Java 日历问题：为什么这两个日期不相等？

Question

import java.io.*;

public class testing {
    public static void main(String a[]) throws Exception{
        Date d1=new Date();
        Thread.sleep(2000);
        Date d2=new Date();
        if(d1.equals(d2)){
            System.out.println("Both equal");
        }else{
            System.out.println("Both not equal");
        }
        Calendar c1=Calendar.getInstance();
        Calendar c2=Calendar.getInstance();
        c1.setTime(d1);
        c2.setTime(d2);
        c1.clear(Calendar.HOUR);
        c1.clear(Calendar.MINUTE);
        c1.clear(Calendar.SECOND);
        c2.clear(Calendar.HOUR);
        c2.clear(Calendar.MINUTE);
        c2.clear(Calendar.SECOND);
        if(c2.compareTo(c1) == 0){
            System.out.println("Cal Equal");
        }else {
            System.out.println("Cal Not Equal");
        }
    }
}

When I run the above code multiple times of which each time (for printing if conditions of date) 'Both not equal' is printed but (for if condition of Calendar) sometimes it prints 'Cal Equal' and sometimes 'Cal Not Equal'. Can anyone please explain me why this is so?

The main reason I was trying this because I want to compare two dates. Both have same day, month and Year but different time of the day when the objects were created. I want them to be compared equal(same). How should I do this?

Answer 1

Truncate the Milliseconds field

Calendars have milliseconds, too. Add this:

c1.clear(Calendar.MILLISECOND);
c2.clear(Calendar.MILLISECOND);

But it's easier to achieve that functionality using DateUtils.truncate() from Apache Commons / Lang

c1 = DateUtils.truncate(c1, Calendar.DATE);
c2 = DateUtils.truncate(c2, Calendar.DATE);

This removes all hours, minutes, seconds and milliseconds.

Answer 2

I believe Calendar also has Millisecond precision. If you want to continue your code, add

c1.clear(Calendar.MILLISECOND);
c2.clear(Calendar.MILLISECOND);

then try your comparison.

Answer 3

改用JodaTime ，它在操作日期和时间方面要好得多（比标准日期和日历）。

Answer 4

你需要清除毫秒

Answer 5

After

Calendar c1=Calendar.getInstance();

add

c1.clear();

That's a bit easier to type than c1.clear(Calendar.MILLISECOND);

Works a treat.

Answer 6

Reset time using the below method before compare dates

private Calendar resetHours(Calendar cal) {
        cal.set(Calendar.HOUR, 0);
        cal.set(Calendar.MINUTE, 0);
        cal.set(Calendar.SECOND,0);
        cal.set(Calendar.HOUR_OF_DAY,0);
        cal.set(Calendar.MILLISECOND,0);
        return cal;
    }