设计模式——工厂方法模式

4.

OO原则：要依赖抽象，不要依赖具体类（依赖倒置原则）

OO模式：工厂方法模式：定义了一个创建对象的接口，但由子类决定要实例化的类是哪一个。工厂方法让类把实例化推迟到子类。

例子：披萨店的披萨生产

public abstract class PizzaStore { abstract Pizza createPizza(String item); public Pizza orderPizza(String type) {Pizza pizza = createPizza(type);System.out.println("--- Making a " + pizza.getName() + " ---");pizza.prepare();pizza.bake();pizza.cut();pizza.box();return pizza;}}

public class NYPizzaStore extends PizzaStore {Pizza createPizza(String item) {if (item.equals("cheese")) {return new NYStyleCheesePizza();} else if (item.equals("veggie")) {return new NYStyleVeggiePizza();} else if (item.equals("clam")) {return new NYStyleClamPizza();} else if (item.equals("pepperoni")) {return new NYStylePepperoniPizza();} else return null;}}

public class ChicagoPizzaStore extends PizzaStore {Pizza createPizza(String item) {        if (item.equals("cheese")) {            return new ChicagoStyleCheesePizza();        } else if (item.equals("veggie")) {            return new ChicagoStyleVeggiePizza();        } else if (item.equals("clam")) {            return new ChicagoStyleClamPizza();        } else if (item.equals("pepperoni")) {            return new ChicagoStylePepperoniPizza();        } else 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("Prepare " + name);System.out.println("Tossing dough...");System.out.println("Adding sauce...");System.out.println("Adding toppings: ");for (String topping : toppings) {System.out.println("   " + topping);}}  void bake() {System.out.println("Bake for 25 minutes at 350");} void cut() {System.out.println("Cut the pizza into diagonal slices");}  void box() {System.out.println("Place pizza in official PizzaStore box");} public String getName() {return name;}public String toString() {StringBuffer display = new StringBuffer();display.append("---- " + name + " ----\n");display.append(dough + "\n");display.append(sauce + "\n");for (String topping : toppings) {display.append(topping + "\n");}return display.toString();}}

public class NYStyleCheesePizza extends Pizza {public NYStyleCheesePizza() { name = "NY Style Sauce 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");}}

public class PizzaTestDrive { public static void main(String[] args) {PizzaStore nyStore = new NYPizzaStore();PizzaStore chicagoStore = new ChicagoPizzaStore(); Pizza pizza = nyStore.orderPizza("cheese");System.out.println("Ethan ordered a " + pizza.getName() + "\n"); pizza = chicagoStore.orderPizza("cheese");System.out.println("Joel ordered a " + pizza.getName() + "\n");pizza = nyStore.orderPizza("clam");System.out.println("Ethan ordered a " + pizza.getName() + "\n"); pizza = chicagoStore.orderPizza("clam");System.out.println("Joel ordered a " + pizza.getName() + "\n");pizza = nyStore.orderPizza("pepperoni");System.out.println("Ethan ordered a " + pizza.getName() + "\n"); pizza = chicagoStore.orderPizza("pepperoni");System.out.println("Joel ordered a " + pizza.getName() + "\n");pizza = nyStore.orderPizza("veggie");System.out.println("Ethan ordered a " + pizza.getName() + "\n"); pizza = chicagoStore.orderPizza("veggie");System.out.println("Joel ordered a " + pizza.getName() + "\n");}}

以下是上述类的关系图：

运用工厂模式前的我们通常设计的对象依赖关系：

运用工厂模式后的对象依赖关系：