JSON parsing (jsonDecode) in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the jsonDecode function do in Flutter?

It converts a JSON string into a Dart object, like a Map or List, so you can use the data in your app.

beginner

Which Dart type does jsonDecode return when parsing a JSON object?

It returns a Map<String, dynamic> representing the JSON object.

intermediate

Why should you handle exceptions when using jsonDecode?

Because if the JSON string is invalid or malformed, jsonDecode throws a FormatException. Handling it prevents app crashes.

beginner

How do you import the library needed to use <code>jsonDecode</code> in Flutter?

You import it with <code>import 'dart:convert';</code> at the top of your Dart file.

beginner

What is a real-life example of using jsonDecode in a mobile app?

When your app gets data from a web server in JSON format, you use jsonDecode to turn that data into Dart objects to show on the screen.

What type of data does jsonDecode expect as input?

A A JSON formatted string

B A Dart Map object

C A List of integers

D An XML string

jsonDecode takes a JSON string and converts it to Dart objects.

Which Dart type is returned when jsonDecode parses a JSON array?

A Map<String, dynamic>

B int

C List<dynamic>

D String

A JSON array becomes a Dart List when decoded.

What happens if you try to decode an invalid JSON string with jsonDecode?

A Throws a FormatException

B Returns null

C Returns an empty Map

D Automatically fixes the JSON

Invalid JSON causes jsonDecode to throw a FormatException.

Which import is required to use jsonDecode in Flutter?

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

B import 'dart:convert';

C import 'dart:io';

D import 'package:http/http.dart';

jsonDecode is in the dart:convert library.

Which of these is a common use of jsonDecode in mobile apps?

A To convert Dart objects to JSON strings

B To handle user input events

C To style UI components

D To parse JSON data received from a web API

jsonDecode is used to parse JSON data from APIs into Dart objects.

Explain how you would use jsonDecode to handle JSON data from a web API in Flutter.

Think about the steps from receiving a JSON string to using it safely in your app.

Describe what types of Dart objects you get after decoding JSON objects and arrays with jsonDecode.

Consider the JSON structure and how it maps to Dart types.

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