Type List vs type ArrayList in Java

Almost always List is preferred over ArrayList because, for instance, List can be translated into a LinkedList without affecting the rest of the codebase.

If one used ArrayList instead of List, it's hard to change the ArrayList implementation into a LinkedList one because ArrayList specific methods have been used in the codebase that would also require restructuring.

You can read about the List implementations here.

You may start with an ArrayList, but soon after discover that another implementation is the more appropriate choice.

I am wondering if anyone uses (2)?

Yes. But rarely for a sound reason (IMO).

And people get burned because they used ArrayList when they should have used List:

• Utility methods like Collections.singletonList(...) or Arrays.asList(...) don't return an ArrayList.

• Methods in the List API don't guarantee to return a list of the same type.

For example of someone getting burned, in https://stackoverflow.com/a/1481123/139985 the poster had problems with "slicing" because ArrayList.sublist(...) doesn't return an ArrayList ... and he had designed his code to use ArrayList as the type of all of his list variables. He ended up "solving" the problem by copying the sublist into a new ArrayList.

The argument that you need to know how the List behaves is largely addressed by using the RandomAccess marker interface. Yes, it is a bit clunky, but the alternative is worse.

Also, how often does the situation actually require using (1) over (2) (i.e. where (2) wouldn't suffice..aside 'coding to interfaces' and best practices etc.)

The "how often" part of the question is objectively unanswerable.

(and can I please get an example)

Occasionally, the application may require that you use methods in the ArrayList API that are not in the List API. For example, ensureCapacity(int), trimToSize() or removeRange(int, int). (And the last one will only arise if you have created a subtype of ArrayList that declares the method to be public.)

That is the only sound reason for coding to the class rather than the interface, IMO.

(It is theoretically possible that you will get a slight improvement in performance ... under certain circumstances ... on some platforms ... but unless you really need that last 0.05%, it is not worth doing this. This is not a sound reason, IMO.)

You can’t write efficient code if you don’t know whether random access is efficient or not.

That is a valid point. However, Java provides better ways to deal with that; e.g.

public <T extends List & RandomAccess> void test(T list) {
    // do stuff
}

If you call that with a list that does not implement RandomAccess you will get a compilation error.

You could also test dynamically ... using instanceof ... if static typing is too awkward. And you could even write your code to use different algorithms (dynamically) depending on whether or not a list supported random access.

Note that ArrayList is not the only list class that implements RandomAccess. Others include CopyOnWriteList, Stack and Vector.

I've seen people make the same argument about Serializable (because List doesn't implement it) ... but the approach above solves this problem too. (To the extent that it is solvable at all using runtime types. An ArrayList will fail serialization if any element is not serializable.)

Finally, I'm not going to say "because its is good style". That "reason" is both a circular argument ("Why is it 'good style'?") and an appeal to an unstated (and probably non-existent!) higher authority ("Who says it is 'good style'?").

(I do think it is good style to program to the interface, but I'm not going to give that as a reason. It is better for you to understand the real reasons and come to the (IMO) correct conclusions for yourself. The correct conclusion may not always be the same ... depending on the context.)

When you write List, you actually tell, that your object implements List interface only, but you don't specify what class your object belongs to.

When you write ArrayList, you specify that your object class is a resizable-array.

So, the first version makes your code more flexible in future.

Look at Java docs:

Class ArrayList - Resizable-array implementation of the List interface.

Interface List - An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted.

Array - container object that holds a fixed number of values of a single type.

(3) Collection myCollection = new ArrayList<?>();

I am using this typically. And only if I need List methods, I will use List. Same with ArrayList. You always can switch to more "narrow" interface, but you can't switch to more "wide".

I think the people who use (2) don't know the Liskov substitution principle or the Dependency inversion principle. Or they really have to use ArrayList.

The only case that I am aware of where (2) can be better is when using GWT, because it reduces application footprint (not my idea, but the google web toolkit team says so). But for regular java running inside the JVM (1) is probably always better.

I would say that 1 is preferred, unless

• you are depending on the implementation of optional behavior* in ArrayList, in that case explicitly using ArrayList is more clear
• You will be using the ArrayList in a method call which requires ArrayList, possibly for optional behavior or performance characteristics

My guess is that in 99% of the cases you can get by with List, which is preferred.

• for instance removeAll, or add(null)

List interface have several different classes - ArrayList and LinkedList. LinkedList is used to create an indexed collections and ArrayList - to create sorted lists. So, you can use any of it in your arguments, but you can allow others developers who use your code, library, etc. to use different types of lists, not only which you use, so, in this method

ArrayList<Object> myMethod (ArrayList<Object> input) {
   // body
}

you can use it only with ArrayList, not LinkedList, but you can allow to use any of List classes on other places where it method is using, it's just your choise, so using an interface can allow it:

List<Object> myMethod (List<Object> input) {
   // body
}

In this method arguments you can use any of List classes which you want to use:

List<Object> list = new ArrayList<Object> ();

list.add ("string");

myMethod (list);

CONCLUSION:

Use the interfaces everywhere when it possible, don't restrict you or others to use different methods which they want to use.