java comparator for two LocalDateTimes

Question

A custom class called MyCustomClass has a property that it s joda LocalDateTime. I need to create a java.util.Comparator class to compare instances of MyCustomClass by their TimeStamp property, which is of type LocalDateTime. I have read several postings on this (including this one), and I have tried all the methods, but none of the methods shown in the answers seem to work. For example, the following several approaches throw compilation errors:

import java.time.temporal.ChronoUnit;
import java.util.Comparator;

import org.joda.time.DateTime;
import org.joda.time.Duration;
import org.joda.time.Period;

import my.app.model.MyCustomClass;

public class MyCustomClassComparator implements Comparator<MyCustomClass>{
    public int compare(MyCustomClass mcc1, MyCustomClass mcc2) {
        //this first attempt throws a return type error for the method.
        return Period.fieldDifference(mcc2.getTimestamp(), mcc1.getTimestamp());
        //This next attempt says that LocalDateTimes are not valid arguments
        Duration.between(mcc2.getTimestamp(), mcc1.getTimestamp());
        //The next approach also says LocalDateTimes are not valid arguments.
        DateTime.parse(mcc2.getTimestamp()), mcc1.getTimestamp()).getSeconds();
        //This boilerplate approach says the minus sign is not valid for LocalDateTime
        return mcc1.getTimestamp() - mcc2.getTimestamp();
    }
}

I intend to use this elsewhere in code like:

List<MyCustomClass> mccs = new ArrayList<MyCustomClass>();
// Sort by time stamp:
Collections.sort(mccs, new MyCustomClassComparator());

How do I write a Comparator class to compare instances of MyCustomClass based on their Joda LocalDateTime properties?

Isn't `LocalDateTime` already `Comparable`? Can't you just use `mcc1.compareTo(mcc2)`? — MadProgrammer, Sep 08 '15 at 01:18
Check this: http://stackoverflow.com/questions/13764106/comparing-two-dates-using-joda-time — akhil_mittal, Sep 08 '15 at 01:21
@MadProgrammer Caveat: Can only do away with `Comparator` if `MyCustomClass` implements `Comparable` with same comparison. — Andreas, Sep 08 '15 at 02:10
@Andreas Not sure I understand. The only need for a `Comparator` is if the default implementation of the `Comparable` isn't what you want, but I can't see anywhere the OP is doing something special about the with their comparision — MadProgrammer, Sep 08 '15 at 02:12
@MadProgrammer OP is not sorting `DateTime`'s, but `MyCustomClass`'es *by* a `DateTime` field. — Andreas, Sep 08 '15 at 02:18

score 11 · Answer 1 · answered Apr 22 '20 at 13:08

11

If all you need is a comparator instance:

Similar to Iłya Bursov's answer, because LocalDateTime implements comparable, you can use Comparator.comparing

Comparator.comparing(MyCustomClass::getTimestamp)

answered Apr 22 '20 at 13:08

Alex Sadler

111
1
2

Iłya Bursov · Answer 2 · 2015-09-08T02:34:44.850

5

LocalDateTime implements comparable interface, so you can implement comparator like:

public class MyCustomClassComparator implements Comparator<MyCustomClass> {
    @Override
    public int compare(final MyCustomClass mcc1, final MyCustomClass mcc2) {
        return mcc1.getTimestamp().compareTo(mcc2.getTimestamp());
    }
}

edited Sep 08 '15 at 02:34

answered Sep 08 '15 at 01:26

Iłya Bursov

23,342
4
33
57

score 1 · Answer 3 · answered Oct 27 '21 at 14:14

Given the fact that Comparator is a functional interface, we could have it with a single lambda expression.

Comparator<LocalDateTime> localDateTimeComparator = (o1, o2) -> o1.compareTo(o2);

//Then use the comparator as always....
LocalDateTime minDate = dates.stream()
                             .min(localDateTimeComparator)
                             .orElseThrow(Assertion::Error)

For reuse, a static instance could be helpful

//interfaces are "more static" than classes with "static members" ;-)
public interface MyDateUtils { 

   Comparator<LocalDateTime> localDateTimeComparator = (o1, o2) -> o1.compareTo(o2);

}

java comparator for two LocalDateTimes

3 Answers3