83 lines
2.3 KiB
Markdown
83 lines
2.3 KiB
Markdown
## 简单工厂（Simple Factory）
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### Intent
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在创建一个对象时不向客户暴露内部细节，并提供一个创建对象的通用接口。
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### Class Diagram
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简单工厂把实例化的操作单独放到一个类中，这个类就成为简单工厂类，让简单工厂类来决定应该用哪个具体子类来实例化。
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这样做能把客户类和具体子类的实现解耦，客户类不再需要知道有哪些子类以及应当实例化哪个子类。客户类往往有多个，如果不使用简单工厂，那么所有的客户类都要知道所有子类的细节。而且一旦子类发生改变，例如增加子类，那么所有的客户类都要进行修改。
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<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/40c0c17e-bba6-4493-9857-147c0044a018.png"/> </div><br>
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### Implementation
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```java
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public interface Product {
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}
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```
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```java
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public class ConcreteProduct implements Product {
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}
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```
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```java
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public class ConcreteProduct1 implements Product {
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}
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```
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```java
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public class ConcreteProduct2 implements Product {
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}
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```
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以下的 Client 类包含了实例化的代码，这是一种错误的实现。如果在客户类中存在这种实例化代码，就需要考虑将代码放到简单工厂中。
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```java
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public class Client {
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public static void main(String[] args) {
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int type = 1;
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Product product;
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if (type == 1) {
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product = new ConcreteProduct1();
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} else if (type == 2) {
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product = new ConcreteProduct2();
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} else {
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product = new ConcreteProduct();
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}
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// do something with the product
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}
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}
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```
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以下的 SimpleFactory 是简单工厂实现，它被所有需要进行实例化的客户类调用。
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```java
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public class SimpleFactory {
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public Product createProduct(int type) {
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if (type == 1) {
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return new ConcreteProduct1();
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} else if (type == 2) {
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return new ConcreteProduct2();
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}
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return new ConcreteProduct();
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}
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}
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```
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```java
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public class Client {
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public static void main(String[] args) {
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SimpleFactory simpleFactory = new SimpleFactory();
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Product product = simpleFactory.createProduct(1);
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// do something with the product
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}
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}
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```
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