Sorting list of objects on datetime (cannot resolve method 'compareTo')

Question

I'm trying to sort an array of objects according to the DateTime of the objects. I've used the following example to create my solution:

Sort objects in ArrayList by date?

The method I use looks like this:

        List<SomeEvent> myEvents = new ArrayList<>();
//...
//left out the part where the List is filled with Objects
//...
        Collections.sort(myEvents, new Comparator<SomeEvent>() {
            public int compare(SomeEvent se1, SomeEvent se2) {
                return se1.getStartTime().compareTo(se2.getStartTime());
            }
        });

I get the error message "Cannot resolve method 'compareTo(com.google.api.client.util.DateTime)'

Any pointers as to what I'm missing?

Edit: My class SomeEvent now implements Comparable as suggested:

public class SomeEvent extends SomeTask implements Comparable<SomeEvent>
{ 
private DateTime startTime; 

private DateTime endTime;


     public DateTime getStartTime() {
         return startTime;
     }

     public DateTime getEndTime() {
         return endTime;
     }

 }

Now, however, I get the hint that I need to either declare my class as abstract (which is not possible in the context it is used) or implement the abstract method 'compareTo(T)' in 'Comparable'.

How can I implement the abstract method?

Your `SomeEvent` class doesn't implmenet comparable interface — Pragnani, Feb 25 '16 at 14:41
@PragnaniKinnera: Ok, thanks for the hint, but if I implement the comparable interface like this: public class SomeEvent extends SomeTask implements Comparable { ... } I am told that I must either declare the class as abstract (which is not an option) or implement the abstract method 'compareTo(t)' in 'Comparable'. How do I do this? — Sorcha, Feb 25 '16 at 15:07
@PragnaniKinnera *"Your `SomeEvent` class doesn't implmenet comparable interface"* ... it doesn't need to. — Tom, Feb 25 '16 at 15:29

Solorad · Answer 1 · 2016-02-25T15:56:44.487

1

Could you try this

Collections.sort(myEvents, new Comparator<SomeEvent>() {
        public int compare(SomeEvent se1, SomeEvent se2) {
            return (int) (se1.getStartTime().getValue() - se2.getStartTime().getValue());
        }
    });

So, I see, you are using not java.util.Date, where compareTo method exists, but com.google.api.client.util.DateTime, in which there is no compareTo method.

edited Feb 25 '16 at 15:56

answered Feb 25 '16 at 15:19

Solorad

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That is definitely my problem, thank you. However, now I have to work out some syntax error, it doesn't work like this quite yet. – Sorcha Feb 25 '16 at 15:49
@Sorcha, I updated answer, could you please check it? – Solorad Feb 25 '16 at 15:57
The current problem is strange - it tells me "unnecessary semicolon" in the third line (the one after getValue()), but tells me that a semicolon is expecte ("; expected") and "unexpected token" at the same time. So right now I've fixed it by casting the DateTime to Date and using the original function. – Sorcha Feb 25 '16 at 15:57

Sorting list of objects on datetime (cannot resolve method 'compareTo')

1 Answers1