Factory method pattern(工廠方法模式)

定義：Define an interface for creating an object,but let subclasses decide which class to
instantiate. Factory Method lets a class defer instantiation to subclasses. (為建立的物件定一個介面，但是是讓子類別去決定要讓哪一個類別要實體化，工廠方法讓一個類別的實體化推遲到子類別。)

Factory method pattern依靠繼承(inheritance)，將建立object的工作委託給子類別實作的factory method。

參與角色(Participants)

• Product(抽象產品)：定義了factory method要建立object的interface，定義這類產品有甚麼方法。
• ConcreteProduct(具體產品)：實作(implement)Product interface。
• Creator(抽象工廠)：宣告factory method，這個method要回傳一個Product type。Creator也可以定義一個預設的factory method，回傳一個預設的ConcreteProduct object。
• ConcreteCreator(實體工廠)：override factory method，實作要回傳哪一種ConcreteProduct instance，包含了實體化object的business logic。

簡單的Java實作

	public class ConcreteCreatorA implements Creator {
	@Override
	public Product createProdut(){
	return new ProductA();
	}
	}

view raw ConcreteCreatorA.java hosted with ❤ by GitHub

	public class ConcreteCreatorB implements Creator {
	@Override
	public Product createProdut(){
	return new ProductB();
	}
	}

view raw ConcreteCreatorB.java hosted with ❤ by GitHub

	public interface Creator {
	// this is a Factory Method.
	Product createProdut();
	}

view raw Creator.java hosted with ❤ by GitHub

	public interface Product {
	String getProductName();
	}

view raw Product.java hosted with ❤ by GitHub

	public class ProductA implements Product{
	public String getProductName() {
	return "Product A";
	}
	}

view raw ProductA.java hosted with ❤ by GitHub

	public class ProductB implements Product{
	public String getProductName() {
	return "Product B";
	}
	}

view raw ProductB.java hosted with ❤ by GitHub

	public class TestFactoryMethod{
	public static void main(String[] args) {
	ConcreteCreatorA creatorA = new ConcreteCreatorA();
	ConcreteCreatorB creatorB = new ConcreteCreatorB();
	Product productA = creatorA.createProdut();
	Product productB = creatorB.createProdut();
	}
	}

view raw TestFactoryMethod.java hosted with ❤ by GitHub

好處：

• 客戶只需要知道如何使用factory method來建立Product object，不需要關心要如何建立。
• 如果建立這個object很複雜的話，可以將建立的過程隱藏起來。
• 很容易的增加Product類別，假設要增加一個ProductC，只要實作ProductC，以及ConcreteCreatorC就可以了。

壞處：

• 類別數增加，增加系統開銷。