Java中的日期比较

Question

I'm trying to compare two dates with the current date. It seems not to work when I try to know if a date is the same as the current date. Here's what I do in my code :

//BeginDate is set earlier

Date myDate= new SimpleDateFormat("dd/MM/yyyy").parse(BeginDate);
Date now = new Date();

    System.out.println("Now : " + now);
    System.out.println("myDate : " + myDate);
    System.out.println("equals : " + myDate.equals(now));
    System.out.println(myDate.compareTo(now));

And I get this in the console :

Now : Thu Dec 29 00:28:45 CET 2011
myDate : Thu Dec 29 00:00:00 CET 2011
equals : false
-1

The first comparison should return true and the second "0" right ? Or am I missing something ?

Answer 1

Comparing dates with either equals() or compareTo() compares the times (hours, minutes, seconds, millis) as well as the dates. Your test is failing because myDate is midnight today, whereas now is a little later than that.

Answer 2

Your comparison is failing because you need to format now so that both dates have the same format and thus may be compared.

Or, if you prefer, you can convert dates into strings and perform the comparison:

    String beginDate = "28/12/2011";
    Calendar now = Calendar.getInstance();
    DateFormat df = new SimpleDateFormat("dd/MM/yyyy");
    String nowStr = df.format(new Date());
    System.out.println("equals : " + beginDate.equals(nowStr));

Answer 3

Are you specifying the milliseconds when creating the dates? If you are, don't. So when creating the dates earlier, only specify the Day, Hour etc, not seconds/milliseconds.

And, change the SimpleDateFormat respectively. That "should" work.

Answer 4

Date object in Java is nothing but a number that represents milliseconds since January 1, 1970, 00:00:00 GMT. It doesn't have any attribute called day, date, month, year etc. That's because date, month, year varies based on the type of calendar and timezone. These attributes belong to Calendar instance.

So, if you have 2 Date objects and you want to compare day of month, month and year then you should create corresponding Calendar instance and compare them separately.

// Parse begin date
DateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");
Date beginDate = dateFormat.parse(beginDateAsString);
// Create calendar instances
Calendar beginDateCalendar = Calendar.getInstance();
beginDateCalendar.setTime(beginDate);
Calendar todayCalendar = Calendar.getInstance();
// Check Equals
boolean dayEquals = todayCalendar.get(Calendar.DAY_OF_MONTH) == beginDateCalendar
        .get(Calendar.DAY_OF_MONTH);
boolean monthEquals = todayCalendar.get(Calendar.MONTH) == beginDateCalendar
        .get(Calendar.MONTH);
boolean yearEquals = todayCalendar.get(Calendar.YEAR) == beginDateCalendar
        .get(Calendar.YEAR);
// Print Equals
System.out.println(dayEquals && monthEquals && yearEquals);

Above code is cumbersome for the current problem but explains how date operations must be done in JAVA.

If you just want to solve the equals problem you have mentioned then the code below will suffice:

String todayAsString = (new SimpleDateFormat("dd/MM/yyyy")).format(new Date());
System.out.println(beginDateAsString.equals(todayAsString));

Answer 5

If you are only going to be dealing with dates between the years 1900 and 2100, there is a simple calculation which will give you the number of days since 1900:

public static int daysSince1900(Date date) {
    Calendar c = new GregorianCalendar();
    c.setTime(date);

    int year = c.get(Calendar.YEAR) - 1900;
    int month = c.get(Calendar.MONTH) + 1;
    int days = c.get(Calendar.DAY_OF_MONTH);

    if (month < 3) {
        month += 12;
        year--;
    }
    int yearDays = (int) (year * 365.25);
    int monthDays = (int) ((month + 1) * 30.61);

    return (yearDays + monthDays + days - 63);
}

Thus, Date (only) comparison can be achieved by checking if the number of days since 1900 of the 2 dates are equal.

NOTE: The above method should have code added to check if the dates are outside the valid range (1/1/1900 - 31/12/2099) and throw an IllegalArgumentException.

And don't ask me where this calculation came from because we've used it since the early '90s.