日期不符

Question

I have a TreeMap of <DDate, Integer> .

DDate just contains month and year (month is 0 indexed, like in Java, JAN = 0);

My compare is not returning the right thing:

@Override
public int compareTo(DDDate o) {

    Calendar cal1 = Calendar.getInstance();
    cal1.setTimeZone(TimeZone.getTimeZone("UTC"));
    cal1.set(year, month, 1); // year is 2012, month is 1

    Calendar cal2 = Calendar.getInstance();
    cal2.setTimeZone(TimeZone.getTimeZone("UTC"));
    cal2.set(o.getYear(), o.getMonth(), 1); // year is 2012, month is 1


    Log.log("COMPARING: " + format.format(cal1.getTime())); // COMPARING: 20120101
    Log.log("COMPARING: " + format.format(cal2.getTime())); // COMPARING: 20120101

    Log.log((cal1.getTime().getTime())); // 1325413927678
    Log.log((cal2.getTime().getTime())); // 1325413927679

    Log.log("WILL RETURN: " + cal1.getTime().compareTo(cal2.getTime())); // WILL RETURN: -1

    return cal1.getTime().compareTo(cal2.getTime());

}

Why is there descrepancy in the two Calendar objects for the same Date? (1325413927678 vs 1325413927679)

Thank you!

FYI: this method works for a while, and then at a certain point, stops working.

PS - I understand this is overkill, I can have a simpler compareTo, but please humor me.

EDIT- FIX

Use JodaTime's LocalDate.

Or do this:

Calendar cal1 = Calendar.getInstance();
cal1.setTimeZone(TimeZone.getTimeZone("UTC"));
cal1.clear();
cal1.set(year, month, 1); // year is 2012, month is 1

Answer 1

You're setting the year, month and day - but that doesn't change the time of day . The internal clock must have ticked between the two calls to Calendar.getInstance , so they have different "milliseconds since the Unix epoch" values.

Personally I'd recommend using Joda Time instead of java.util.Calendar and java.util.Date - that way you can represent what you're actually interested in, such as LocalDate .

Answer 2

Because you are making them at two different times.

Calendar cal1 = Calendar.getInstance(); // time now 
cal1.setTimeZone(TimeZone.getTimeZone("UTC")); // time ticking
cal1.set(year, month, 1); // year is 2012, month is 1 // time ticking

Calendar cal2 = Calendar.getInstance(); // time two ticks later

You have changed the day and month and year, however the hour minutes and seconds stayed and keep on moving and chaning.

As mentioned here
http://docs.oracle.com/javase/7/docs/api/java/util/Calendar.html#set(int , int, int, int, int)
The other field values are retained and you should use clear() to set them to zero. Additionally as Jon recomended I would say use Joda time it is much better.

Answer 3

You should initialize the calendar properly to "no time" (0 ticks), this should fix the issue.

cal1.setTime(new Date(0));
cal1.set(year, month, 1);

Answer 4

Try using the clear() method before setting the date:

final Calendar cal1 = Calendar.getInstance();
cal1.clear();
cal1.setTimeZone(TimeZone.getTimeZone("UTC"));
cal1.set(2012, 1, 1); // year is 2012, month is 1

final Calendar cal2 = Calendar.getInstance();
cal2.clear();
cal2.setTimeZone(TimeZone.getTimeZone("UTC"));
cal2.set(2012, 1, 1); // year is 2012, month is 1

cal1.getTime().compareTo(cal2.getTime()); // Returns 0

Answer 5

Jon Skeet's answer is correct.

Here is some example code showing how to use Joda-Time to do a comparison of dates. Much easier in Joda-Time rather than Java's bundled Date/Calendar classes. Takes only 3 lines of code to both instantiate and compare.

// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.

// Joda-Time - The popular alternative to Sun/Oracle's notoriously bad date, time, and calendar classes bundled with Java 7 and earlier.
// http://www.joda.org/joda-time/

// Joda-Time will become outmoded by the JSR 310 Date and Time API introduced in Java 8.
// JSR 310 was inspired by Joda-Time but is not directly based on it.
// http://jcp.org/en/jsr/detail?id=310

// By default, Joda-Time produces strings in the standard ISO 8601 format.
// https://en.wikipedia.org/wiki/ISO_8601

// New DateTime instance with default time zone. For specific zone, call "withZone()" method.
org.joda.time.DateTime dateTimeX = new org.joda.time.DateTime();
org.joda.time.DateTime dateTimeY = new org.joda.time.DateTime(2012, 1, 1, 0, 0); // First of January.

// To compare the date only while ignoring the time, set identical time component on both.
// Call withTimeAtStartOfDay() to set same time value.
Boolean isSameDate = dateTimeX.withTimeAtStartOfDay().isEqual(dateTimeY.withTimeAtStartOfDay());