Factory pattern là gì?

Factory tách "tạo object" khỏi "dùng object". Caller không gọi new ConcreteClass() mà gọi factory; factory quyết định trả về concrete nào.

Vì sao cần: thay implementation, ẩn complexity, cache/pool, dễ test (mock factory).

3 biến thể chính:

1. Simple Factory:

class ShapeFactory {
  static Shape create(String type) {
    return switch (type) {
      case "circle" -> new Circle();
      case "square" -> new Square();
      default -> throw new IllegalArgumentException();
    };
  }
}

**2.

Factory Method (GoF):** subclass quyết định concrete class.

abstract class Dialog {
  public void render() { createButton().onClick(); }
  protected abstract Button createButton();
}
class MacDialog extends Dialog {
  protected Button createButton() { return new MacButton(); }
}

3. Abstract Factory (GoF): factory tạo họ object liên quan (UI theme: button + checkbox cùng style).

Static factory method trên class (hiện đại, ưa chuộng): LocalDate.now(), List.of(...), Optional.empty(), Integer.valueOf(5) (có cache cho -128..127).

Khi không dùng:
- Class đơn giản, 1 cách tạo → new thẳng. Đừng over-engineer.
- Có DI framework (Spring) — container đã là factory siêu cấp.

Factory là nền tảng cho Dependency Injection — caller phụ thuộc interface, không phụ thuộc concrete.

Factory separates "creating objects" from "using objects". Callers do not call new ConcreteClass(); they ask a factory which decides what to return.

Why use it: swap implementations, hide complexity, caching/pooling, easier testing.

Three main variants:

1. Simple Factory:

class ShapeFactory {
  static Shape create(String type) {
    return switch (type) {
      case "circle" -> new Circle();
      case "square" -> new Square();
      default -> throw new IllegalArgumentException();
    };
  }
}

**2.

Factory Method (GoF):** subclasses decide the concrete class.

abstract class Dialog {
  public void render() { createButton().onClick(); }
  protected abstract Button createButton();
}
class MacDialog extends Dialog {
  protected Button createButton() { return new MacButton(); }
}

3. Abstract Factory (GoF): factory creating families of related objects (UI theme: button + checkbox of the same style).

Static factory methods on the class (modern, preferred): LocalDate.now(), List.of(...), Optional.empty(), Integer.valueOf(5) (cached for -128..127).

When not to use:
- Simple class, one way to create → just use new. Avoid over-engineering.
- Have a DI framework (Spring) — the container is already a super-factory.

Factories underpin Dependency Injection — callers depend on interfaces, not concrete classes.