89 lines
3.7 KiB
Markdown
89 lines
3.7 KiB
Markdown
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---
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layout: pattern
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title: Factory Method
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folder: factory-method
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permalink: /patterns/factory-method/
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categories: Creational
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tags:
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- Extensibility
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- Gang Of Four
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---
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## Also known as
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# 或称
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虚拟构造器
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## 目的
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为创建一个对象定义一个接口，但是让子类决定实例化哪个类。工厂方法允许类将实例化延迟到子类。
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## 解释
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真实世界例子
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> 铁匠生产武器。精灵需要精灵武器，而兽人需要兽人武器。根据客户来召唤正确类型的铁匠。
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通俗的说
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> 它为类提供了一种把实例化的逻辑委托给子类的方式。
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维基百科上说
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> 在基于类的编程中，工厂方法模式是一种创建型设计模式用来解决创建对象的问题，而不需要指定将要创建对象的确切类。这是通过调用工厂方法创建对象来完成的，而不是通过调用构造器。该工厂方法在接口中指定并由子类实现，或者在基类实现并可以选择由子类重写。
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**程序示例**
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以上面的铁匠为例，首先我们有铁匠的接口和一些它的实现。
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```java
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public interface Blacksmith {
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Weapon manufactureWeapon(WeaponType weaponType);
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}
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public class ElfBlacksmith implements Blacksmith {
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public Weapon manufactureWeapon(WeaponType weaponType) {
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return ELFARSENAL.get(weaponType);
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}
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}
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public class OrcBlacksmith implements Blacksmith {
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public Weapon manufactureWeapon(WeaponType weaponType) {
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return ORCARSENAL.get(weaponType);
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}
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}
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```
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现在随着客户的到来，会召唤出正确类型的铁匠并制造出要求的武器。
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```java
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var blacksmith = new ElfBlacksmith();
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blacksmith.manufactureWeapon(WeaponType.SPEAR);
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blacksmith.manufactureWeapon(WeaponType.AXE);
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// Elvish weapons are created
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```
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## 类图
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## 适用性
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使用工厂方法模式当
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* 一个类无法预料它所要必须创建的对象的类
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* 一个类想要它的子类来指定它要创建的对象
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* 类将责任委派给几个帮助子类中的一个，而你想定位了解是具体之中的哪一个
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## Java中的例子
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* [java.util.Calendar](http://docs.oracle.com/javase/8/docs/api/java/util/Calendar.html#getInstance--)
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* [java.util.ResourceBundle](http://docs.oracle.com/javase/8/docs/api/java/util/ResourceBundle.html#getBundle-java.lang.String-)
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* [java.text.NumberFormat](http://docs.oracle.com/javase/8/docs/api/java/text/NumberFormat.html#getInstance--)
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* [java.nio.charset.Charset](http://docs.oracle.com/javase/8/docs/api/java/nio/charset/Charset.html#forName-java.lang.String-)
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* [java.net.URLStreamHandlerFactory](http://docs.oracle.com/javase/8/docs/api/java/net/URLStreamHandlerFactory.html#createURLStreamHandler-java.lang.String-)
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* [java.util.EnumSet](https://docs.oracle.com/javase/8/docs/api/java/util/EnumSet.html#of-E-)
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* [javax.xml.bind.JAXBContext](https://docs.oracle.com/javase/8/docs/api/javax/xml/bind/JAXBContext.html#createMarshaller--)
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## 鸣谢
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* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
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* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
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* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)
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