Creating lists and sets with fixed elements

One-line initializations for Java lists and sets with given elements.

Initializing an array with fixed elements is straightforward:

new String[] {"a", "b"};

Given some context information, the compiler spares you from redundantly defining the type:

String[] array = {"a", "b"};

If you need lists or sets instead of arrays, you have the choice between several alternatives:

List.of("a", "b");
Stream.of("a", "b").toList();
Stream.of("a", "b").collect(Collectors.toList());
Stream.of("a", "b").collect(Collectors.toCollection(ArrayList::new));
Arrays.asList("a", "b");
		
Set.of("a", "b");
Stream.of("a", "b").collect(Collectors.toSet());
Stream.of("a", "b").collect(Collectors.toCollection(HashSet::new));

These alternatives behave differently with respect to null elements and mutability, as summarized in the following table:

Method	accepts null	modifiable
List.of(...)	no	no
Stream.of(...).toList()	not guaranteed	no
Stream.of(...).collect(Collectors.toList())	not guaranteed	not guaranteed
Stream.of(...).collect(Collectors.toCollection(...))	depends1	depends1
Arrays.asList(...)	yes	partly2
Set.of(...)	no	no
Stream.of(...).collect(Collectors.toSet());	not guaranteed	not guaranteed
Stream.of(...).collect(Collectors.toCollection(...))	depends1	depends1

Note #1: Depends on the provided factory method.

Note #2: Mutations that don’t change the list’s size are supported.