Question

Let’s suppose I’m using a library for which I don’t know the source code. It has a method that returns a List, like so:

public List<SomeObj> getObjs() { ... }

I’m wondering if this is a good idea:

ArrayList<SomeObj> objs = (ArrayList<SomeObj>) getObjs();

If, for example, the concrete implementation of the List inside getObjs() is a LinkedList then wouldn’t there be some kind of type discrepancy?

Answer 1

No, it is not a good idea. You should always use the interface (List) to declare your list variable unless you for some reason need specific behaviour from ArrayList.
Also, if you do, you need to be really sure that the list returned is an ArrayList. In this case, the promised contract of getObjs() is only that the return type is some kind of List, so you shouldn’t assume anything else. Even if the List returned now would be ArrayList , there is nothing preventing the implementer of getObjs() to later change the type of List returned, which would then break your code.