how to implement comparator in java?

Question

Stop(Id, Name) is a java class, and i want to store these stop objects in a java.util.Set and those objects should be sorted according to the Id of Stop . this is my comparator

public class StopsComparator implements Comparator{

    @Override
    public int compare(Object o1, Object o2) {
        // TODO Auto-generated method stub
        Stop stop1 = (Stop)o1;
        Stop stop2 = (Stop)o2;

        return stop1.getStopId().compareTo(stop2.getStopId());
    }
}


 private Set<Stop> stops = new TreeSet<Stop>(new StopsComparator());

but its not giving correct result?

Answer 1

Does Stop implement an equals method that works on the same field as your comparator? If not then that will lead to problems. You also might want to switch to have your object implement Comparable (although that wouldn't fix the problem you're seeing).

Once you implement an equals() method, then you should also implement a hashCode() method that works on the same field.

Findbugs would have probably told you these things. Its extremely useful.

Answer 2

The following code works for me -

public class Stop {

    private Long id;
    private String name;

    public Stop(Long id, String name) {
        this.id = id;
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Stop{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }

    private static class StopComparator implements Comparator<Stop> {


        public int compare(Stop o1, Stop o2) {
            return o1.getId().compareTo(o2.getId());
        }
    }

    public static void main(String[] args) {
        Set<Stop> set = new TreeSet<Stop>(new StopComparator());
        set.add(new Stop(102L, "name102"));
        set.add(new Stop(66L, "name66"));
        set.add(new Stop(72L, "name72"));
        System.out.println(set);
    }
}

prints -

[Stop{id=66, name='name66'}, Stop{id=72, name='name72'}, Stop{id=102, name='name102'}]

Ofc you need to implement equals and hashcode so that class behaves consistently in each Set implementation, but for TreeSet this should work as is since TreeSet relies on compareTo method while performing add , remove or contains operations (instead of equals like HashSet ).

Answer 3

This is from the Comparator docs:

The ordering imposed by a comparator c on a set of elements S is said to be consistent with equals if and only if c.compare(e1, e2)==0 has the same boolean value as e1.equals(e2) for every e1 and e2 in S.

Caution should be exercised when using a comparator capable of imposing an ordering inconsistent with equals to order a sorted set (or sorted map). Suppose a sorted set (or sorted map) with an explicit comparator c is used with elements (or keys) drawn from a set S. If the ordering imposed by c on S is inconsistent with equals, the sorted set (or sorted map) will behave "strangely." In particular the sorted set (or sorted map) will violate the general contract for set (or map), which is defined in terms of equals.

I would recommend to try implementing equals and hashCode .