I have been learning Kotlin lately, and playing with delegates and reified functions. For those that don’t know yet, a delegate is an object-oriented design or programming pattern. As Grady Booch stated: “ is a way to make composition as powerful for reuse as inheritance”

Our code generator generated kotlin data classes to be used as POJOs/DTOs from jooq, but being as paranoid as possible with database normalization some business needs involving fields from more than a few distinct POJOs. We are not using nor will use lazy fetching nor collections / entity trees even on the near future — I could probably write an entry about why it is bad in the near future; so some boilerplate code was involved. To be completely honest, I was trying to write some reified code to try to make a wrapper/delegate pattern on the data classes, but I gave up before the third or fourth tunnel deep in the rabbit hole.

The main problem and advantage with data classes in Kotlin is that they are final. You cannot inherit from them. So you cannot override them, which is good, but in my case, just wanting to add some flavor to already existing objects was a pain:

What I have above is a scaffolding setup for users based on a tenant. Say my service is used by tenants whose users interact through their API with my platform. I have my User, the ExternalUser and the Tenants tables, which translate to the rough POJOs in kotlin (yes, data classes in kotlin generate those pesky getters, setters, equals and hashcode methods for you). Each of my tenants users have some credit (we all want to make money, yay!).

So what happens when I need to know the credit a given external user holds within a given tenant? Either I have to write a new POJO or delegate it (hint: same effort):

The above code, through pretty, won’t compile (d’oh!) but fortunately, we potentially can use delegates and reified inline functions to work around it:

A reified function is a function that will allow typed values based on where and how it is used. The compiler will guess the proper types and make its magic for us. I have implemented two functions, one accepts a block that returns any value T and other one accepts a value T. The good thing of them is that one can extend the functionality easily:

I have removed the value based delegate since you can use the functional now (much more readable!), and we added another type of users that have bonus credit (use your imagination here). For the sake of the reader I added a main function, so you can run this code whenever you want or in the online kotlin interpreter (just paste the code there and hit run).

So in my real case I called my decorated classes DTO (how original, isn’t it?) instead of using delegation, and I added some flavor to them because I needed to transform to a custom library JSON object that I could not modify.

What is the deal then and why did I write my own DTO as the first example instead? When writing the tests, I personally find easier to mock a given object and not all the delegation or delegate ones, but on the other hand, delegation allows you to skip that part of the tests (well, the delegated object will have its own tests, right?). I’m still playing around with Kotlin, so I have not yet made up my mind about which approach to use.

Performance wisely speaking, even having caches between your data layer and your service, you will need to query either the caches or the database for all the objects. While in the past minimizing data packets was a gain, I don’t think it is anymore, specially as I have stated that the databases are properly normalized and no redundant information sits there. Of course if you are using a document-based database you will probably have other issues and needs but surely you’ll have learnt something from this.

Originally published in Medium