Object-Oriented Programming (OOP) has been a mainstream programming paradigm since about 40 years now. That’s quite a bit of time. So it’s worth asking: how did the paradigm evolve over time? I would say, looking back, that there has been 3 eras.
Era 1: Modelling the world with objects (1980-1995)
The idea with the object-oriented paradigm is to model the world in objects. The poster child of the object paradigm from this era is Smalltalk, where everything is an object. Objects send messages to each other and live in a parmanent, persisted state. This approach is great to model transient domain, e.g. GUI components. The problem with this approach is that it doesn’t work very well for busines entities. In Smalltalk everything is an object and everyting is persistent. In other language, regular programs are started and stopped. You only want to persist the business entites. Persisting a subset of the objects is possible with object databases. This is a challenging problem though, similar to the serialization of object graphs. Searching and navigating heaps of objects is also no so easy. For these reasons, business applications have frequently relied on relational database to persist their state.
Era 2: Objects and enterprise applications (1995-2005)
Using relational database to persist object graph leads to the so called “object-relational impedance mismatch“. It manifest itself in the difficulty to have a rich domain model that is persistent at the same time. The simplest approach to reduce the mismatch is to have a simpler domain model – just data structure – that can be persisted easily. But this in turns means that you move towards a more procedural style of programing again. This style of programming is well capture in the pattern “anemic domain model” of Martin Fowler.
The Java Enterpise Plattform is a major technology of this era. The Java Enterprise Plattform embraced object-orientation with the concept of enterprise java beans (EJB). Prio to EJB3, entity beans and session beans where objects that were persisted by the application server itself, with a vague ressemblance to an object database. Every operations would be carried as a synchronous operation over the network. The technology proved however to be hard to use because of the associated network costs. Another mismatch. Starting with EJB3, entity beans were turned into regular objects persisted with an object-relational mapper.
Other approach to object-orientation exists. Domain-driven design promotes rich domain model in accordance with the concept of “modelling the world as objects”. To solve the object-relational mismatch, the domain model is kept separate from the model used for persistence. So called repositories take care of dealing with the mismatch.
The actor pardigm can be seen as a special form of object-orientation where objects communicate asynchronously. Actors are stateful domain entites. This avoids the problem of networking but doesn’t provide out of the box a solution for persistence. Some way to solve it is through event sourcing of object serialisation.
The heavy use of inheritance is also a characteristic of this era. Object-orientation promised reuse, which we thought meant inheritance. This missed the point. Reuse is promoted with good interfaces, which doesn’t stricly needs inheritance. With time, we learned to use interfaces, inheritance, composition and parametric polymorphism (aka generics) in a sane way.
With the previous learnings, the use of object orientation stabilized to a form mixing object-oriented data structures (think lists, data transfer objects, etc.) and object-oriented components (think of a business service, an HTTP server, or framework). This isn’t the revolution promised in the first era, but it makes good uses of objects and encapsulation to improve upon procedural programming.
Era 3: Objects and functional programming (2005-now)
Scala started exploring object-orientation and functional programming more in detail around 2005. Both paradigms might look contradictory at first (object-oriented programming is about mutability, functional programming about immutability). But both blend in actually quite well, at least the basics like list transformations (think map and flatMap). This isn’t actually a big suprise, given that lambda has been there from the beginning in Smalltalk, it’s just that Java didn’t have them initially.
This exploration continued with Kotlin and with Java itself. Java finally added lambda to the language and there are many more explorations going in with incubating projects. For instance pattern matching and OO play along quite well too. Developpers found an appropriate balance between immutable constructs and mutable ones.
What we have now, is what we can call “FP in the small, OO in the large”. Objects shine at encaspulating whole components or services. The object-oriented data structures that are used internally don’t need necessary to be mutable, though. They can be transformed and manipulated using idioms from functional programming.
It’s I think we’re we stand now, and where we will stay for a few more years until we’ve fully explored this space.