How to sort objects by field value?

Question

If I have a ArrayList of Columns and Columns have the field disparityRating which is an int how do I sort the arraylist of column by the value of the disparityRating field?

I have tried implementing the comparable interface on my column class but when I do and change the compare to parameter to Column like this:

@Override
public int compareTo(Column o) {
    return 0;
}

It comes up with an error saying method does not override method from its superclass

My main class has an arraylist of columns

Collection<Column> columns = new ArrayList<>();

and so I want to be able to sort them by using

Collections.sort(columns);

score 2 · Accepted Answer · answered Dec 06 '16 at 21:07

2

Add "implements Comparable".

public class ... extends ... implements Comparable<Column> {

    @Override
    public int compareTo(Column o) {
       return Integer.compare(disparityRating, o.disparityRating);
    }
}

answered Dec 06 '16 at 21:07

Dan Armstrong

469
2
5

Ok thanks but then what do I run in main because Collections.sort(columns); doesn't work – Moulie415 Dec 06 '16 at 21:12
Sorry, no worries now i've sorted it. – Moulie415 Dec 06 '16 at 21:23
Needed to change Collection columns = new ArrayList<>(); to ArrayList columns = new ArrayList<>(); – Moulie415 Dec 06 '16 at 21:24
Glad you found it. Please consider the answer below using Comparator for when the sort criteria are not the "natural ordering" of the objects. – Dan Armstrong Dec 06 '16 at 21:35

score 1 · Answer 2 · answered Dec 06 '16 at 21:25

1

Or you could :

 Collections.sort(ListOfCols, new Comparator<Column>() {
                    @Override
                    public int compare(Column o1,Column o2) {
                       return o2.getRating().compareTo(o1.getRating());
                        }
                    });

answered Dec 06 '16 at 21:25

Joey

203
1
11

1

Yes, this is useful when ordering by something that doesn't represent the "natural ordering" of the objects. – Dan Armstrong Dec 06 '16 at 21:34

How to sort objects by field value?

2 Answers2