Factory constructors return am instance of the class, but it doesn't necessarily create a new instance. Factory constructors might return an instance that already exists, or a sub-class.

Rules for factory constructors:

• Factory constructors do use the return key word.
• You cannot refer to 'this' within the factory constructor.

    
class Cat {
    String name;
    String color;
    // factory constructor that returns a new instance 
    factory Cat.fromJson(Map json) {
        return Cat(json['name'], json['color']);
    }
}