Factory Design Pattern

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Aliases 📌: Virtual constructor

Description 📚: we can use it to control how a class instances are created, for instance, to "hide" which class we effectively created.

public A someMethod() { return new XXX(); }

➡️ We say that the return type is A but the class instantiated may be a subclass or a class implementing A.

Advantages ✅

  • More flexible, we can change the instantiated class or the constructor call easily.

Disadvantages 🚫

  • We may create have to create a lot of classes

Notes 📝

  • An example would be SomeClass#iterator() method in Java

Java implementations

// ------------ factory related ------------- //
// should manage the creation of RPG Units, UnitCreator.java
public interface UnitCreator {

    // kind of units
    enum UnitName { HERO, MONSTER, NPC }

    // factory method, @see RPGUnit
    RPGUnit getUnit(UnitName name);

}

// Concrete and real creator
public class Game implements UnitCreator {
    // create unit
    @Override
    public RPGUnit getUnit(UnitName name) {
        switch (name){
            case HERO: return new Hero();
            // not done
            case NPC: case MONSTER: default: 
                throw new IllegalArgumentException("not implemented yet");
        }
    }

    // test
    public static void main(String[] args) {
        Game game = new Game();
        // we "don't know" what unit go created
        RPGUnit hero = game.getUnit(UnitName.HERO);
        System.out.println(hero); // Hero
    }
}

// ------------ created ------------- //
// abstract unit, RPGUnit.java
public abstract class RPGUnit {
    // print unit name
    public abstract String toString();
}

// Hero.java
public class Hero extends RPGUnit {
    @Override
    public String toString() { return "Hero"; }
}