Creational Patterns CSE 8313

2 Creational Patterns Class creational pattern ◦ uses inheritance to vary the class that is instantiated Object creational pattern ◦ delegates instantiation to another object Systems evolve to depend more on object composition than class inheritance Two recurring themes: ◦ Encapsulate knowledge about which concrete classes the system uses ◦ Hide how instances of these classes are created and put together

The Maze Game Popular videogame Centerpiece: a class that generates maze layouts ◦ creates random mazes to be solved ◦ different for every game ◦ MazeGame

Maze creation process Invoked for each game

5 Creational Patterns – Maze Example class MapSite { public: virtual void Enter() = 0; }; enum Direction {North, South, East, West};

6 Creational Patterns – Maze Example class Room : public MapSite { public: Room(int roomNo); MapSite* GetSide(Direction) const; void SetSide(Direction, MapSite*); virtual void Enter(); private: MapSite* _sides[4]; int _roomNumber; }; class Wall : public MapSite { public: Wall(); virtual void Enter(); }; class Door : public MapSite { public: Door(Room* = 0, Room* = 0); virtual void Enter(); Room* OtherSideFrom(Room*); private: Room* _room1; Room* _room2; bool _isOpen; };

7 Creational Patterns – Maze Example class Maze { public: Maze(); void AddRoom(Room*); Room* RoomNo(int) const; private: //... }; Maze* MazeGame::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; }

Maze creation process public class MazeGame { public 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(MapSite.NORTH, new Wall()); r1.setSide(MapSite.EAST, theDoor); r1.setSide(MapSite.SOUTH, new Wall()); r1.setSide(MapSite.WEST, new Wall()); r2.setSide(MapSite.NORTH, new Wall()); r2.setSide(MapSite.EAST, new Wall()); r2.setSide(MapSite.SOUTH, new Wall()); r2.setSide(MapSite.WEST, theDoor); return aMaze; } //... }

Change: game extensions New Features ◦ add new types of mazes to the game … ◦ … without changing the overall logic according to which the game works ◦ in particular how it creates the mazes Example: besides regular mazes ◦ Add enchanted mazes ◦ Add bombed mazes ◦ … etc.

Solutions with current code public class MazeGame { public 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(MapSite.NORTH, new Wall()); r1.setSide(MapSite.EAST, theDoor); r1.setSide(MapSite.SOUTH, new Wall()); r1.setSide(MapSite.WEST, new Wall()); r2.setSide(MapSite.NORTH, new Wall()); r2.setSide(MapSite.EAST, new Wall()); r2.setSide(MapSite.SOUTH, new Wall()); r2.setSide(MapSite.WEST, theDoor); return aMaze; } //... } 1. Duplicate code of createMaze() ◦ createEnchantedMaze() ◦ createBombedMaze() 2. Add switch/case statements every time a constructor is invoked ◦ based on some flag variable 3. … 4. Re-factor!

Refactoring maze creation Factory Methods Client still invokes this method

12 Creational Patterns – Maze Example CreateMaze calls virtual functions instead of constructor calls to create the rooms, walls, and doors (Factory Method pattern) CreateMaze is passed an object as a parameter to use to create rooms, walls, and doors (Abstract Factory pattern) CreateMaze is passed an object that can create a new maze in its entirety using operations for adding rooms, doors, and walls to the maze it builds (Builder pattern) CreateMaze is parameterized by various prototypical room, door, and wall objects to be copied and added to the maze (Prototype pattern)

Class Creational Pattern

Factory methods Each of the factory methods wraps the invocation of corresponding constructor A set of methods that can be inherited and overridden Examples (See Code): Room makeRoom(int id) { return new Room(id); } Wall makeWall() { return new Wall(); }

Creating the maze

Build Enchanted Products

Enchanted Maze Creator createMaze() can still be invoked to create regular mazes or enchanted mazes without modification

Enchanted Maze Creator public class EnchantedMazeGame extends MazeGame { public Room makeRoom(int n) { return new EnchantedRoom(n);} public Wall makeWall() { return new EnchantedWall();} public Door makeDoor(Room r1, Room r2) { return new EnchantedDoor(r1, r2);} }

Build Bombed Mazes r

21 We made createMaze() just slightly more complex, but a lot more flexible! Consider this EnchantedMazeGame class: public class EnchantedMazeGame extends MazeGame { public Room makeRoom(int n) {return new EnchantedRoom(n);} public Wall makeWall() {return new EnchantedWall();} public Door makeDoor(Room r1, Room r2) {return new EnchantedDoor(r1, r2);} } The createMaze() method of MazeGame is inherited by EnchantedMazeGame and can be used to create regular mazes or enchanted mazes without modification!

Properties of this solution The client component in the game that invokes the creation of mazes does not need to change It interacts with different mazes creator classes ◦ Depending which extension has been selected by the player ◦ in exactly the same way as in the original game Caveat: ◦ Recall we need a “global” flag that tells us which MazeCreator subclass we need to instantiate in every game

The Factory Method pattern - structure

Advantages The Creator provides a factory method that substitute constructor of ConcreteProducts ◦ The business logic of product creation, initialization etc. can be wholly encapsulated in those methods The client of Creator can ask for the production of different Products in a uniform way ◦ And use them uniformly (all derive from main Product super-class) ◦ Without needing to know the nitty-gritty details

The Factory Method pattern Classification: ◦ Creational purpose; Class scope Context: dynamic creation of different types of objects depending on context, transparent the client Problem: a client class needs to instantiate one of many derivations of another class, but does not know which one. Solution: define an interface for creation, and delegate to a derived class of that interface the decision of what class to instantiate and how Consequences: ◦ Need for parallel Product/Creator hierarchies ◦ The logic of creating a particular types of product is encapsulated in each Creator

26 Intent: ◦ Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. The Factory Method pattern

27 Motivation: consider a framework for applications that can present multiple documents to the user The createDocument() method is a factory method. The Factory Method pattern

28 Use Factory Method pattern when ◦ a class can't anticipate the class of objects it must create ◦ a class wants its subclasses to specify the objects it creates The Factory Method pattern

29 Structure and Participants Collaborations ◦ Creator relies on its subclasses to implement the factory method so that it returns an instance of the appropriate ConcreteProduct (Document) (MyDocument) (Application) (My Application) The Factory Method pattern

30 So what exactly does it mean when we say that "the Factory Method Pattern lets subclasses decide which class to instantiate?" ◦ It means that Creator class is written without knowing what actual ConcreteProduct class will be instantiated. The ConcreteProduct class which is instantiated is determined solely by which ConcreteCreator subclass is instantiated and used by the application. ◦ It does not mean that somehow the subclass decides at runtime which ConreteProduct class to create The Factory Method pattern

31 Example 1: Clients can also use factory methods. The factory method in this case is createManipulator() The Factory Method pattern

32 Example 2: MazeGame Revisit class MazeGame { public: Maze* CreateMaze(); // factory methods: virtual Maze* MakeMaze() const { return new Maze; } virtual Room* MakeRoom(int n) const { return new Room(n); } virtual Wall* MakeWall() const { return new Wall; } virtual Door* MakeDoor(Room* r1, Room* r2) const { return new Door(r1, r2); } }; The Factory Method pattern

33 The reason this works is that the createMaze() method of MazeGame defers the creation of maze objects to its subclasses. That's the Factory Method pattern at work! In this example, the correlations are: ◦ Creator => MazeGame ◦ ConcreteCreator => EnchantedMazeGame (MazeGame is also a ConcreteCreator) ◦ Product => MapSite ◦ ConcreteProduct => Wall, Room, Door, EnchantedWall, EnchantedRoom, EnchantedDoor The Factory Method pattern

34 Consequences ◦ Benefits  Code is made more flexible and reusable by the elimination of instantiation of application-specific classes  Code deals only with the interface of the Product class and can work with any ConcreteProduct class that supports this interface ◦ Liabilities  Clients might have to subclass the Creator class just to instantiate a particular ConcreteProduct Implementation Issues ◦ Creator can be abstract or concrete ◦ 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. The Factory Method pattern

Factory Method in the real world Example: iterator() in Java Collections Depending on the Collection type being used, it returns the right iterator object ◦ which implements the right traversal algorithm for that Collection

Object Creational Patterns

Abstract object instantiation Add one more level of abstraction on top of OO languages What’s the use of the extra abstraction layer?

Object Creational Patterns - motivation Evolution and extendibility of the system Do not hardcode object creation ◦ Type selection is static when using constructor ◦ Prepare for more types of similar objects to enter the design The extra layer of abstraction enables to configure the system dynamically ◦ Depending on the configuration, the system will create those new types

Analogy: factory Imagine a company with many different products in the same product family ◦ and 1 production plant: a factory The more flexible the plant, the more successful the company’s business!

Analogy: factory You want the capability of making different products in the same production plant ◦ Simply by hitting a switch The production procedure followed by the factory is the same ◦ independent from the product being produced ◦ the switch controls what machinery is activated during the production process Result: a different final product

Abstract Factory pattern Abstract Factory Similar to Factory method Let’s see the difference in our Maze game example …

MazeGame Abstract Factory

 The createMaze() now method takes a MazeFactory reference as a parameter

Enchanted Feature

Bombed Feature

Abstract Factory - structure

Dependency Inversion Principle

Abstract Factory vs. Factory Method Slightly more elegant than Factory Method in our example Where is the difference? In fact, very similar to the Factory Method pattern ◦ in Abstract Factory, a class delegates the responsibility of object instantiation to another one via composition ◦ the Factory Method pattern uses inheritance to handle the desired object instantiation.

When to use Abstract Factory Pattern When a system should be independent of how its products are created, composed, and represented When a class can't anticipate the class of objects it must create When a system must use just one of a multiple families of product objects When a family of related product objects is designed to be used together, and you need to enforce this constraint

The Abstract Factory pattern Classification: ◦ Creational purpose; Class scope Context: there are multiple libraries or sets of classes that are to be chosen depending on context Problem: families of related objects need to be instantiated together Solution: coordinates the creation of objects of the same family. Client remains agnostic on the procedure and the rules about which object is in which family Consequences: ◦ The logic of creating a particular object family is kept hidden from client ◦ Enforces family rules ◦ Supporting new product requires changing the AbstractFactory interface

Real-world example: A GUI toolkit that supports multiple look-and-feels

The Factory Method Pattern defines an interface for creating an object, but let subclasses decide which class to instantiate. Factory method lets a class defer instantiation to the subclasses The Abstract Factory Pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes.

Bullet Points All factories encapsulate object creation Factory Method relies on inheritance: object creation is delegated to subclasses which implement the factory method to create objects All factory patterns promote loose coupling by reducing the dependency of your application on concrete classes

Bullet Points The intent of Factory Method is to allow a class to defer instantiation to its subclasses The intent of Abstract Factory is to create families of related objects without having to depend on their concrete classes.

Class recap Creational patterns ◦ Factory method ◦ Abstract Factory ◦ Builder

Design Principles Open Close Principle Dependency Inversion ◦ A. High-level modules should not depend on low-level modules. Both should depend on abstractions. ◦ B. Abstractions should not depend upon details. Details should depend upon abstractions. Information Hiding