List.of() or Collections.emptyList()

Question

As an special case of List.of(...) or Collections.unmodifiableList() - what is the preferred Java 9 way of pointing to an empty and immutable list?

Keep writing

Collections.emptyList();

or switch to

List.of();

Answer 1

Collections.emptyList() does not need to create a new object for each call; it's typical, as in OpenJDK, to just return the singleton EMPTY_LIST object. Additionally, it's clearer that you intend to mean an empty list rather than having forgotten to fill in a placeholder.

Use emptyList(); it's both faster (up to Java target level 1.9) and more readable.

Answer 2

What is the preferred Java 9 way of pointing to an empty and immutable list?

The difference is rather subtle so "preferred" depends on what you want to achieve. Some behavioral differences:

• List.of will throw an exception on contains(null) invocations.
• You can deserialize emptyList() on JDK 8 and previous, but not List.of.

In terms or conveying that you want an empty list, emptyList() might look better, but this is just a temporary convention. If developers start using List.of() (which is much shorter than Collections.emptyList()) then it will become a known and accepted way, it's just new. If you think about it, there are some constructs we use which do not always convey what they do by themselves, but we got accustomed to them.

So there is no strictly preferred way. If the behavior does not matter use whatever you want.

Answer 3

1. emptyList() creates a new empty list instance only once
2. there is no difference on Readability: maybe List.of() is shorten than Collections.emptyList() but you can use a static import like import static java.util.Collections.emptyList; and then write only emptyList()