20 years ago there were no generics, so you couldn’t have implemented it that way. You could have written a class OptionalLong { long value; boolean isSet; } at the time and that would have only a single allocation overhead. Alternatively, have an array of longs and a boolean array marking which ones are set, with a trivial wrapper object over that for essentially zero overhead.

Java’s tradeoffs are maintainability in huge teams over multiple years with relatively fast performance even if you write your code very naively, with top notch tooling, observability, etc. In the rare case you have to optimize in the hot loops you can allow to have less readable code like I mentioned.