简单工厂模式

1. 概念

• 定义：由一个工厂对象决定创建出哪一种对象的实例
• 优缺点：
  • 优点：只需要传入一个参数，就可以获取你所需要的对象，而无需知道细节
  • 缺点：工厂的职责过重，增加新的产品，需要修改工厂的判断逻辑，违反开闭原则

2. 代码

需求： 需要根据电影类型创建电影

2.1 版本1：常规写法

public class JavaVideo extends Video {
    @Override
    public void produce() {
        System.out.println("录制Java课程视频");
    }
}

public class PythonVideo extends Video {
    @Override
    public void produce() {
        System.out.println("录制Python课程视频");
    }
}

public class Test {
    public static void main(String[] args) {
        PythonVideo pythonVideo = new PythonVideo();
        pythonVideo.produce();
    }
}

2.2 版本2： 使用简单工厂模式改造

public class VideoFactory {
    public Video getVideo(String type){
        if("java".equalsIgnoreCase(type)){
            return new JavaVideo();
        }else if("python".equalsIgnoreCase(type)){
            return new PythonVideo();
        }
        return null;
    }
}

public class Test {
    public static void main(String[] args) {
        VideoFactory videoFactory = new VideoFactory();
        Video video = videoFactory.getVideo("python");
        if(video == null){
            return;
        }
        video.produce();
    }
}

2.3 版本3: 基于反射改造，避免工厂里逻辑太复杂

public class VideoFactory {
    public Video getVideo(Class c){
        Video video = null;
        try {
            video = (Video) Class.forName(c.getName()).newInstance();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        return video;
    }
}

public class Test {
    public static void main(String[] args) {
        VideoFactory videoFactory = new VideoFactory();
        Video video = videoFactory.getVideo(JavaVideo.class);
        if(video == null){
            return;
        }
        video.produce();
    }
}

3 JDK的应用

JDK1.8的应用：java.util.Calendar

private static Calendar createCalendar(TimeZone zone,
                                           Locale aLocale)
    {
        CalendarProvider provider =
            LocaleProviderAdapter.getAdapter(CalendarProvider.class, aLocale)
                                 .getCalendarProvider();
        if (provider != null) {
            try {
                return provider.getInstance(zone, aLocale);
            } catch (IllegalArgumentException iae) {
                // fall back to the default instantiation
            }
        }

        Calendar cal = null;

        if (aLocale.hasExtensions()) {
            String caltype = aLocale.getUnicodeLocaleType("ca");
            if (caltype != null) {
                cal = switch (caltype) {
                    case "buddhist" -> new BuddhistCalendar(zone, aLocale);
                    case "japanese" -> new JapaneseImperialCalendar(zone, aLocale);
                    case "gregory"  -> new GregorianCalendar(zone, aLocale);
                    default         -> null;
                };
            }
        }
        if (cal == null) {
            // If no known calendar type is explicitly specified,
            // perform the traditional way to create a Calendar:
            // create a BuddhistCalendar for th_TH locale,
            // a JapaneseImperialCalendar for ja_JP_JP locale, or
            // a GregorianCalendar for any other locales.
            // NOTE: The language, country and variant strings are interned.
            if (aLocale.getLanguage() == "th" && aLocale.getCountry() == "TH") {
                cal = new BuddhistCalendar(zone, aLocale);
            } else if (aLocale.getVariant() == "JP" && aLocale.getLanguage() == "ja"
                       && aLocale.getCountry() == "JP") {
                cal = new JapaneseImperialCalendar(zone, aLocale);
            } else {
                cal = new GregorianCalendar(zone, aLocale);
            }
        }
        return cal;
    }