fromJson
- Dart course
- Advanced
- Lesson 39/83
- 5 min
Learn to create objects with fromJson.
The fromJson factory
fromJson is a factory method that builds an instance from a JSON Map. It provides type-safe access; safer than dynamic.
Example: factory User.fromJson(Map<String, dynamic> m) => User(m['name'] as String, m['age'] as int);. Each JSON field is cast.
Usage: var u = User.fromJson(jsonDecode(s) as Map<String, dynamic>);. Now u.name works with type safety.
Null-safe fromJson
JSON fields may be missing; therefore fromJson is written null-safe. m['name'] as String? makes it nullable, or a default is given: m['name'] ?? 'guest'.
For numeric fields, (m['age'] as num).toInt() is safe; the int/double distinction in JSON can be ambiguous.
For nested objects, the inner class's fromJson is called: Address.fromJson(m['address'] as Map<String, dynamic>).
Practice Define User.fromJson
Define class User { String name; int age; User(this.name, this.age); factory User.fromJson(Map<String, dynamic> m) => User(m['name'] as String, m['age'] as int); }. Inside main add var u = User.fromJson({'name': 'Ayşe', 'age': 30}); and print(u.name);.
Starter code
void main() {
// define the User class with fromJson (outside main)
// create an instance via fromJson; print name
}Sample solution
class User {
String name;
int age;
User(this.name, this.age);
factory User.fromJson(Map<String, dynamic> m) =>
User(m['name'] as String, m['age'] as int);
}
void main() {
var u = User.fromJson({'name': 'Ayşe', 'age': 30});
print(u.name);
}Expected output
Ayşe
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the fromJson parts
| Term | Meaning |
|---|---|
factory |
Factory keyword |
Map<String, dynamic> |
Input type |
as String |
Type cast |
User(...) |
Instance creation |
2 What does fromJson return?
- String
- Map
- Class instance Correct answer
- Future
Explanation: fromJson returns a class instance.
3 Type cast
m['name'] ____ Stringm['name'] as StringExplanation: 'as String' casts dynamic to String.