1. FACTORY METHOD

2. Design Pattern Space Purpose ScopeCreationalStructuralBehavioral ClassFactory MethodAdapterInterpreter Template Method ObjectAbstract factory Builder Prototype Singleton Adapter Bridge Composite Facade Flyweight Proxy Chain of Responsibility Command Iterator Mediator Memento State Strategy Visitor Observer

3. Intent  Define an interface for creating an object, but let subclasses decide which class to instantiate  Like other creational patterns, it deals with the problem of creating objects (products) without specifying the exact class of object that will be created  More generally, the term Factory Method is often used to refer to any method whose main purpose is to create objects

4. Problem  A framework needs to standardize the architectural model for a range of applications, but allow for individual applications to define their own domain objects and provide for their instantiation.

5. Applicability  The factory pattern can be used when:  The creation of an object makes reuse impossible without significant duplication of code.  The creation of an object requires access to information or resources that should not be contained within the composing class.  The lifetime management of the generated objects must be centralized to ensure a consistent behavior within the application.

6. Structure

7. Example: Maze Game

8. Maze Class Version 1  Now a maze game has to make a maze, so we might have something like: class MazeGame { public Maze createMaze() { Maze aMaze = new Maze(); Room r1 = new Room( 1 ); Room r2 = new Room( 2 ); Door theDoor = new Door( r1, r2); aMaze.addRoom( r1 ); aMaze.addRoom( r2 ); r1.setSide( North, new Wall() ); r1.setSide( East, theDoor ); r1.setSide( South, new Wall() ); r1.setSide( West, new Wall() ); r2.setSide( North, new Wall() ); r2.setSide( East, new Wall() ); r2.setSide( South, new Wall() ); r2.setSide( West, theDoor ); return aMaze; }

9. Problem  How do we make Other Mazes? Solution Idea 1 - Subclass MazeGame, override createMaze

10. class BombedMazeGame extends MazeGame { public Maze createMaze() { Maze aMaze = new Maze(); Room r1 = new RoomWithABomb( 1 ); Room r2 = new RoomWithABomb( 2 ); Door theDoor = new Door( r1, r2); aMaze.addRoom( r1 ); aMaze.addRoom( r2 ); r1.setSide( North, new BombedWall() ); r1.setSide( East, theDoor ); r1.setSide( South, new BombedWall() ); r1.setSide( West, new BombedWall() ); … Note the amount of cut and paste!

11. How do we make Other Mazes? Idea 2 - Factory Method class MazeGame { public Maze makeMaze() { return new Maze(); } public Room makeRoom(int n ) { return new Room( n ); } public Wall makeWall() { return new Wall(); } public Door makeDoor(Room r1, Room r2) { return new Door(r1, r2); } public Maze CreateMaze() { Maze aMaze = makeMaze(); Room r1 = makeRoom( 1 ); Room r2 = makeRoom( 2 ); Door theDoor = makeDoor( r1, r2); aMaze.addRoom( r1 ); aMaze.addRoom( r2 );

12. r1.setSide( North, makeWall() ); r1.setSide( East, theDoor ); r1.setSide( South, makeWall() ); r1.setSide( West, makeWall() ); r2.setSide( North, makeWall() ); r2.setSide( East, makeWall() ); r2.setSide( South, makeWall() ); r2.setSide( West, theDoor ); return aMaze; }

13.  Now subclass MazeGame override make methods  CreateMaze method stays the same class BombedMazeGame extends MazeGame { public Room makeRoom(int n ) { return new RoomWithABomb( n ); } public Wall makeWall() { return new BombedWall(); }

14. Implementation Two major varieties  Creator class is an abstract class:  Does not provide default implementation of factory methods abstract class MazeGame { public Maze makeMaze(); public Room makeRoom(int n ); public Wall makeWall(); public Door makeDoor();

15.  Creator is a concrete class  Provides a default implementation for the factory method class MazeGame { public Maze makeMaze() { return new Maze(); } public Room makeRoom(int n ) { return new Room( n ); } public Wall makeWall() { return new Wall(); } public Door makeDoor(Room r1, Room r2) { return new Door(r1, r2); }  "Create objects in a separate operation so that subclasses can override the way they're created"

16. Another Example

17. interface IIceCream { string Functionality(); } class ChocolateIceCream : IIceCream { public string Functionality() { return "Chocolate Ice cream"; } } class VanillaIceCream : IIceCream { public string Functionality() { return "Vanilla Ice cream"; } } class StrawberryIceCream : IIceCream { public string Functionality() { return "Strawberry Ice cream"; } } /* client */ static class Factory { /// This is the Factory method /// public static IIceCream Get(int id) { switch (id) { case 0: return new ChocolateIceCream(); case 1: return new VanillaIceCream(); case 2: return new StrawberryIceCream(); default: return null; } } }

18. Consequences  Benefits  More flexible and reusable code by the elimination of instantiation of application-specific classes  Deals only with interface of Product class and can work with any ConcreteProduct class that supports this interface  Implementation Issues  Should the factory method be able to create multiple kinds of products? If so, then the factory method has a parameter (possibly used in an if-else!) to decide what object to create.