Home/ Questions/Q 231969
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-11T19:58:39+00:00

when programming in Java I practically always, just out of habit, write something like

  • 0

when programming in Java I practically always, just out of habit, write something like this:

public List<String> foo() {
    return new ArrayList<String>();
}

Most of the time without even thinking about it. Now, the question is: should I always specify the interface as the return type? Or is it advisable to use the actual implementation of the interface, and if so, under what circumstances?

It is obvious that using the interface has a lot of advantages (that’s why it’s there). In most cases it doesn’t really matter what concrete implementation is used by a library function. But maybe there are cases where it does matter. For instance, if I know that I will primarily access the data in the list randomly, a LinkedList would be bad. But if my library function only returns the interface, I simply don’t know. To be on the safe side I might even need to copy the list explicitly over to an ArrayList:

List bar = foo();
List myList = bar instanceof LinkedList ? new ArrayList(bar) : bar;

but that just seems horrible and my coworkers would probably lynch me in the cafeteria. And rightfully so.

What do you guys think? What are your guidelines, when do you tend towards the abstract solution, and when do you reveal details of your implementation for potential performance gains?

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    For instance, if I know that I will
    primarily access the data in the list
    randomly, a LinkedList would be bad.
    But if my library function only
    returns the interface, I simply don’t
    know. To be on the safe side I might
    even need to copy the list explicitly
    over to an ArrayList.

    As everybody else has mentioned, you just mustn’t care about how the library has implemented the functionality, to reduce coupling and increasing maintainability of the library.

    If you, as a library client, can demonstrate that the implementation is performing badly for your use case, you can then contact the person in charge and discuss about the best path to follow (a new method for this case or just changing the implementation).

    That said, your example reeks of premature optimization.

    If the method is or can be critical, it might mention the implementation details in the documentation.

    • 0