Verify JSON Structure: Before processing JSON, acquaint yourself with its structure to avoid assumptions about lists or maps. This can prevent type mismatches crucially.

Utilize Dart's Type System: Leverage Dart's type system to explicitly define expected data structures. The sample code demonstrates assigning the decoded JSON to a `Map`:

import 'dart:convert';

void main() {
  String jsonString = '{"name": "Flutter", "version": "2.0"}';
  Map<String, dynamic> parsedJson = jsonDecode(jsonString); // Correct assignment

  print(parsedJson['name']); // Accessing map values safely.
}

By adhering to these suggestions, developers can effectively mitigate and comprehend potential runtime type errors which may impact the application's data handling practices. Understanding the distinct types in Dart makes parsing and utilization of different data structures a seamless process.

What Causes type '_InternalLinkedHashMap' is not a subtype of type 'List' in Flutter

Understanding the Error

This error typically occurs when there is a mismatch between the expected data structure and the actual data structure being used within the Flutter application. Specifically, it arises when the code expects a `List` but receives a `_InternalLinkedHashMap` instead.

In Flutter, when dealing with data serialization such as converting JSON data, the deserialization process can result in unexpected data types. If the JSON structure represents an object, Dart will interpret it as a `_InternalLinkedHashMap` rather than a `List`.

Common Scenarios Where This Error Occurs

**Parsing JSON Data:** If a JSON array is expected but a JSON object is returned, the JSON decoding process will yield a map, causing this type mismatch.

\`\`\`dart
var response = await http.get(Uri.parse(url));
var data = jsonDecode(response.body);

// Expected a list but received a map
List<dynamic> myList = data; // This will trigger the error
\`\`\`

**Data Structural Assumptions:** Sometimes developers may assume a uniform data structure for external data but the data may vary in structure, leading to incorrect assumptions.

\`\`\`dart
var jsonData = '''
  {
    "users": {"name": "John", "age": 30}
  }
''';

var decodedData = jsonDecode(jsonData);

// Incorrect assumption that 'users' would be a list
List<dynamic> usersList = decodedData['users']; // Error occurs here
\`\`\`

**API Response Changes:** When an external API changes its response format without notifying the client application, it can lead to such type mismatches. For instance, changing from an array of objects to a single nested object.

**Misinterpretation of Dynamic Types:** Developers might misinterpret dynamic types when working with APIs. If documentation is unclear or misread, this can lead to incorrect type casting.

\`\`\`dart
dynamic apiResponse = getApiResponse();

// Assuming 'apiResponse' is a list but it might be a map
List<dynamic> items = apiResponse; // Leads to error if apiResponse is a map
\`\`\`

Conclusion

This error highlights the importance of understanding data structures returned by external data sources. Proper data type checking and testing with different datasets can help prevent such errors.

How to Fix type '_InternalLinkedHashMap' is not a subtype of type 'List' in Flutter

Identify the Source of the Issue

Check the API response or data source to understand the expected data structure. If you anticipated a list but received a map, revision might be needed in the decoding logic.

Use debugging tools or log the response data to confirm its structure prior to processing it further in your application.

Revise the Parsing Logic

Ensure that JSON decoding methods (`jsonDecode`) correctly handle the map response. You can adapt the code to parse into a `Map` instead of directly using it as a `List`.

import 'dart:convert';

void someFunction(String jsonResponse) {
  var parsedData = jsonDecode(jsonResponse);
  Map<String, dynamic> dataMap = parsedData; // Directly use as a map

  // If you originally expected a list
  List<dynamic>? dataList = dataMap['items'] as List<dynamic>?; // Convert specific field to a list
}

Handle the Conversion Appropriately

If the data you need is nested within a map, access it using key lookups and then process it as required.

In case the wrong structure was anticipated, refactor code patterns for data utilization, whether for iteration or accessing individual items.

List<dynamic> items = dataMap['items'] ?? [];

for (var item in items) {
  // Process each item
}

Modify the Data Model

Create data models for deserialization purposes via `fromJson` methods that appropriately interpret the nested map structures.

Adjust models to reflect actual underlying data types, ensuring the map values translate seamlessly into desired objects or collections.

class Item {
  final String name;
  final int value;

  Item({required this.name, required this.value});

  factory Item.fromJson(Map<String, dynamic> json) {
    return Item(name: json['name'], value: json['value']);
  }
}

List<Item> items = (dataMap['items'] as List).map((item) => Item.fromJson(item)).toList();

Consider Using Try-Catch Blocks

Implement error handling to gracefully address conversion issues. This approach cushions the application against unexpected JSON structures.

try {
  List<dynamic> items = dataMap['items'] ?? [];
  // Additional processing
} catch (e) {
  print('Error processing data: $e');
}

Review Testing and Optimization

Conduct unit tests on decoding and conversion functions for various scenarios to anticipate structural variances in JSON schemas.

Use mock data closely imitating actual responses to ensure your code adapts seamlessly to real-world use cases.