• Creational design pattern
• For framework/library implementation
  • Framework phrased in terms of interfaces and abstract classes
  • Framework only knows when to instantiate, not what
  • Concrete subclasses of the framework decide what to instantiate

Problem illustration: Application and Document are a framework for implementing document-processing applications.

What kind of specific Document should application create?

• Machine to manufacture plastic toys
• Molds define diferent shapes
• Machine injects plastic into molds of desired shape

• All products share the same interface Product
• Implement a Creator with factory method (return type Product)
• Add one subclass of Creator for each subclass of Product
• Replace references to concrete product constructors with calls to factory method
• Naming convention: use a method name that suggests factory method, e.g., Class doMakeClass

public abstract class MazeGame {
  protected abstract Room doMakeRoom();
}
public abstract class Room {}

// Concrete creator and product: dungeon game
public class DungeonCrawler extends MazeGame {
  @Override protected Room doMakeRoom() {
    return new Chamber();
  }
}
public class Chamber extends Room {}

// Concrete creator and product: enchanted maze game
public class EnchantedMazeGame extends MazeGame {
  @Override protected Room doMakeRoom() {
    return new EnchantedRoom();
  }
}
public class EnchantedRoom extends Room {}

Before: Two test cases are implemented similarly

public class XMLBuilderTests extends TestCase {
  public void testAddRootSibling() {
    XMLBuilder builder = new XMLBuilder("orders");
    builder.addBelow("orders", "order");
    try {
      builder.addSibling("orders", "customer");
      fail("Expected to fail: add sibling to root");
    } catch (InvalidArgumentException e) { /* expected exception */ }
  }
}

public class DOMBuilderTests extends TestCase {
  public void testAddRootSibling() {
    DOMBuilder builder = new DOMBuilder("orders");
    builder.addBelow("orders", "order");
    try {
      builder.addSibling("orders", "customer");
      fail("Expected to fail: add sibling to root");
    } catch (InvalidArgumentException e) { /* expected exception */ }
  }
}