Are there any use cases for List when Deques and Arrays are available?

Question

I've been thinking about this over the past few weeks, and I've come up with no good arguments. My perspective is from Java, but if anyone has any language-specific cases outside of this language, I'd love to hear them.

It seems to me that the benefit of using a List over a Deque comes from the fact that one can access the elements within directly via index numbers. While I can see the use in something like a UI (e.g. having a drag-and-drop sortable list), when talking about pure code interaction I see three cases for this:

1. Iteration. Iterating with a for loop using get(i) and size() is expensive in most implementations, and can usually be better done with iterator(), which is present in every Iterable collection.

2. Lookup by index. This usually requires a table of indices and a fixed length list, in which case one would get better performance out of an array.

3. Operating on the front or back of a List. This is what Deque was designed for, and it doesn't require any calls to size().

Collections support is nice, but the only thing I could find offhand that was implemented in Collections but not Arrays was a shuffle() function, which is fairly simple for an experienced programmer to implement (or delegate to Collections, since the overhead for non-primitive arrays isn't too bad IMO).

I feel that everything that one would need a List for can be better filled by either a Deque or an array. I've done some searching for comparisons, but the only info I've found either doesn't really discuss Deques or is written as a "Welcome to Programming" thing and doesn't offer a direct comparison of use cases. I've looked over my code for the past few years and haven't found any Lists outside of UI elements; I usually use a Set or a BlockingQueue for storing variable-length data.

Answer 1

A List is general and flexible.

Generalized structures are ideal for return values in business, web, and desktop applications. It's not optimal to return a Deque or an Array if your consumers will be converting to List in most cases anyway. It's potentially less efficient. And, it clutters your consumers' codebases with boilerplate conversions and/or utility libraries to deal with your "more efficient" interfaces.

To be clear: Some structures, like Deque's and Array's are certainly more efficient for particular use cases, but a List is sufficiently efficient for most use cases. List's may be over-used. But, in many domains, the performance hit of using a List over a more well-suited structure is negligible — not even worth a second thought.

Compound that with work-effort versus business value: Choosing a data structure that supports the precise interface I need and no more requires mental effort and time. And those are costs to the business. Sometimes there's a corresponding increase in value. But, in many applications there isn't — neither the business nor the customers will see any difference if you swap your List out for a more-optimized Deque when you're piping 50 rows from the database through your model to your view. (Or whatever.)

That said, there are use-cases that are well-served by List's.

The "need" for a List is more obvious in long-lived, stateful applications, where collections can stick around and be mutated over a long period of time. A text editor, for example, shouldn't automatically create a data[MAX_ROWS][MAX_COLS] array of arrays. And, you certainly could create an array of arrays with some default sizes and scaling behaviors and add methods for insertions and deletions. But, that's precisely what the List interface already does for you. (And the existing implementation already well-tested and optimized!)

My "2 cents": Unless your domain is small, very well-defined, or performance-critical, start with general collections and optimize later. The flexibility is immediate valuable in most business and web applications. Whereas the performance edge is negligible.

And don't forget about stateful applications, where a List interface is more often precisely what you'll end up reinventing if you've defaulted to an array or Deque...

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^{Examples of things where you might favor efficiency over flexibility might include things like system code (OS, driver, firmware), a VM, or other types of "platform" code — like a modern web browser's internal DOM implementation or JavaScript execution engine.}

Answer 2

Is there a reason to use java.util.List instead of using java.util.Deque?

Yes.

java.util.List has methods to access elements by position. java.util.Deque does not. If you are implementing an algorithm that needs to access elements by position, then Deque is not going to work. The contrapositive is true as well: If your algorithm only needs to look at the items in sequence (i.e., if you can use an Iterator), and if it only needs to mutate the sequence at the two ends (or one end, or not at all), then a Deque will work for you.

The guiding principle is pretty simple: Choose an interface that defines the methods that your application needs to call.

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Is java.util.List a well-designed inteface with a clear purpose?

IMO, No. It tries to be too many different things. If I want you to call my function that will randomly access the list elements, then I am not going to allow you to pass in just any List. I am going to make you pass in an ArrayList, or maybe, if we are collaborating on a project where it makes sense to have a lot of custom classes, we will define an interface that defines exactly those operations that my code is going to perform on your list.