Simplify Immutable Collections in Java: From Collections.unmodifiable to List.of & Set.of

Conventional Approach

Before Java 9, immutable collections were created by constructing a mutable collection and then wrapping it with Collections.unmodifiableSet or Collections.unmodifiableList:

// immutable Set
Set<String> set = new HashSet<>();
set.add("a");
set.add("b");
set.add("c");
set = Collections.unmodifiableSet(set);

// immutable List
List<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);
list = Collections.unmodifiableList(list);

Java 8 Style

Using the Stream API, the code can be shortened:

Set<String> set = Collections.unmodifiableSet(Stream.of("a", "b", "c").collect(toSet()));
List<Integer> list = Collections.unmodifiableList(Stream.of(1, 2, 3).collect(toList()));

Java 9 Style

Java 9 introduces factory methods that create immutable collections directly:

Set<String> set = Set.of("a", "b", "c");
List<Integer> list = List.of(1, 2, 3);

Complex collections such as maps are also supported:

Map<String, String> map = Map.of("a", "1", "b", "2", "c", "3");

Note that Map.of requires an even number of arguments, each pair representing a key and a value:

Map.of()
Map.of(k1, v1)
Map.of(k1, v1, k2, v2)
Map.of(k1, v1, k2, v2, k3, v3)
...

Difference from Arrays.asList

Key differences between List.of (introduced in Java 9) and Arrays.asList are: List.of creates an immutable collection, whereas Arrays.asList returns a mutable list backed by an array.

Both prohibit add and remove, but Arrays.asList allows set to modify elements, while List.of throws java.lang.UnsupportedOperationException. List.of does not permit null elements, while Arrays.asList accepts them.