Model classes from JSON in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a model class in Flutter when working with JSON?

A model class is a simple Dart class that represents the structure of JSON data. It helps convert JSON into Dart objects and vice versa, making it easier to work with data in the app.

beginner

What is the purpose of the factory constructor named fromJson in a model class?

The <code>fromJson</code> factory constructor creates an instance of the model class from a JSON map. It reads JSON keys and assigns their values to the class properties.

beginner

Why do we use the toJson method in a model class?

The <code>toJson</code> method converts the model class instance back into a JSON map. This is useful when sending data to a server or saving it in JSON format.

intermediate

How do you handle nested JSON objects in Flutter model classes?

For nested JSON objects, create separate model classes for each nested structure. Use their <code>fromJson</code> and <code>toJson</code> methods inside the parent model class to convert nested data.

beginner

What Dart type is commonly used to represent JSON maps in model classes?

The Dart type Map<String, dynamic> is used to represent JSON maps because JSON keys are strings and values can be any type.

Which method in a Flutter model class converts JSON data into a Dart object?

A toJson

B initState

C build

D fromJson

The fromJson factory constructor reads JSON and creates a Dart object.

What Dart type is used to represent JSON data in model classes?

A List<dynamic>

B String

C Map<String, dynamic>

D int

JSON objects are represented as Map in Dart.

Why do we create separate model classes for nested JSON objects?

A To handle nested data clearly and convert it easily

B To make code longer

C Because Dart requires it

D To avoid using JSON

Separate classes help manage nested JSON data clearly and convert it properly.

What does the toJson method do in a Flutter model class?

A Converts Dart object to JSON map

B Converts JSON to Dart object

C Builds UI widgets

D Starts the app

toJson converts the Dart object back into JSON format.

Which keyword is used to create a constructor that returns an instance from JSON?

A async

B factory

C static

D final

The factory keyword defines a constructor that can return an instance, often used for fromJson.

Explain how to create a Flutter model class from JSON data.

Think about how you turn a map of data into a Dart object and back.

Describe how to handle nested JSON objects in Flutter model classes.

Imagine JSON like a family tree with smaller parts inside bigger parts.

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