When to use a List over an Array in Java?

Question

In Java, when would it be preferential to use a List rather than an Array?

Answer 1

I see the question as being the opposite-

When should you use an Array over a List?

Only you have a specific reason to do so (eg: Project Constraints, Memory Concerns (not really a good reason), etc.)

Lists are much easier to use (imo), and have much more functionality.

Note: You should also consider whether or not something like a Set, or another datastructure is a better fit than a List for what you are trying to do.

Each datastructure, and implmentation, has different pros/cons. Pick the ones that excel at the things that you need to do.

If you need get() to be O(1) for any item? Likely use an ArrayList, Need O(1) insert()? Possibly a Linked List. Need O(1) contains()? Possibly a Hashset.

TLDR: Each data structure is good at some things, and bad at others. Look at your objectives and choose the data structure that best fits the given problem.

Edit:

One thing not noted is that you're better off declaring the variable as its interface (i.e. List or Queue) rather than its implementing class. This way, you can change the implementation at some later date without changing anything else in the code.

As an example:

List<String> myList = new ArrayList<String>(); 

vs

List<String> myList = new LinkedList<String>(); 

Note that myList is a List in both examples. --R. Bemrose

Answer 2

Always prefer lists.

Arrays when

1. Varargs for a method ( I guess you are forced to use Arrays here ).
2. When you want your collections to be covariant ( arrays of reference types are covariant ).
3. Performance critical code.

Answer 3

Most people have answered it already.

There are almost no good reason to use an array instead of List. The main exception being the primitive array (like int[]). You cannot create a primitive list (must have List<Integer>).

The most important difference is that when using List you can decide what implementation will be used. The most obvious is to chose LinkedList or ArrayList.

I would like to point out in this answer that choosing the implementation gives you very fine grained control over the data that is simply not available to array:

1. You can prevent client from modifying your list by wrapping your list in a Collection.unmodifiableList
2. You can synchronize a list for multithreading using Collection.synchronizedList
3. You can create a fixed length queue with implementation of LinkedBlockingQueue
4. ... etc

In any case, even if you don't want (now) any extra feature of the list. Just use an ArrayList and size it with the size of the array you would have created. It will use an Array in the back-end and the performance difference with a real array will be negligible. (except for primitive arrays)