Model classes from JSON in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Correct JSON parsing in Flutter model

Given this JSON string representing a user:

{"id": 1, "name": "Alice", "email": "[e-mail]"}

Which Dart code correctly parses this JSON into a User model?

Flutter

class User {
  final int id;
  final String name;
  final String email;

  User({required this.id, required this.name, required this.email});

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

A factory User.fromJson(Map<String, dynamic> json) => User(id: json['id'], name: json['name'], email: json['email']);

B User.fromJson(Map<String, dynamic> json) => User(id: json['id'].toString(), name: json['name'], email: json['email']);

C User.fromJson(Map<String, dynamic> json) => User(id: json['id'], name: json['name'], email: json['email'].toInt());

D User.fromJson(Map<String, dynamic> json) => User(id: json['id'], name: json['name'], email: json['email']);

💡 Hint

Check the data types expected by the User constructor and how JSON values are accessed.

Option A correctly maps the JSON fields to the User model fields with matching types. Option A misses the factory keyword. Option A wrongly converts id to string. Option A tries to convert email to int causing error.

❓ ui_behavior

intermediate

Displaying parsed model data in Flutter UI

You have parsed a User model from JSON. Which Flutter widget code correctly displays the user's name and email in a column?

Flutter

final user = User(id: 1, name: "Alice", email: "[e-mail]");

A Row(children: [Text(user.name), Text(user.email)])

B Column(children: [Text(user.name), Text(user.email.toString())])

C Column(children: [Text(user.name), Text(user.email)])

D Column(children: [Text(user.id), Text(user.email)])

💡 Hint

Use a vertical layout and display the correct fields as strings.

Option C uses Column to stack name and email vertically and displays correct string fields. Option C uses Row which lays out horizontally. Option C shows id which is int, causing error in Text widget. Option C unnecessarily calls toString() on email which is already a string.

❓ lifecycle

advanced

When to parse JSON in Flutter widget lifecycle

You receive JSON data asynchronously and want to parse it into a model and display it. Which Flutter widget lifecycle method is best to parse JSON after data is fetched?

A didChangeDependencies()

B initState()

C build()

D dispose()

💡 Hint

Consider when you want to initialize state once before UI builds.

initState() runs once when the widget is inserted in the tree, ideal for starting async fetch and parsing. build() runs many times and should be pure. didChangeDependencies() runs on dependency changes, not ideal here. dispose() is for cleanup.

❓ navigation

advanced

Passing model data between Flutter screens

You have parsed a User model from JSON on Screen A. How do you pass this User object to Screen B when navigating?

A Both B and C are valid ways to pass the model.

B Navigator.pushNamed(context, '/screenB', arguments: user);

C Navigator.push(context, MaterialPageRoute(builder: (_) => ScreenB(user: user)));

D You cannot pass objects between screens in Flutter.

💡 Hint

Flutter supports passing data via constructor or route arguments.

Option A passes the User object directly via constructor. Option A passes it via named route arguments. Both are valid Flutter navigation methods to pass model data. Option A is false.

🔧 Debug

expert

Identify the cause of JSON parsing runtime error

Given this Dart code snippet parsing JSON:

final jsonString = '{"id": "1", "name": "Bob", "email": "[e-mail]"}';
final Map userMap = jsonDecode(jsonString);
final user = User.fromJson(userMap);

The User model expects id as int but JSON has id as string. What error will occur?

A No error, code runs fine

B FormatException: Invalid JSON format

C NullPointerException

D TypeError: String cannot be assigned to int

💡 Hint

Check the type mismatch between JSON field and model field.

The JSON has id as string "1" but User expects int id. This causes a TypeError at runtime when assigning string to int field. FormatException would occur if JSON was malformed. NullPointerException is not typical in Dart.

Practice

1. What is the main purpose of a model class when working with JSON in Flutter?

easy

A. To handle user input events

B. To display UI elements on the screen

C. To convert JSON data into Dart objects and back

D. To manage app navigation between screens

Solution

1. Step 1: Understand the role of model classes

   Model classes are designed to represent data structures in Dart that match JSON data.

2. Step 2: Identify the purpose related to JSON

   They convert JSON strings into Dart objects and convert Dart objects back to JSON for easy data handling.

3. Final Answer:

   To convert JSON data into Dart objects and back -> Option C

4. Quick Check:

   Model class = JSON conversion [OK]

Hint: Model classes handle JSON to Dart object conversion [OK]

Common Mistakes:

• Confusing model classes with UI widgets
• Thinking model classes manage navigation
• Assuming model classes handle user input

2. Which of the following is the correct syntax for a factory constructor named fromJson in a Dart model class?

easy

A. factory Model.fromJson(Map json) { return Model(json['id']); }

B. Model.fromJson(Map json) { return Model(json['id']); }

C. factory Model.fromJson(Map json) { Model(json['id']); }

D. Model.fromJson(Map json) => Model(json['id']);

Solution

1. Step 1: Recall factory constructor syntax

   In Dart, a factory constructor uses the keyword 'factory' and returns an instance, often with a return statement.

2. Step 2: Check each option for correct syntax

   factory Model.fromJson(Map json) { return Model(json['id']); } correctly uses 'factory', has the parameter, and returns a new Model instance inside braces.

3. Final Answer:

   factory Model.fromJson(Map json) { return Model(json['id']); } -> Option A

4. Quick Check:

   Factory constructor syntax = factory Model.fromJson(Map json) { return Model(json['id']); } [OK]

Hint: Factory constructors need 'factory' keyword and return statement [OK]

Common Mistakes:

• Omitting the 'factory' keyword
• Not returning an instance
• Using arrow syntax without 'factory'

3. Given this Dart model class snippet, what will print(user.name); output?

class User {
  final String name;
  User(this.name);

  factory User.fromJson(Map json) {
    return User(json['name']);
  }
}

void main() {
  final json = {'name': 'Alice'};
  final user = User.fromJson(json);
  print(user.name);
}

medium

A. null

B. Alice

C. User Instance

D. Error: No such method

Solution

1. Step 1: Understand fromJson factory usage

   The factory constructor takes the JSON map and extracts the 'name' value, passing it to the User constructor.

2. Step 2: Trace the print statement

   The user object has name set to 'Alice', so printing user.name outputs 'Alice'.

3. Final Answer:

   Alice -> Option B

4. Quick Check:

   JSON 'name' value = printed name [OK]

Hint: fromJson sets fields from JSON keys [OK]

Common Mistakes:

• Expecting print to show object instead of field
• Confusing factory constructor with normal constructor
• Assuming null if key missing

4. Identify the error in this Dart model class code for JSON parsing:

class Product {
  final int id;
  final String name;

  Product(this.id, this.name);

  factory Product.fromJson(Map json) {
    return Product(json['id'], json['title']);
  }
}

medium

A. The JSON key 'title' does not match the field 'name'

B. The constructor parameters are in wrong order

C. Missing semicolon after constructor

D. Factory constructor should not return Product instance

Solution

1. Step 1: Compare JSON keys with class fields

   The class has a field 'name' but the factory uses 'json['title']' which does not match.

2. Step 2: Understand impact of key mismatch

   This causes the 'name' field to get null or wrong value, leading to incorrect data mapping.

3. Final Answer:

   The JSON key 'title' does not match the field 'name' -> Option A

4. Quick Check:

   JSON keys must match field names [OK]

Hint: JSON keys must match model field names exactly [OK]

Common Mistakes:

• Using wrong JSON keys
• Ignoring field and parameter order
• Thinking factory can't return instance

5. You have a JSON list of users: [{'id':1,'name':'Bob'},{'id':2,'name':'Eve'}]. How do you convert this list into a Dart list of User objects using a model class with fromJson factory?

hard

A. User.fromJson(jsonList[0])

B. User.fromJson(jsonList)

C. jsonList.forEach((json) => User.fromJson(json))

D. jsonList.map((json) => User.fromJson(json)).toList()

Solution

1. Step 1: Understand JSON list to Dart list conversion

   Each JSON map in the list must be converted to a User object using fromJson.

2. Step 2: Use map and toList for conversion

   Using map applies fromJson to each item, and toList collects results into a Dart list.

3. Final Answer:

   jsonList.map((json) => User.fromJson(json)).toList() -> Option D

4. Quick Check:

   Map each JSON to User, then toList [OK]

Hint: Use map + fromJson + toList to convert JSON list [OK]

Common Mistakes:

• Passing whole list to fromJson expecting batch conversion
• Using forEach without collecting results
• Converting only first item