工厂模式在JDK-Calendar 应用

• 编写案例代码

package com.atguigu.jdk;
import java.util.Calendar;
 
public class Factory {
 
	public static void main(String[] args) {
	    // getInstance 是 Calendar 静态方法
	    Calendar cal = Calendar.getInstance();
	    // 注意月份下标从0开始，所以取月份要+1
	    System.out.println("年:" + cal.get(Calendar.YEAR));
	    System.out.println("月:" + (cal.get(Calendar.MONTH) + 1));       
	    System.out.println("日:" + cal.get(Calendar.DAY_OF_MONTH));
	    System.out.println("时:" + cal.get(Calendar.HOUR_OF_DAY));
	    System.out.println("分:" + cal.get(Calendar.MINUTE));
	    System.out.println("秒:" + cal.get(Calendar.SECOND));
	}
 
}

• 查看getInstance方法

    public static Calendar getInstance()
    {
        Locale aLocale = Locale.getDefault(Locale.Category.FORMAT);
        return createCalendar(defaultTimeZone(aLocale), aLocale);   // 根据时区来创建
    }

• 查看createCalendar方法

    private static Calendar createCalendar(TimeZone zone,
                                           Locale aLocale)
    {
        CalendarProvider provider =
            LocaleProviderAdapter.getAdapter(CalendarProvider.class, aLocale)   // 根据传入的参数aLocale获取provider
                                 .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()) {        // 根据传入的参数aLocale构建不同的Calendar 
            String caltype = aLocale.getUnicodeLocaleType("ca");
            if (caltype != null) {
                switch (caltype) {
                case "buddhist":
                cal = new BuddhistCalendar(zone, aLocale);
                    break;
                case "japanese":
                    cal = new JapaneseImperialCalendar(zone, aLocale);
                    break;
                case "gregory":
                    cal = new GregorianCalendar(zone, aLocale);
                    break;
                }
            }
        }
        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;
    }

• 工厂模式的意义

将实例化对象的代码提取出来，放到一个类中统一管理和维护，达到和主项目的依赖关系的解耦。从而提高项目的扩展和维护性

• 三种工厂模式 (简单工厂模式、工厂方法模式、抽象工厂模式)

• 设计模式的依赖抽象原则

创建对象实例时，不要直接 new 类, 而是把这个new 类的动作放在一个工厂的方法中，并返回。有的书上说，变量不要直接持有具体类的引用。
不要让类继承具体类，而是继承抽象类或者是实现interface(接口)
不要覆盖基类中已经实现的方法