Factory Method Pattern

Concepts

• Doesn't expose instantiation logic
• Defer to subclasses
• Common interface
• Specified by architecture, implemented by user
• Examples: 
      Calendar, ResourceBundle, NumberFormat

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Design

• Factory is responsible for lifecycle
• Common Interface
• Concrete Classes
• Parameterized create method

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Everyday Example - Calendar

package com.pattern.factory;

import java.util.Calendar;

public class FactoryEverydayDemo {

	public static void main(String[] args) {

		Calendar cal = Calendar.getInstance();
		System.out.println(cal);
		System.out.println(cal.get(Calendar.DAY_OF_MONTH));
	}
}

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Exercise Factory

• Create Pages
• Create Website
• Create Concrete Classes
• Create Factory
• Enum
  

工厂static方法

Blog, Shop子类

Website抽象父类

ContactPage, CarPate子类

Page父类

执行结果：

package com.pattern.factory;

public class FactoryDemo {

	public static void main(String[] args) {
		Website site = WebsiteFactory.getWebsite(WebsiteType.BLOG);
		
		System.out.println(site.getPages());
		
		site = WebsiteFactory.getWebsite(WebsiteType.SHOP);
		
		System.out.println(site.getPages());
	}
}

package com.pattern.factory;

public class WebsiteFactory {

	public static Website getWebsite(WebsiteType siteType) {
		switch (siteType) {
		case BLOG: {
			return new Blog();
		}
		case SHOP: {
			return new Shop();
		}	

		default:
			return null;
		}
	}
}

package com.pattern.factory;

public class Blog extends Website {

	@Override
	public void createWebsite() {
		pages.add(new ContactPage());
	}

}

package com.pattern.factory;

import java.util.ArrayList;
import java.util.List;

public abstract class Website {

	protected List<Page> pages = new ArrayList<>();

	public List<Page> getPages() {
		return pages;
	}
	
	public Website() {
		this.createWebsite();
	}
	
	public abstract void createWebsite();
}

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Pitfalls

• Complexity
• Creation in subclass
• Refactoring

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Contrast
Singleton

• Returns same instance(One constructor method - no args)
• No Interface
• No subclasses

Factory

• Returns various instances(Multiple constructors)
• Interface driven
• Subclasses
• Adaptable to environment more easily

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Factory Summary

• Parameter Driven
• Solves complex creation
• A little complex
• Opposite of a Singleton

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AbstractFactory Pattern
https://blog.csdn.net/u012596785/article/details/79704008