jdk8u-jdk

Note that for enumerable domains of less than 64, using a long plus bit ops has been a standard Set implementation for quite a while. For example, Java's EnumSet uses a long if the enum has less than 64 values: https://github.com/frohoff/jdk8u-jdk/blob/master/src/share/c... Where add() uses `|= bitmask`, remove() uses `&= ~bitmask`, and size() uses a count of the 1's in the long. Adding XOR as an efficient toggle would be interesting, but unnecessary to keep this O(n), if I understand correctly. It's just toggling the value, so (albeit with an extra branch), you could implement it as: if (!set.contains(val)) { set.add(val); } else { set.remove(val); }

Here's the code in question: https://github.com/frohoff/jdk8u-jdk/blob/master/src/share/c...

Maybe my memory is a little rusty or I glossed over a bit too much, but I was thinking of how hotspot does lambdas from here[0]. It seems to use the Invokedyanmic Bootstrap method to spin an InnerClass at runtime. To be fair, it's a hotspot thing and not in the JVM spec. [0]: https://github.com/frohoff/jdk8u-jdk/blob/master/src/share/c...