java - Calendar.getInstance() 和 GregorianCalendar.getInstance() 之间的区别

Question

自从首次引入它来替换 Date 以来，我一直在使用静态方法 Calendar.getInstance() 来获取新的 Calendar目的。我从来没有遇到过问题，但出于某种原因我只是想知道使用 GregorianCalendar.getInstance() 方法是否更好。

我的程序是否有可能在某个地方或某个 JVM 中运行，父类(super class)版本将返回一个我不期望的对象类？ Calendar GregorianCalendar 是否曾被广泛使用？

正如我所说，我目前没有遇到任何问题，但我一直在寻求改进我的做法。

Answer 1

是的，Calendar 可以返回特定于语言环境的日历。来自源头。

/**
 * 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;
}