5: Factories: encapsulating object creation

When you discover that you need to add new types to a system, the most sensible first step is to use polymorphism to create a common interface to those new types. This separates the rest of the code in your system from the knowledge of the specific types that you are adding. New types may be added without disturbing existing code ... or so it seems. At first it would appear that the only place you need to change the code in such a design is the place where you inherit a new type, but this is not quite true. You must still create an object of your new type, and at the point of creation you must specify the exact constructor to use. Thus, if the code that creates objects is distributed throughout your application, you have the same problem when adding new types—you must still chase down all the points of your code where type matters. It happens to be the creation of the type that matters in this case rather than the use of the type (which is taken care of by polymorphism), but the effect is the same: adding a new type can cause problems.

The solution is to force the creation of objects to occur through a common factory rather than to allow the creational code to be spread throughout your system. If all the code in your program must go through this factory whenever it needs to create one of your objects, then all you must do when you add a new object is to modify the factory.

Since every object-oriented program creates objects, and since it’s very likely you will extend your program by adding new types, I suspect that factories may be the most universally useful kinds of design patterns.

Simple Factory method

As an example, let’s revisit the Shape system. One approach is to make the factory a static method of the base class:

//: c05:shapefact1:ShapeFactory1.java
// A simple static factory method.
package c05.shapefact1;
import java.util.*;
import com.bruceeckel.test.*;

abstract class Shape {
  public abstract void draw();
  public abstract void erase();
  public static Shape factory(String type) {
    if(type.equals("Circle")) return new Circle();
    if(type.equals("Square")) return new Square();
    throw new RuntimeException(
      "Bad shape creation: " + type);
  }
}

class Circle extends Shape {
  Circle() {} // Friendly constructor
  public void draw() { 
    System.out.println("Circle.draw"); 
  }
  public void erase() { 
    System.out.println("Circle.erase"); 
  }
}

class Square extends Shape {
  Square() {} // Friendly constructor
  public void draw() { 
    System.out.println("Square.draw"); 
  }
  public void erase() { 
    System.out.println("Square.erase"); 
  }
}

public class ShapeFactory1 extends UnitTest {
  String shlist[] = { "Circle", "Square", 
    "Square", "Circle", "Circle", "Square" };
  List shapes = new ArrayList();
  public void test() {
    for(int i = 0; i < shlist.length; i++)
      shapes.add(Shape.factory(shlist[i]));
    Iterator i = shapes.iterator();
    while(i.hasNext()) {
      Shape s = (Shape)i.next();
      s.draw();
      s.erase();
    }
  }
  public static void main(String args[]) {
    new ShapeFactory1().test();
  }
} ///:~

The factory( ) takes an argument that allows it to determine what type of Shape to create; it happens to be a String in this case but it could be any set of data. The factory( ) is now the only other code in the system that needs to be changed when a new type of Shape is added (the initialization data for the objects will presumably come from somewhere outside the system, and not be a hard-coded array as in the above example).

To encourage creation to only happen in the factory( ), the constructors for the specific types of Shape are made “friendly,” so factory( ) has access to the constructors but they are not available outside the package.

Polymorphic factories

The static factory( ) method in the previous example forces all the creation operations to be focused in one spot, so that’s the only place you need to change the code. This is certainly a reasonable solution, as it throws a box around the process of creating objects. However, the Design Patterns book emphasizes that the reason for the Factory Method pattern is so that different types of factories can be subclassed from the basic factory (the above design is mentioned as a special case). However, the book does not provide an example, but instead just repeats the example used for the Abstract Factory (you’ll see an example of this in the next section). Here is ShapeFactory1.java modified so the factory methods are in a separate class as virtual functions. Notice also that the specific Shape classes are dynamically loaded on demand:

//: c05:shapefact2:ShapeFactory2.java
// Polymorphic factory methods.
package c05.shapefact2;
import java.util.*;
import com.bruceeckel.test.*;

interface Shape {
  void draw();
  void erase();
}

abstract class ShapeFactory {
  protected abstract Shape create();
  private static Map factories = new HashMap();
  public static void 
  addFactory(String id, ShapeFactory f) {
    factories.put(id, f);
  }
  // A Template Method:
  public static final 
  Shape createShape(String id) {
    if(!factories.containsKey(id)) {
      try {
        // Load dynamically
        Class.forName("c05.shapefact2." + id);
      } catch(ClassNotFoundException e) {
        throw new RuntimeException(
          "Bad shape creation: " + id);
      }
      // See if it was put in:
      if(!factories.containsKey(id))
        throw new RuntimeException(
          "Bad shape creation: " + id);
    }
    return 
      ((ShapeFactory)factories.get(id)).create();
  }
}

class Circle implements Shape {
  private Circle() {}
  public void draw() { 
    System.out.println("Circle.draw"); 
  }
  public void erase() { 
    System.out.println("Circle.erase");
  }
  private static class Factory 
  extends ShapeFactory {
    protected Shape create() { 
      return new Circle(); 
    }
  }
  static {
    ShapeFactory.addFactory(
      "Circle", new Factory());
  }
}

class Square implements Shape {
  private Square() {} 
  public void draw() { 
    System.out.println("Square.draw"); 
  }
  public void erase() { 
    System.out.println("Square.erase"); 
  }
  private static class Factory 
  extends ShapeFactory {
    protected Shape create() { 
      return new Square(); 
    }
  }
  static {
    ShapeFactory.addFactory(
      "Square", new Factory());
  }
}

public class ShapeFactory2 extends UnitTest {
  String shlist[] = { "Circle", "Square", 
    "Square", "Circle", "Circle", "Square" };
  List shapes = new ArrayList();
  public void test() {
    // This just makes sure it will complete 
    // without throwing an exception.
    for(int i = 0; i < shlist.length; i++)
      shapes.add(
        ShapeFactory.createShape(shlist[i]));
    Iterator i = shapes.iterator();
    while(i.hasNext()) {
      Shape s = (Shape)i.next();
      s.draw();
      s.erase();
    }
  }
  public static void main(String args[]) {
    new ShapeFactory2().test();
  }
} ///:~

Now the factory method appears in its own class, ShapeFactory, as the create( ) method. This is a protected method which means it cannot be called directly, but it can be overridden. The subclasses of Shape must each create their own subclasses of ShapeFactory and override the create( ) method to create an object of their own type. The actual creation of shapes is performed by calling ShapeFactory.createShape( ), which is a static method that uses the Map in ShapeFactory to find the appropriate factory object based on an identifier that you pass it. The factory is immediately used to create the shape object, but you could imagine a more complex problem where the appropriate factory object is returned and then used by the caller to create an object in a more sophisticated way. However, it seems that much of the time you don’t need the intricacies of the polymorphic factory method, and a single static method in the base class (as shown in ShapeFactory1.java) will work fine.

Notice that the ShapeFactory must be initialized by loading its Map with factory objects, which takes place in the static initialization clause of each of the Shape implementations. So to add a new type to this design you must inherit the type, create a factory, and add the static initialization clause to load the Map. This extra complexity again suggests the use of a static factory method if you don’t need to create individual factory objects.

Abstract factories

The Abstract Factory pattern looks like the factory objects we’ve seen previously, with not one but several factory methods. Each of the factory methods creates a different kind of object. The idea is that at the point of creation of the factory object, you decide how all the objects created by that factory will be used. The example given in Design Patterns implements portability across various graphical user interfaces (GUIs): you create a factory object appropriate to the GUI that you’re working with, and from then on when you ask it for a menu, button, slider, etc. it will automatically create the appropriate version of that item for the GUI. Thus you’re able to isolate, in one place, the effect of changing from one GUI to another.

As another example suppose you are creating a general-purpose gaming environment and you want to be able to support different types of games. Here’s how it might look using an abstract factory:

//: c05:Games.java
// An example of the Abstract Factory pattern.
import com.bruceeckel.test.*;

interface Obstacle {
  void action();
}

interface Player {
  void interactWith(Obstacle o);
}

class Kitty implements Player {
  public void interactWith(Obstacle ob) {
    System.out.print("Kitty has encountered a ");
    ob.action();
  }
}

class KungFuGuy implements Player {
  public void interactWith(Obstacle ob) {
    System.out.print("KungFuGuy now battles a ");
    ob.action();
  }
}

class Puzzle implements Obstacle {
  public void action() { 
    System.out.println("Puzzle"); 
  }
}

class NastyWeapon implements Obstacle {
  public void action() { 
    System.out.println("NastyWeapon"); 
  }
}

// The Abstract Factory:
interface GameElementFactory {
  Player makePlayer();
  Obstacle makeObstacle();
}

// Concrete factories:
class KittiesAndPuzzles 
implements GameElementFactory {
  public Player makePlayer() { 
    return new Kitty();
  }
  public Obstacle makeObstacle() {
    return new Puzzle();
  }
}

class KillAndDismember 
implements GameElementFactory {
  public Player makePlayer() { 
    return new KungFuGuy();
  }
  public Obstacle makeObstacle() {
    return new NastyWeapon();
  }
}

class GameEnvironment {
  private GameElementFactory gef;
  private Player p;
  private Obstacle ob;
  public GameEnvironment(
    GameElementFactory factory) {
    gef = factory;
    p = factory.makePlayer(); 
    ob = factory.makeObstacle();
  }
  public void play() { p.interactWith(ob); }
}

public class Games extends UnitTest {
  GameElementFactory
    kp = new KittiesAndPuzzles(),
    kd = new KillAndDismember();
  GameEnvironment 
    g1 = new GameEnvironment(kp),
    g2 = new GameEnvironment(kd);
  // These just ensure no exceptions are thrown:
  public void test1() { g1.play(); }
  public void test2() { g2.play(); }
  public static void main(String args[]) {
    Games g = new Games();
    g.test1();
    g.test2();
  }
} ///:~

In this environment, Player objects interact with Obstacle objects, but there are different types of players and obstacles depending on what kind of game you’re playing. You determine the kind of game by choosing a particular GameElementFactory, and then the GameEnvironment controls the setup and play of the game. In this example, the setup and play is very simple, but those activities (the initial conditions and the state change) can determine much of the game’s outcome. Here, GameEnvironment is not designed to be inherited, although it could very possibly make sense to do that.

This also contains examples of Double Dispatching and the Factory Method, both of which will be explained later.

Exercises

Add a class Triangle to ShapeFactory1.java
Add a class Triangle to ShapeFactory2.java
Add a new type of GameEnvironment called GnomesAndFairies to GameEnvironment.java
Modify ShapeFactory2.java so that it uses an Abstract Factory to create different sets of shapes (for example, one particular type of factory object creates “thick shapes,” another creates “thin shapes,” but each factory object can create all the shapes: circles, squares, triangles etc.).

chris@sunpack.com Last Update:09/08/2001