Encapsulation is the ability we give our classes (and objects created from them) to hide the implementation details of their inner workings and control the consistency of the internal state and invariants.
It happens all the time that we expose some attribute in our class to the outside world in the form of a collection like the following:
This is actually pretty common and I think a bad practice, we tend to create private fields in our classes and as a reflex create setters and getters immediately without thinking about the consequences of exposing the state that way.
If we simply return the reference to the attribute of the object there is a lot of danger there because now others have a copy of the reference to that attribute that is supposed to be private and under the control of the owner object.
Imagine if some code that gets that list of emails from our class now tries to modify the content of that list by adding or removing some objects from it. How can we control that? what if there is some kind of limit on the number of emails a Contact can have? How can we ensure only valid emails are added?
What if, instead, we write the following:
Now users of our class will get a list of emails that cannot be modified. New Email objects cannot be added or removed (but still the objects in the list can be changed if the public methods in the Email class allow that 😉). You could also create a copy of the list of emails by instantiating a new ArrayList sending the list of emails as an argument to the constructor new ArrayList<>(this.emails) but I still think it is better to fail when trying to modify the list.
Probably it is even better to just expose public methods with just the exact functionality that we want to allow like the following:
With the last implementation, we are hiding more and then have even more control. For example, let's say a Contact can have thousands of Emails (I know, it is unlikely, but let's say that is possible), then we can even change internally the way we store Emails from a List to a Map and then be able to have better performance for some operations, like removing an Email.
Do you have suggestions on how to improve the information hiding here? please comment!