java – Calendar.getInstance()和GregorianCalendar.getInstance(

自从第一次引入替换Date以来,我一直在使用静态方法Calendar.getInstance()来获取一个新的Calendar对象.我从来没有遇到任何问题,但我只是想知道如果最好使用GregorianCalendar.getInstance()方法,

是否可能我的程序将运行在某些地方或某些JVM的某个地方超类版本将返回一个我不期望的对象类？有没有一个执行日历,而不是格雷戈里安卡尔日历,曾经使它广泛使用？

正如我所说,我目前没有问题,但我一直在寻求改善我的做法.

解决方法

/**
 * Gets a calendar using the default time zone and locale. The
 * <code>Calendar</code> returned is based on the current time
 * in the default time zone with the default
 * {@link Locale.Category#FORMAT FORMAT} locale.
 *
 * @return a Calendar.
 */
public static Calendar getInstance()
{
    return createCalendar(TimeZone.getDefault(),Locale.getDefault(Locale.Category.FORMAT));
}

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) {
            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;
}

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