CODEWITHSUNDEEP
javacomparedomain-model
The Archetypal Paul
The Archetypal Paul

Reputation: 41769

What to do for objects that are equal but not identical?

This is either a Java coding design question, or a domain modelling question, I'm not sure yet!

Background, simplified as I can't share all the detail:

The system I'm working on is deployed in multiple instances (e.g. at different customers, and at one customer there may be development, test, preprod, prod instances)

The system configuration is a list of ConfigArtefact<T>, where T indicates that it might be a database connection configuration, or a predefined-query, or....

ConfigArtefacts are named. The names are semantically meaningful/well-known (e.g there could be an object for "Console.translations.en", "Console.translations.fr") or "Application.Database.connection.credentials", "Reporting.Database.connection.credentials") and are distinct for each deployment- no two different ConfigArtefacts will have the same name for a given deployment

ConfigArtefacts have other attributes (e.g. for the database, username and password) depending on the type used for <T>. The value of the attributes could be different in different deployments of this system.

There's no natural ordering of artefacts, even ones of the same type <T>. Where some arbitrary ordering is needed, I use the name.

Goal:

I need to write something that compares the configuration of two deployments of this system and identify Artifacts that have been added, removed, or changed. In order to find the same artefact on each deployment, I need to compare by name only (I always know what type of artefact I'm working with). In order to say if they've changed, I need to compare by all other attributes.

So, two kinds of comparison. One can be modelled with equals/hashcode, but not the other. Which should use equals()? (I think the one by name, as then added and deleted are just set subtraction, using one of the many collection libraries).

Would that be the normal choice? And if so, is there a conventional name for the other ("full compare") one? I'm considering identicalTo() (so two objects are changed if one.identicalTo(two) is false)

Upvotes: 0

Views: 119

Answers (1)

Bohemian
Bohemian

Reputation: 425278

Your design is flawed - you have misused equals() by only comparing name.

If all attributes must be compared to know if the object has changed, then it is not true that objects with the same name are “equal”, because the use of the word “changed” implies there’s a difference, and if there’s a difference they’re not equal.

Finding something by using an identifier is different to two objects being equal if they have the same identifier.

Implement equals() and hashCode() using all attributes that matter for determining if an object is “different”.

To facilitate convenience and performance, populate a Map<String, ConfigArtefact<?>> for each environment using the name as the key.

Finding differences between 2 such maps is a fairly trivial O(n) task.

Upvotes: 1

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