自定义比较器不排序时间

Question

I am working on a project where I am comparing the date and time in a custom Comparator. I actually concatenated the date with date and time. When I debugged the issue, I realized that time is not getting sorted. Here is the snippet of my code from my Comparator.

Date dateObject1= new Date();
Date dateObject2  = new Date();

try {
    dateObject1 = sdf.parse(date1  + "T" + time1);
    dateObject2 = sdf.parse(date2  + "T" + time2);
} catch (Exception e) { }

if (dateObject1.compareTo(dateObject2) > 0)
    return 1;
else if (dateObject1.compareTo(dateObject2) < 0)
    return -1;
else
    return 0; 

Test cases: 1. date1 - 2019-12-13 , date2 - 2019-12-13 time1 - 08:00:00, time2 - 12:00:00

When i debugged the issue I found it's returning 0 for the above test case. I am not sure why it's happening but I intent to return -1 such that it's sorted in ascending order. Please advice.

Answer 1

Your problem is here

} catch (Exception e) { }

You initialize your 2 dates, both of which get initialized to the current time ( System.currentTimeMillis() ).

Date dateObject1= new Date();
Date dateObject2  = new Date();

Your parsing then fails, but you swallow the exception so you never noticed it.

Then you try to sort two dates which are either exactly same, or separated by a couple of milliseconds, but are certainly unrelated to the actual timestamps that you're trying to sort.

Check the exception, fix the parsing, and then it will work.

Answer 2

java.time and Comparator.comparing … thenComparing

I don't know what your Java version is. The following snippet works on Java 8 and above. The most important ideas can be applied on Java 6 and 7 too.

    List<MyObject> listToBeSorted = Arrays.asList(
            new MyObject("2019-12-12", "11:53:50"),
            new MyObject("2019-12-11", "13:07:05"),
            new MyObject("2019-12-13", "05:02:16"),
            new MyObject("2019-12-11", "09:54:57"),
            new MyObject("2019-12-12", "05:53:52"),
            new MyObject("2019-12-13", "06:56:08"),
            new MyObject("2019-12-12", "02:31:55"),
            new MyObject("2019-12-11", "09:28:16"),
            new MyObject("2019-12-11", "20:58:55"));

    Comparator<MyObject> cmpr = Comparator.comparing(MyObject::getDate)
            .thenComparing(MyObject::getTime);

    listToBeSorted.sort(cmpr);

    listToBeSorted.forEach(System.out::println);

Output is:

 MyObject [date=2019-12-11, time=09:28:16] MyObject [date=2019-12-11, time=09:54:57] MyObject [date=2019-12-11, time=13:07:05] MyObject [date=2019-12-11, time=20:58:55] MyObject [date=2019-12-12, time=02:31:55] MyObject [date=2019-12-12, time=05:53:52] MyObject [date=2019-12-12, time=11:53:50] MyObject [date=2019-12-13, time=05:02:16] MyObject [date=2019-12-13, time=06:56:08]

You will observe that the objects have been sorted by date and objects with the same date also by time. Here is the MyObject class that I used:

public class MyObject {

    LocalDate date;
    LocalTime time;

    public MyObject(String dateString, String timeString) {
        date = LocalDate.parse(dateString);
        time = LocalTime.parse(timeString);
    }

    public LocalDate getDate() {
        return date;
    }

    public LocalTime getTime() {
        return time;
    }

    @Override
    public String toString() {
        return "MyObject [date=" + date + ", time=" + time + "]";
    }

}

The two key messages are:

1. Don't keep your dates and times as strings in your objects. Keep proper date and time objects. It may require parsing strings when you build your objects, but everything else gets noticeably easier.
2. Don't use Date and SimpleDateFormat at all. Use classes from java.time, the modern Java date and time API. In this case LocalDate and LocalTime . The SimpleDateFormat and Date classes are poorly designed and long outdated, the former in particular notoriously troublesome. The modern API is so much nicer to work with.

The advantage of the Comparator methods comparing and thenComparing is not so much that code gets considerably shorter. The really important gain is that writing comparators in this style is much less error prone, and the code reads more naturally.

What went wrong in your code?

The problem is in the line that you posted in a comment:

    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd"); 

This formatter only parses the date from each string and ignores the time. It's one of many confusing traits of SimpleDateFormat that it is happy to parse only a part of the string and doesn't draw our attention to the fact that some of the text is ignored — in this case the T and the entire time.

Link

Oracle tutorial: Date Time explaining how to use java.time.

Answer 3

Just return the value of the comparison since that is what you return anyway.

Date dateObject1= new Date();
Date dateObject2  = new Date();
try {
       dateObject1 = sdf.parse(date1  + "T" + time1);
       dateObject2 = sdf.parse(date2  + "T" + time2);
} catch (Exception e) {
     e.printStackTrace();  // always print these.  They are there to help you.
}
return dateObject1.compareTo(dateObject2);