java - 时代困惑，需要澄清

Question

给定:

private Calendar calendarInstance = Calendar.getInstance();

public long inMillis() {
    calendarInstance.set(year, month, day, hour, min);
    return calendarInstance.getTimeInMillis();
}

据我了解，结果返回自纪元以来的时间，以毫秒为单位

The current time as UTC milliseconds from the epoch.

既然我的测试总是设置相同的对象，为什么随着时间的推移结果会不同？

detailedMoment = new MomentInTime(2012, 11, 1, 19, 9);
detailedMoment.inMillis() // gives different results as time passes by

更新:

由于以下原因，我继续猜测自己

在同一时间我得到

1_351_796_940 // http://www.epochconverter.com
1_354_410_540 // my number

Answer 1

我认为你应该使用 clear() .如果您这样做，它每次都会返回准确的毫秒数。

public long inMillis() {
    calendarInstance.clear();
    calendarInstance.set(year, month, day, hour, min);
    return calendarInstance.getTimeInMillis();
}

来自 Java 文档

Sets all the calendar field values and the time value (millisecond offset from the Epoch) of this Calendar undefined. This means that isSet() will return false for all the calendar fields, and the date and time calculations will treat the fields as if they had never been set. A Calendar implementation class may use its specific default field values for date/time calculations. For example, GregorianCalendar uses 1970 if the YEAR field value is undefined.

示例程序

public class MomentInTime {

private static Calendar calendarInstance = Calendar.getInstance();

public static long inMillis() {
    calendarInstance.clear();
    calendarInstance.set(2012, 10, 1, 19, 9);
    return calendarInstance.getTimeInMillis();
}

public static void main(String[] args) throws InterruptedException {
    for (int i = 0; i < 10; i++) {
        System.out.println(inMillis()/1000);
        Thread.sleep(300);
    }
}
}

输出:

 1351777140