JAVA : How to remove duplicates from list using stream

Question

I want to remove duplicates from a list based on a property(mail) of the items in the list.
I did this :

acteurs = acteurs.stream().distinct().collect(Collectors.toList());

The equals method of Acteur

@Override
public boolean equals(Object o) {
    if (this.getMail().equals(((Acteur)o).getMail())) {
        return true;
    }
    if (this == o) {
        return true;
    }
    if (o == null || getClass() != o.getClass()) {
        return false;
    }
    Acteur acteur = (Acteur) o;
    if(acteur.id == null || id == null) {
        return false;
    }
    return Objects.equals(id, acteur.id);
}

The equals method should be called during the instruction

acteurs = acteurs.stream().distinct().collect(Collectors.toList());

But it isn't.
Where am I wrong?

UPDATE :
My solution :

List<Acteur> dto3s =acteurs.stream()
            .collect(Collectors.collectingAndThen(
                Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Acteur:: getMail))),
                ArrayList::new));

what should be the best method to remove duplicates acteur (who have same 'mail' value) then? — thomas, Mar 06 '20 at 10:44
Also, your `equals` method can easily throw a `ClassCastException`. — Slaw, Mar 06 '20 at 10:45

score 2 · Answer 1 · answered Mar 06 '20 at 10:39

2

You broke equals/hashCode contract.

You need to implement hashCode according to requirements.

answered Mar 06 '20 at 10:39

talex

17,973
3
29
66

he is not using a Hasmap – Aditya Rewari Mar 06 '20 at 10:42
so would hascode come into picture for List?? @talex – Aditya Rewari Mar 06 '20 at 10:43
You can't tell from this piece of code. – Rob Audenaerde Mar 06 '20 at 10:44
2

@AdityaRewari `HashMap` used behind the scene in `distinct` method. – talex Mar 06 '20 at 10:48
@RobAu You right, but it is good guess. – talex Mar 06 '20 at 10:49
@talex which of the List implementation is using hashcode ? – Aditya Rewari Mar 06 '20 at 10:52
3

@AdityaRewari None of them. The `Stream#distinct()` operation is currently implemented using a hash-based, unique-enforcing collection. – Slaw Mar 06 '20 at 10:54
This is the correct answer IF OP has not overriden `hashCode()` – Smile Mar 06 '20 at 10:59
@Smile there no other explanation to this behaviour. Either hashCode not implemented or it implemented wrong. – talex Mar 06 '20 at 11:04

shahaf · Answer 2 · 2020-03-06T11:05:13.167

you can use the following solution, where you create a map by the distinct key mail and get the map's values

acteurs.stream().collect(Collectors.toMap(a -> a.getMail(), Function.identity(), (a1, a2) -> a1)).values();

the third arg (a1, a2) -> a1 means that whenever it get a conflicts i.e need to add to the map an entry (key,value) with the same key it will use the map's current value, i.e no-op

keep in mind, this will only compare by mail property, if you want to compare by a more complex way using the map logic, you might need to implement hash() and equals() properly

score 0 · Answer 3 · answered Mar 06 '20 at 10:41

0

Your equals method seems not correct because you need to "return false" if the "if condition" is not fulfilled

answered Mar 06 '20 at 10:41

Dd__Mad

116
8

OP has updated the question. This answer is no longer relevant. – Smile Mar 06 '20 at 11:00

JAVA : How to remove duplicates from list using stream

3 Answers3