overriding equals in persistant objects

dan_james · **Posted:** Wed Jan 07, 2004 5:48 pm

Hi,

I want to override the equals() method in my persistant objects; I am of the understanding that Hibernate uses the id attribute to check equality; so I am assuming that there will be no side-effects if I override equals() ?

many thanks

Dan

dimas · **Posted:** Wed Jan 07, 2004 8:09 pm

Hmm... I don't think sentence "Hibernate uses the id attribute to check equality" is correct at all. for example, if Hibernate uses object id to lookup it in the cache could you say it was "an equality check"? I doubt.

When you put an object in Set for example, object's equals() method is used to check for equality. And Hibernate's Set is just an extension to JDK's Set - so there is nothing about id.

Anyway, as long as you follow standard Java rules about equals()/hashCode(), it should be safe to override them. For some classes (like composite-id) you even MUST do it.

dan_james · **Posted:** Thu Jan 08, 2004 3:16 pm

OK,

I guess a reason why I ask would be usefull.......

I have a facade class which has two methods ......

void addProduct(Product product)
List listProducts(..);

I am writing a junit test which in the setUp method adds a product and in the 'testListProducts' method, among other things tests to see if that product is in the list. However, as we all know, we cannot use == and also there is a convenient method in List i.e. contains(..) .

So in order for my original product and my listed product to be equal (in the business sense and for the contains method) I want to override equals(..).

The reason I mention testing equality with the id attribute .... when I was looking at the source code for session.saveOrUpdate, if it wanted to decide if to update rather than save, it did an equality check using the id element declared in the Product.hbm.xml file (in my case).

After this post I went ahead and added the equals(..) and all seems OK so far, I will repost if I hit any problems...

many thanks
dan

christian · **Posted:** Thu Jan 08, 2004 3:21 pm

You should not use the database identifer value to test for equality. Implement equals()/hashcode() just like you would without Hibernate: By finding the best set of (unique) attributes in your entity. It doesn't have to be as good as a candidate key (really unique) attribute for a database table, but it definitely should be stable enough to use it for a short period in a Set.

For example: Most business classes often have a "created: Date" attribute. This ist just fine if you use it together with another attribute, for example a product name. It is unlikely that two products with the same name have been created in the same millisecond. Not good enough for a database, but good enough for the 30 seconds two products may be in the same Set on the client.

dimas · **Posted:** Fri Jan 09, 2004 7:55 am

JFYI in our project we ended up with ID-based equals/hashCode to avoid instantiation of lazy/proxy objects when they are get inserted into Set for example.

But like you we needed a way to compare objects property-by-property which was needed for Unit tests only. Our solution may... seem strange at least :) We made all our classes Comparable and their compareTo method performs property-by property comparison.

So when we need to check if object is really the same, we use

object.compareTo(another) == 0

Below an example of a class

Code:

public class Task implements Comparable
{
    private Long id;
    private String name;
    private Double estimatedCost;
    private Company company;

    /* Comparison */

    public int hashCode()
    {
        return (id == null) ? 0 : id.hashCode();
    }

    public boolean equals(Object object)
    {
        if (object == this)
            return true;
        else if ( !(object instanceof Task) )
            return false;

        Task other = (Task) object;

        return Util.equals(id, other.getId());
    }

    public int compareTo(Object object)
    {
        if (object == this)
            return 0;

        Task other = (Task) object;
        int result;

        result = Util.compare(id, other.getId());
        if (result != 0)
            return result;

        result = Util.compare(name, other.getName());
        if (result != 0)
            return result;

        result = Util.compare(estimatedCost, other.getEstimatedCost());
        if (result != 0)
            return result;

        result = Util.compareAssociation(company, other.getCompany());
        if (result != 0)
            return result;

        return result;
    }
...
}

christian · **Posted:** Fri Jan 09, 2004 7:58 am

I wouldn't call this a good practice. You should of course not include "associations" in equals(), thus preventing lazy loading issues. But you should never use the identifier: Transient objects don't have an identifier value.