Encapsulation is the core principle of object-oriented programming that makes objects solid, cohesive, trustworthy, etc. But what exactly is encapsulation? Does it only protect against access to private attributes from outside an object? I think it's much more. Encapsulation leads to the absence of naked data on all levels and in all forms.
This is what naked data is (C code):
t is the data, which is publicly accessible by the code around it. Anyone can modify it or read it.
Why is that bad? For one reason: tight and hidden coupling.
The code around
t inevitably makes a lot of assumptions about the data. For example, both lines after
int t decided that the temperature is in Fahrenheit. At the moment of writing, this may be true, but this assumption couples the code with the data. If tomorrow we change
t to Celsius, the code won't know about this change. That's why I call this coupling hidden.
If we change the type of
int to, say,
printf line won't print anything after the decimal point. Again, the coupling is there, but it's hidden. Later on, we simply won't be able to find all the places in our code where we made these or other assumptions about
This will seriously affect maintainability.
And this is not a solution, as you can imagine (Java now):
It looks like an object, but the data is still naked. Anyone can retrieve
t from the object and decide whether it's Fahrenheit or Celsius, whether it has digits after the dot or not, etc. This is not encapsulation yet!
The only way to encapsulate
t is to make sure nobody can touch it either directly or by retrieving it from an object. How do we do that? Just stop exposing data and start exposing functionality. Here is how, for example:
We don't allow anyone to retrieve
t anymore. All they can do is convert temperature to text. If and when we decide to change
t to Celsius, we will do it just once and in one place: in the class
If we need other functions in the future, like math operations or conversion to Celsius, we add more methods to class
Temperature. But we never let anyone touch or know about
This idea is close to "printers instead of getters," which we discussed earlier, though from a much wider perspective. Here I'm saying that any data elements that escape objects are naked and lead to maintainability problems.
The question is how we can work entirely without naked data, right? Eventually we have to let objects exchange data, don't we? Yes, that's true. But not entirely. I'll explain that in my next post.