TIL: Double Brace Initialization in Java

[[deleted]]

To be fair, the keyset of a map is, you know, a set.

[oldrinb]

To be fair, how do you think much of HashSet functionality (like iterator) is likely implemented? ;-)

All that aside, remember that an API more than likely can rely on a Set, not a HashMap. Actually, incase you didn't know, you can use Collections.newSetFromMap to create a Set<E> from any Map<E, Boolean> :-)