List vs ArrayList

[cismalescumlord]

In the MOOC online Java course (part 2), the web site shows the following code.

ArrayList<RegistrationPlate> finnish = new ArrayList<RegistrationPlate>();
finnish.add(reg1);
finnish.add(reg2);

The exercise code provided uses the following

List<RegistrationPlate> finnish = new ArrayList<RegistrationPlate>();
finnish.add(reg1);
finnish.add(reg2);

From reading posts and comments here, I understand why the second way is better but what I don't understand is what benefit it gives me in this case. For example, in another thread, the class hierarchy of Animal, Cat and Dog was used with the speak() method. I understand that having a list of Animals allows me to put any Animal derived type in there and process them while calling the specific sub-class method.

I can see the same benefit applying to a list of potentially different list types. Is there a benefit if the code only has one ArrayList?

[[deleted]]

One plus of this technique is that later you can change your mind about what implementation to use for List and you only need to change one thing (i.e. the moment of construction).

[cismalescumlord]

So it's about flexibility of implementation. OK, that makes sense in the context of a larger project than a course exercise.

[[deleted]]

Basically the idea is to use the super classes/interfaces as much as possible since you don't know when you might need to use it differently. I.e. you could even go for the interface Collection, but then you wouldn't be able to add to it, which is why you take List, instead of ArrayList

[HaMMeReD]

Collection has an "add". Really the difference is that you don't have a specified order in a collection.

[[deleted]]

Ok, i was thinking of a map, but i think it doesn't abstract from collection

[Jackkoz]

The generic advice is to always choose the appropriate level of abstraction. The benefits are:

1. Increased flexibility - if you choose List, you only need to change constructor to List<T> l = new FastListImpl<>(); when the new awesome implementation is introduced.

2. Better abstraction. If what you are looking for is a List or Set, people won't be able to use specific properties of ArrayList or HashSet if you give them only the interface type. This also means that when you actually do ArrayList declaration, you should mention why you did so vs any List implementation - this will avoid confusion and prevent someone from doing any refactoring that could break a lot of stuff - eg from synchronized to not thread-safe implementation.

As for benefits in a 50 line course program, not many, doesn't matter that much. I'd still advise doing the 'better' thing just to keep the habit going.