JSON parsing (jsonDecode) in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Flutter widget after JSON parsing?

Given this Flutter code snippet that parses JSON and displays a name, what will the Text widget show?

Flutter

import 'dart:convert';
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    const jsonString = '{"name": "Alice", "age": 30}';
    final Map<String, dynamic> user = jsonDecode(jsonString);
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Text(user['name']),
        ),
      ),
    );
  }
}

A The app shows: Alice

B The app shows: {name: Alice, age: 30}

C The app shows: null

D The app crashes with a runtime error

💡 Hint

Look at how jsonDecode converts the JSON string into a Dart Map and how the 'name' key is accessed.

jsonDecode converts the JSON string into a Map. Accessing user['name'] returns the string 'Alice', which is displayed by the Text widget.

📝 Syntax

intermediate

Which option correctly parses a JSON string into a Dart Map?

Choose the correct Dart code snippet that parses the JSON string '{"key": "value"}' into a Map.

A final Map data = jsonDecode('{key: "value"}');

B final Map<String, String> data = jsonDecode('{key: value}');

C final data = jsonDecode('{"key": value}');

D final Map<String, dynamic> data = jsonDecode('{"key": "value"}');

💡 Hint

JSON keys and string values must be in double quotes.

Option D uses valid JSON syntax with double quotes around keys and string values, and correctly assigns the result to a Map.

🔧 Debug

advanced

What error does this code raise when parsing invalid JSON?

Consider this Dart code: final jsonString = '{"name": "Bob", age: 25}'; final user = jsonDecode(jsonString); What error will occur?

Flutter

final jsonString = '{"name": "Bob", age: 25}';
final user = jsonDecode(jsonString);

A TypeError: Cannot assign int to String

B No error, user is parsed correctly

C FormatException: Unexpected character at position 16

D NullPointerException at jsonDecode

💡 Hint

Check JSON syntax: keys must be in double quotes.

The JSON string is invalid because the key 'age' is not in double quotes. jsonDecode throws a FormatException indicating unexpected character.

🧠 Conceptual

advanced

What type does jsonDecode return when parsing a JSON array?

If you parse the JSON string '[1, 2, 3]' using jsonDecode in Dart, what is the runtime type of the returned object?

Flutter

final jsonString = '[1, 2, 3]';
final result = jsonDecode(jsonString);

A Map<String, dynamic>

B List<dynamic>

C String

D int

💡 Hint

Think about how JSON arrays map to Dart types.

jsonDecode returns a List when parsing a JSON array, because JSON arrays correspond to Dart lists.

❓ lifecycle

expert

When should you call jsonDecode in a Flutter app to parse API response data?

You fetch JSON data from a web API in a Flutter app. To parse the JSON string into Dart objects, when is the best time to call jsonDecode?

A Immediately after receiving the response string, before updating any UI

B After the app is closed and restarted

C Only when the user taps a button to refresh the UI

D Inside the build() method of a widget

💡 Hint

Consider when you have the raw JSON string and when you need Dart objects.

You should parse the JSON string right after receiving it, so you can update your app state and UI with Dart objects. Parsing inside build() is inefficient and can cause errors.

Practice

1. What does the jsonDecode function do in Flutter?

easy

A. It converts a JSON string into Dart objects like Map or List.

B. It converts Dart objects into a JSON string.

C. It formats JSON strings for better readability.

D. It sends JSON data to a web server.

Solution

1. Step 1: Understand jsonDecode purpose

   The jsonDecode function takes a JSON formatted string and parses it into Dart objects such as Map or List.

2. Step 2: Differentiate from encoding

   Converting Dart objects back to JSON string is done by jsonEncode, not jsonDecode.

3. Final Answer:

   It converts a JSON string into Dart objects like Map or List. -> Option A

4. Quick Check:

   jsonDecode = JSON string to Dart objects [OK]

Hint: jsonDecode parses JSON text into Dart objects [OK]

Common Mistakes:

• Confusing jsonDecode with jsonEncode
• Thinking jsonDecode formats JSON for display
• Assuming jsonDecode sends data over network

2. Which import is required before using jsonDecode in Flutter?

easy

A. import 'dart:io';

B. import 'dart:convert';

C. import 'package:flutter/material.dart';

D. import 'dart:async';

Solution

1. Step 1: Identify the library for JSON functions

   The jsonDecode function is part of the dart:convert library.

2. Step 2: Confirm correct import statement

   To use jsonDecode, you must import dart:convert with import 'dart:convert';.

3. Final Answer:

   import 'dart:convert'; -> Option B

4. Quick Check:

   dart:convert import needed for jsonDecode [OK]

Hint: Always import dart:convert for jsonDecode [OK]

Common Mistakes:

• Forgetting to import dart:convert
• Importing dart:io instead
• Assuming Flutter imports include jsonDecode

3. What is the output of this Flutter code?

import 'dart:convert';
void main() {
  String jsonString = '{"name":"Anna","age":25}';
  var user = jsonDecode(jsonString);
  print(user['age']);
}

medium

A. Anna

B. SyntaxError

C. 25

D. null

Solution

1. Step 1: Decode JSON string to Map

   The jsonDecode converts the JSON string into a Map with keys 'name' and 'age'.

2. Step 2: Access the 'age' key value

   Accessing user['age'] returns the integer 25.

3. Final Answer:

   25 -> Option C

4. Quick Check:

   jsonDecode output is Map, user['age'] = 25 [OK]

Hint: jsonDecode returns Map; access keys like a dictionary [OK]

Common Mistakes:

• Trying to access JSON string directly without decoding
• Expecting a string instead of integer for 'age'
• Using wrong key name in the Map

4. What is wrong with this Flutter code snippet?

import 'dart:convert';
void main() {
  String jsonString = '{name:"John", age:30}';
  var data = jsonDecode(jsonString);
  print(data['name']);
}

medium

A. print statement syntax is incorrect

B. Missing import for jsonDecode

C. jsonDecode cannot parse strings with numbers

D. JSON string keys must be in double quotes

Solution

1. Step 1: Check JSON string format

   JSON keys and string values must be enclosed in double quotes. Here, keys name and age are not quoted properly.

2. Step 2: Identify parsing error cause

   Because keys lack double quotes, jsonDecode will throw a FormatException.

3. Final Answer:

   JSON string keys must be in double quotes -> Option D

4. Quick Check:

   JSON keys need double quotes for valid parsing [OK]

Hint: JSON keys must be double-quoted strings [OK]

Common Mistakes:

• Using single quotes for JSON keys
• Forgetting quotes around string values
• Assuming jsonDecode accepts invalid JSON

5. Given this JSON string representing a list of users:

String jsonString = '[{"id":1,"name":"Alice"},{"id":2,"name":"Bob"}]';

How do you parse it to get the name of the second user in Flutter?

hard

A. var users = jsonDecode(jsonString); print(users[1]['name']);

B. var users = jsonDecode(jsonString); print(users['1']['name']);

C. var users = jsonDecode(jsonString); print(users[2]['name']);

D. var users = jsonDecode(jsonString); print(users[1].name);

Solution

1. Step 1: Decode JSON string to List of Maps

   The JSON string represents a list with two user objects. jsonDecode converts it to a List of Maps.

2. Step 2: Access second user and their name

   List indices start at 0, so the second user is at index 1. Accessing users[1]['name'] gives 'Bob'.

3. Final Answer:

   var users = jsonDecode(jsonString); print(users[1]['name']); -> Option A

4. Quick Check:

   List index 1, then key 'name' = 'Bob' [OK]

Hint: List index starts at 0; use users[1]['name'] for second item [OK]

Common Mistakes:

• Using string '1' as index instead of integer 1
• Accessing index 2 which is out of range
• Trying to access Map keys with dot notation