Java: Arrays vs ArrayLists (and other Lists)

An array (something like int[]) is a built in type while ArrayList is a regular class part of the Java standard library.

When to use which?

Sometimes you must use an array. For example:

• An API method takes an array as argument or returns an array
• You need to work with primitives for performance reasons

Unless you have a specific reason to use an array (such as those mentioned above), use a List, such as an ArrayList.

Resizing

Once you've created an array, it can't be resized. You can for instance not append an element to the end of an array, or remove an element.

Most list types (including ArrayList) provide List.add and List.remove which allows it to grow and shrink.

Insertion

Both arrays and lists allow you to update the content at a specific index.

arr[5] = 17;
list.set(5, 17);

Lists however, also allows you to insert an element in the middle, shifting all subsequent elements from index i to i + 1.

list.add(5, 17);  // insert 17 at index 5

Primitives

An arrray can store primitive values. Lists can not. You can have a List of integers, but you'll have to use List<Integer>. List<int> doesn't work. (Support for this is under way and might be available in something like Java 13. See JEP 218: Generics over Primitive Types.)

Generic elements

You can't create an array of generic elements. This won't compile:

// Error: generic array creation
Set<String>[] sets = new Set<String>[3];

This however works fine:

List<Set<String>> sets = new ArrayList<Set<String>>();

There is however a simple workaround for the array case. See Java: Generic array creation.

Immutability

There's no way to make the elements of an array "final". If you write for instance

final int[] arr = { 1, 2, 3 };

Something like arr[1] = 77 is still allowed.

Element immutability can be achieved for lists by doing:

List<Integer> list = Collections.unmodifiableList(Arrays.asList(1, 2, 3));

Covariance

A String[] is a subtype of Object[]. This allows us to write:

String[] strings = new String[1];
Object[] objects = strings;
objects[0] = new Object();

Although it throws an exception when executed!

A List<String> is however not a subtype of a List<Object>. This for example does not compile:

List<String> strings = new ArrayList<>();
List<Object> objects = strings;  // Error: Incompatible types
objects.add(new Object());

Put in fancy computer science terms: Arrays are covariant, while lists are not.

List methods

You can't use List methods on arrays… WRONG. There's in fact a bridge from the array world over to the List world. The bridge is called Arrays.asList. This method returns a List implementation backed by the provided array, which means that modifications to the list affect the original array.

Here's an example using List.replaceAll on an array:

String[] strs = { "a", "b", "c", "d" };
Arrays.asList(strs).replaceAll(s -> s + s);
// strs == { "aa", "bb", "cc", "dd" }

Note however, that since the backing array can't shrink or grow, operations such as List.add will throw UnsupportedOperationException.