【设计模式】之工厂方法（Factory Method）

无论是工厂方法还是抽象工厂，所谓工厂就是生产之用，工厂方法的作用就是生产一个对象。

工厂方法的定义为：为创建对象定义一个接口，但是让子类决定实例化哪一个类。工厂方法让一个类把实例化推迟到子类。

英文定义为：Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defter instantiation to subclasses.

《Header First》介绍一个OO设计的原则----Depend Upon abstractions. Do not depend upon concrete classes. 可以在<GOF>中找到类似说法Frameworks use abstract classes to define and maintain relationships between objects. 可以看出，工厂方法将抽象与实现分离开来，可以让客户端不用关心对象的具体实现，只需关心对象见的关系。

抽象工厂的结构图如下

如果父类中的工厂方法没有实现内容，就用到了模板方法（template method）的设计模式，如果有实现内容，则工厂方法提供了生成一个默认对象的方法，这样增加了工厂方法的灵活性，子类可以无需override。

在平行继承的结构中，工厂方法可以起到联系两个继承体系的作用；

可以在工厂方法中使用lazy initialization.

工厂方法使用应该是非常广泛，下面给出的示例代码来自<Header First>

public abstract class PizzaStore {
    public Pizza orderPizza(String type) {
        Pizza pizza;

        pizza = createPizza(type);
        
        pizza.prepare();
        pizza.bake();
        pizza.cut();
        pizza.box();

        return pizza;
    }

    protected abstract Pizza createPizza(String type);
}

public class NYPizzaStore extends PizzaStore {
    public Pizza createPizza(String type) {
        if (type.equals("cheese")) {
            return new NYStyleCheesePizza();
        }
        return null;
    }
}

public class ChicagoPizzaStore extends PizzaStore  {
    protected Pizza createPizza(String type) {
        if (type.equals("cheese")) {
            return new ChicagoStyleCheesePizza();
        }
        return null;
    }
}
import java.util.ArrayList;
public abstract class Pizza {
    String name;
    String dough;
    String sauce;
    ArrayList<String> toppings = new ArrayList<String>();

    void prepare() {
        System.out.println("Preparing " + name);
        System.out.println("Tossing dough...");
        System.out.println("Adding sauce...");
        System.out.println("Adding toppings: ");
        for (int i = 0; i < toppings.size(); i++) {
            System.out.println("    " + toppings.get(i));
        }
    }

    void bake() {
        System.out.println("Bake for 25 minutes at 350");
    }

    void cut() {
        System.out.println("Cutting the pizza into diagonal slices");
    }

    void box() {
        System.out.println("Place pizza in official PizzaStore box");
    }

    public String getName() {
        return name;
    }
}

public class NYStyleCheesePizza extends Pizza {
    public NYStyleCheesePizza() {
        name = "NY Style Sause and Cheese Pizza";
        dough = "Thin Crust Dough";
        sauce = "Marinara Sauce";
        toppings.add("Grated Reggiano Cheese");
    }
}


public class ChicagoStyleCheesePizza extends Pizza {
    public ChicagoStyleCheesePizza() {
        name = "Chicago Style Deep Dish Cheese Pizza";
        dough = "Extra Thick Crust Dough";
        sauce = "Plum Tomato Sauce";
        toppings.add("Shredded Mozzarella Cheese");
    }

    void cut() {
        System.out.println("Cutting the pizza into square slices");
    }
}
import java.util.ArrayList;
public abstract class Pizza {
    String name;
    String dough;
    String sauce;
    ArrayList<String> toppings = new ArrayList<String>();

    void prepare() {
        System.out.println("Preparing " + name);
        System.out.println("Tossing dough...");
        System.out.println("Adding sauce...");
        System.out.println("Adding toppings: ");
        for (int i = 0; i < toppings.size(); i++) {
            System.out.println("    " + toppings.get(i));
        }
    }

    void bake() {
        System.out.println("Bake for 25 minutes at 350");
    }

    void cut() {
        System.out.println("Cutting the pizza into diagonal slices");
    }

    void box() {
        System.out.println("Place pizza in official PizzaStore box");
    }

    public String getName() {
        return name;
    }
}

factory method 与 abstract factory的区别是 factory method只生产一个产品，而abstract factory则生产的是families of products.

abstract factory为生产产品集中的每一个产品时可以使用factory method。