Wzorzec Factory Method

Cel wzorca Factory Method

Problem bez wzorca Factory Method

// Client knows too much:
Shape shape;
if (type.equals("circle")) shape = new Circle(r);
else if (type.equals("rect")) shape = new Rectangle(w, h);
// Every new shape requires editing this code.

Abstrakcyjna klasa twórcy

public abstract class Dialog {
    public void render() {
        Button btn = createButton(); // factory method
        btn.onClick();
    }
    protected abstract Button createButton();
}

Konkretni twórcy

public class WindowsDialog extends Dialog {
    @Override
    protected Button createButton() { return new WindowsButton(); }
}
public class WebDialog extends Dialog {
    @Override
    protected Button createButton() { return new HtmlButton(); }
}

Interfejs produktu

public interface Button {
    void onClick();
    void render();
}
public class WindowsButton implements Button {
    public void onClick() { System.out.println("Windows click"); }
    public void render() { System.out.println("Render Windows button"); }
}

Statyczne metody fabrykujące a wzorzec Factory Method

// Static factory (NOT the GoF pattern)
LocalDate d = LocalDate.of(2024, 6, 15);
// GoF Factory Method uses a class hierarchy

Parametryzowana metoda fabrykująca

public class ShapeFactory {
    public static Shape create(String type, double... dims) {
        return switch (type) {
            case "circle"    -> new Circle(dims[0]);
            case "rectangle" -> new Rectangle(dims[0], dims[1]);
            default -> throw new IllegalArgumentException("Unknown: " + type);
        };
    }
}

Factory Method z typami generycznymi

public abstract class Repository<T, ID> {
    public abstract T findById(ID id);
    protected abstract T createEmpty(); // factory method
}

Rejestrowanie fabryk za pomocą mapy

Map<String, Supplier<Notification>> registry = new HashMap<>();
registry.put("email", EmailNotification::new);
registry.put("sms",   SmsNotification::new);
Notification n = registry.getOrDefault(type, DefaultNotification::new).get();

Factory Method a Abstract Factory

Przykład z praktyki: fabryka loggerów

Logger log = LoggerFactory.getLogger(MyService.class);
log.info("Service started");

Szybki test

Podsumowanie