Deserialization of JavaScript array to Java LinkedHashSet using Jackson and Spring doesn't remove duplicates

Question

Let's say I have this client-side JSON input:

{
   id: "5",
   types: [
      {id: "1", types:[]},
      {id: "2", types:[]},
      {id: "1", types[]}
   ]
}

I have this class:

class Entity {
    private String id;
    private Set<Entity> types = new LinkedHashSet<>();

    public String getId() {
        return this.id;
    }

    public String setId(String id) {
        this.id = id;
    }

    public Set<Entity> getTypes() {
        return types;
    }

    @JsonDeserialize(as=LinkedHashSet.class)
    public void setTypes(Set<Entity> types) {
        this.types = types;
    }

    @Override
    public boolean equals(Object o){
        if (o == null || !(o instanceof Entity)){
            return false;
        }
        return this.getId().equals(((Entity)o).getId());
    }
}

I have this Java Spring endpoint where I pass the input in the body of a POST request:

@RequestMapping(value = "api/entity", method = RequestMethod.POST)
public Entity createEntity(@RequestBody final Entity in) {
    Set<Entity> types = in.getTypes();
    [...]
}

I would like in:

Set<Entity> types = in.getTypes();

to have only two entries in the correct order... since one of them is a duplicate based on the id... Instead I get duplicates in the LinkedHashSet (!)

I thought from the code I have that removing duplicates would work automatically, but apparently it is not.

This question has a broader context than Why do I need to override the equals and hashCode methods in Java? since it is using implicit Jackson serialization through Java Spring.

Thanks! it seems I am missing that - let me add it and see if it works — Michail Michailidis, Dec 31 '16 at 08:18
@GurwinderSingh It worked!! You can add that as an answer and I will accept it if you want :) — Michail Michailidis, Dec 31 '16 at 08:22
for this simple scenario and since I might have different classes with the same id I used: ```@Override public int hashCode() { return ("entity-"+this.getId()).hashCode(); }``` — Michail Michailidis, Dec 31 '16 at 08:37
Possible duplicate of [Why do I need to override the equals and hashCode methods in Java?](http://stackoverflow.com/questions/2265503/why-do-i-need-to-override-the-equals-and-hashcode-methods-in-java) — Raedwald, Dec 31 '16 at 08:50

score 6 · Accepted Answer · edited May 23 '17 at 11:33

Only overriding equals method will not work because hash-based collections use both the equals and hashCode method to see if two objects are the same. You'll need to override hashCode() method in Entity class as both hashCode() and equals() method are required to be properly implemented to work with Hash based collections.

If your requirement is that if the some or all of the fields of two objects of the Entity class are same, then the two objects are to be considered equivalent, in that case, you'll have to override both equals() and hashCode() method.

For e.g. - if only id field in the Entity class is required to determine if the two objects are equal, then you'll override equals(), something like this:

@Override
public boolean equals(Object o) {
    if (this == o)
        return true;
    if (o instanceof Entity){
        Entity that = (Entity) o;
        return this.id == null ? that.id == null : this.id.equals(that.id);
    }
    return false;

}

but along with it, the hashCode() method need to be overridden in a way to produce same hashcode if id has the same value, maybe something like this:

@Override
public int hashCode() {
    int h = 17;
    h = h * 31 + id == null ? 0 : id.hashCode();
    return h;
}

Only now it'll work properly with Hash based collections, as both these methods are used to identify an object uniquely.

More on this:

score 3 · Answer 2 · edited Dec 31 '16 at 08:31

3

Assuming that if the members of Entity class i.e. the id and type are same then the object of class Entity is same is completely wrong unless and until you override the hashcode() and equals() function explicitly.

If you do not override the hashCode() and equals() function in your Entity class then the two objects will be different even though they have same data within their members.

edited Dec 31 '16 at 08:31

Michail Michailidis

11,792
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answered Dec 31 '16 at 08:29

user2004685

9,548
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1

well I forgot ```hashCode()``` - I thought that hash based collections were using ```equals()``` to determine if it is the same object.. – Michail Michailidis Dec 31 '16 at 08:31
1

@MichailMichailidis You should always consider overriding both `hashCode()` as well as `equals()` and not just one. I hope this resolves your problem :) – user2004685 Dec 31 '16 at 08:32
1

yeap I figured it out based on Gurwinder's answer that came a bit earlier :) Thanks – Michail Michailidis Dec 31 '16 at 08:33
1

@MichailMichailidis Cheers! :) – user2004685 Dec 31 '16 at 08:34

score 3 · Answer 3 · edited Nov 09 '19 at 09:36

In Java, objects equality is determined by overriding the equals() and hashcode() contract.

There are default implementations of equals() and hashCode() in Object. If you don't provide your own implementation, those will be used. For equals(), this means an == comparison: the objects will only be equal if they are exactly the same object.

Answer to your question : Objects in LinkedHashSet are inheriting default behaviour of equals() and hashcode() methods from Object class. Override equals() and hashcode() of Entity class of LinkedHashSet

See below this for default behaviour of hashcode() and equals().

Deserialization of JavaScript array to Java LinkedHashSet using Jackson and Spring doesn't remove duplicates

3 Answers3