Which Set implementation in Collectors.toSet()? [duplicate]

Question

I have the following code:

Stream.of(1, 4, 5).collect(Collectors.toSet());

From Javadoc of toSet() method one can read:

There are no guarantees on the type (...) of the Set returned

I took a look at the actual implementation of toSet() method and at a first sight it looks like HashSet is always returned (at least in JDK 11).

I know that implementation can change in the future without violation of the contract but is there currently any situation when different implementation than HashSet is returned?

Answer 1

As of JDK 11, the type of Set returned by Collectors.toSet() is a HashSet. This can, of course, change in future versions of Java, and you should therefore not rely on this.

If you wish to use a specific type of Set, you should use Collectors.toCollection instead, and provide a lambda to create a collection of your choosing.

Answer 2

Use Collectors.toCollection for different Set implementation:

// Accumulate names into a TreeSet
Set<String> set = people.stream().map(Person::getName).collect(Collectors.toCollection(TreeSet::new));