What is the difference between List.of and Arrays.asList?

Java 9 introduced new factory methods for lists, List.of:

List<String> strings = List.of("first", "second");

What's the difference between the previous and the new option? That is, what's the difference between this:

Arrays.asList(1, 2, 3);

and this:

List.of(1, 2, 3);

Arrays.asList returns a mutable list while the list returned by List.of is immutable:

List<Integer> list = Arrays.asList(1, 2, null);
list.set(1, 10); // OK

List<Integer> list = List.of(1, 2, 3);
list.set(1, 10); // Fails with UnsupportedOperationException

Arrays.asList allows null elements while List.of doesn't:

List<Integer> list = Arrays.asList(1, 2, null); // OK
List<Integer> list = List.of(1, 2, null); // Fails with NullPointerException

contains behaves differently with nulls:

List<Integer> list = Arrays.asList(1, 2, 3);
list.contains(null); // Returns false

List<Integer> list = List.of(1, 2, 3);
list.contains(null); // Fails with NullPointerException

Arrays.asList returns a view of the passed array, so the changes to the array will be reflected in the list too. For List.of this is not true:

Integer[] array = {1,2,3};
List<Integer> list = Arrays.asList(array);
array[1] = 10;
System.out.println(list); // Prints [1, 10, 3]

Integer[] array = {1,2,3};
List<Integer> list = List.of(array);
array[1] = 10;
System.out.println(list); // Prints [1, 2, 3]

#The differences between Arrays.asList and List.of

See the JavaDocs and this talk by Stuart Marks (or previous versions of it).

I'll be using the following for the code examples:

List<Integer> listOf = List.of(...);
List<Integer> asList = Arrays.asList(...);
List<Integer> unmodif = Collections.unmodifiableList(asList);

##Structural immutability (Or: unmodifiability)

Any attempt to structurally change List.of will result in an UnsupportedOperationException. That includes operations such as add, set and remove. You can, however, change the contents of the objects in the list (if the objects are not immutable), so the list is not "completely immutable".

This is the same fate for unmodifiable lists created with Collections.unmodifiableList. Only this list is a view of the original list, so it can change if you change the original list.

Arrays.asList is not completely immutable, it does not have a restriction on set.

listOf.set(1, "a");  // UnsupportedOperationException
unmodif.set(1, "a"); // UnsupportedOperationException
asList.set(1, "a");  // modified unmodif! unmodif is not truly unmodifiable

Similarly, changing the backing array (if you hold it) will change the list.

Structural immutability comes with many side-effects related to defensive coding, concurrency and security which are beyond the scope of this answer.

##Null hostility

List.of and any collection since Java 1.5 do not allow null as an element. Attempting to pass null as an element or even a lookup will result in a NullPointerException.

Since Arrays.asList is a collection from 1.2 (the Collections Framework), it allows nulls.

listOf.contains(null);  // NullPointerException
unmodif.contains(null); // allowed
asList.contains(null);  // allowed

##Serialized form

Since List.of has been introduced in Java 9 and the lists created by this method have their own (binary) serialized form, they cannot be deserialized on earlier JDK versions (no binary compatibility). However, you can de/serialize with JSON, for example.

##Identity

Arrays.asList internally calls new ArrayList, which guarantees reference inequality.

List.of depends on internal implementation. The instances returned can have reference equality, but since this is not guaranteed you can not rely on it.

asList1 == asList2; // false
listOf1 == listOf2; // true or false

Worth mentioning that lists are equal (via List.equals) if they contain the same elements in the same order, regardless of how they were created or what operations they support.

asList.equals(listOf); // true i.f.f. same elements in same order

##Implementation (warning: details can change over versions)

If the number of elements in the list of List.of is 2 or less, the elements are stored in fields of a specialized (internal) class. An example is the list that stores 2 elements (partial source):

static final class List2<E> extends AbstractImmutableList<E> {
    private final E e0;
    private final E e1;

    List2(E e0, E e1) {
        this.e0 = Objects.requireNonNull(e0);
        this.e1 = Objects.requireNonNull(e1);
    }
}

Otherwise they are stored in an array in a similar fashion to Arrays.asList.

###Time and Space efficiency

The List.of implementations which are field-based (size<2) perform slightly faster on some operations. As examples, size() can return a constant without fetching the array length, and contains(E e) does not require iteration overhead.

Constructing an unmodifiable list via List.of is also faster. Compare the above constructor with 2 reference assignments (and even the one for arbitrary amount of elements) to

Collections.unmodifiableList(Arrays.asList(...));

which creates 2 lists plus other overhead. In terms of space, you save the UnmodifiableList wrapper plus some pennies. Ultimately, the savings in the HashSet equivalent are more convincing.

---

Conclusion time: use List.of when you want a list that doesn't change and Arrays.asList when you want a list that can change (as shown above).

Let summarize the differences between List.of and Arrays.asList

1. List.of can be best used when data set is less and unchanged, while Arrays.asList can be used best in case of large and dynamic data set.

2. List.of take very less overhead space because it has field-based implementation and consume less heap space, both in terms of fixed overhead and on a per-element basis. while Arrays.asList take more overhead space because while initialization it creates more objects in heap.

3. Collection returned by List.of is immutable and hence thread-safe while Collection returned by Arrays.asList is mutable and not thread safe. (Immutable collection instances generally consume much less memory than their mutable counterparts.)

4. List.of doesn't allow null elements while Arrays.asList allows null elements.

Apart from the above answers there are certain operations on which both List::of and Arrays::asList differ:

+----------------------+---------------+----------+----------------+---------------------+
|      Operations      | SINGLETONLIST | LIST::OF | ARRAYS::ASLIST | JAVA.UTIL.ARRAYLIST |
+----------------------+---------------+----------+----------------+---------------------+
|          add         |       ❌      |     ❌  |        ❌      |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
|        addAll        |       ❌      |     ❌  |        ❌      |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
|         clear        |       ❌      |     ❌  |        ❌      |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
|        remove        |       ❌      |     ❌  |        ❌      |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
|       removeAll      |       ❗️       |     ❌   |        ❗️       |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
|       retainAll      |       ❗️       |     ❌  |        ❗️        |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
|      replaceAll      |       ❌      |     ❌  |        ✔️       |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
|          set         |       ❌      |     ❌  |        ✔️       |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
|         sort         |       ✔️       |     ❌   |        ✔️      |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
|  remove on iterator  |       ❌      |     ❌  |        ❌      |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+
| set on list-iterator |       ❌      |     ❌  |        ✔️       |          ✔️          |
+----------------------+---------------+----------+----------------+---------------------+

1. ✔️ means the method is supported
2. ❌ means that calling this method will throw an UnsupportedOperationException
3. ❗️ means the method is supported only if the method's arguments do not cause a mutation, e.g. Collections.singletonList("foo").retainAll("foo") is OK but Collections.singletonList("foo").retainAll("bar")throws an UnsupportedOperationException

More about Collections::singletonList Vs. List::of

> Arrays.asList(1, 2, 3);

A fixed-sized List will be create:

public static void main(String[] args) {
        List<Integer> asList = Arrays.asList(1, 2, 3, 4, 5);
        asList.add(6);    // java.lang.UnsupportedOperationException
        asList.remove(0); // java.lang.UnsupportedOperationException
        asList.set(0, 0); // allowed
}

> List.of(1, 2, 3);

A Immutable(Java 9)/Unmodifiable(Java 11) List will be create:

public static void main(String[] args) {
    List<Integer> listOf = List.of(1, 2, 3, 4, 5);
    listOf.add(6);    // java.lang.UnsupportedOperationException
    listOf.remove(0); // java.lang.UnsupportedOperationException
    listOf.set(0, 0); // java.lang.UnsupportedOperationException
}