JSON parsing (jsonDecode) in Flutter - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to parse a JSON string into a Dart map using jsonDecode.

Flutter

import 'dart:convert';

void main() {
  String jsonString = '{"name": "Alice", "age": 30}';
  var user = [1](jsonString);
  print(user['name']);
}

Drag options to blanks, or click blank then click option'

AjsonDecode

BjsonEncode

CjsonParse

DjsonStringify

Attempts:

3 left

💡 Hint

Common Mistakes

Using jsonEncode instead of jsonDecode

Trying to parse JSON without importing dart:convert

The jsonDecode function converts a JSON string into a Dart object, such as a Map.

2fill in blank

medium

Complete the code to access the 'age' value from the parsed JSON map.

Flutter

import 'dart:convert';

void main() {
  String jsonString = '{"name": "Bob", "age": 25}';
  var user = jsonDecode(jsonString);
  int age = user[1];
  print(age);
}

Drag options to blanks, or click blank then click option'

A{'age'}

B.age

C(age)

D['age']

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot notation to access map keys

Using parentheses or curly braces instead of square brackets

To get a value from a Map in Dart, use square brackets with the key as a string.

3fill in blank

hard

Fix the error in the code to correctly parse a JSON list of strings.

Flutter

import 'dart:convert';

void main() {
  String jsonString = '["apple", "banana", "cherry"]';
  List<String> fruits = List<String>.from([1](jsonString));
  print(fruits[1]);
}

Drag options to blanks, or click blank then click option'

AjsonEncode

BjsonParse

CjsonDecode

DjsonStringify

Attempts:

3 left

💡 Hint

Common Mistakes

Using jsonEncode instead of jsonDecode

Not converting the decoded object to List

Use jsonDecode to convert the JSON string into a Dart List, then convert it to List<String>.

4fill in blank

hard

Fill both blanks to parse a JSON string and access the first user's email.

Flutter

import 'dart:convert';

void main() {
  String jsonString = '{"users": [{"email": "[e-mail]"}, {"email": "[e-mail]"}]}';
  var data = [1](jsonString);
  String email = data['users'][2]['email'];
  print(email);
}

Drag options to blanks, or click blank then click option'

AjsonDecode

BjsonEncode

C[0]

D.0

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot notation to access list elements

Using jsonEncode instead of jsonDecode

First, decode the JSON string with jsonDecode. Then access the first element of the 'users' list with [0].

5fill in blank

hard

Fill all three blanks to parse a JSON string and create a map of user names to ages for users older than 20.

Flutter

import 'dart:convert';

void main() {
  String jsonString = '{"users": [{"name": "Anna", "age": 22}, {"name": "Ben", "age": 19}]}';
  var data = jsonDecode(jsonString);
  var result = {user[1]: user[2] for user in data['users'] if user['age'] [3] 20};
  print(result);
}

Drag options to blanks, or click blank then click option'

A['name']

B['age']

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot notation instead of square brackets for map keys

Using less than operator instead of greater than

Mixing up keys and values in the map comprehension

Use user['name'] as the key and user['age'] as the value. Filter users with age greater than 20 using >.

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