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Factory

  • Intent:

    Encapsulate the conditional creation of different objects that share a common super class

  • How it works:

    Create a factory class that has a method that can be called for object creation. This method then returns the proper object. The proper object is chosen at runtime.

  • Use when:

    • you do not know ahead of time which class object you need
    • all of the potential classes are in the same subclass hierarchy
    • you want to centralize class selection code
    • you do not want the client to need to know every subclass
    • you want to encapsulate object creation

  • Structure:

    class diagram

  • Example Code:

    View on Github

    void main(List<String> args) {
  Animal animal = AnimalFactory().getAnimal(args[0]);
  animal.makeNoise();
}

class AnimalFactory{
  Animal getAnimal(String name){
    if(name == "cat"){
      return Cat(name);
    }
    else if(name == "dog"){
      return Dog(name);
    }
    throw Exception("Error: The name " + name + " is unknown!");
  }
}

abstract class Animal{
  String name;
  
Animal(this.name);

  void makeNoise();
}

class Dog extends Animal{
  Dog(String name) : super(name);

  @override
  void makeNoise() {
    print("WauWau");
  }
}

class Cat extends Animal{
  Cat(String name) : super(name);

  @override
  void makeNoise() {
    print("Miauuu");
  }
}