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.
