What is the role of a factory constructor in Dart compared with a generative constructor?

Short answer

A generative constructor creates an instance of the same class, while a factory can return an existing instance, an instance of a subclass, or run selection logic before returning.

Explanation

Use a generative constructor when you want to create an object and initialize its fields directly. A factory has no direct access to this before returning, but it suits caching, parsing, or choosing an implementation. Having a factory does not automatically mean there is a singleton; the decision depends on the class logic.

Code example

class User {
  final String id;
  User._(this.id);

  factory User.fromJson(Map<String, Object?> json) {
    return User._(json["id"]! as String);
  }
}

Common mistakes

• Treating a factory as always a singleton.
• Forgetting that a factory cannot use this before the object is created.

Follow-up questions

• When do you choose List, Set, or Map in Dart?

Sources

• Dart language — Constructors