Fair point! Thanks for the contribution.

One question would be though whether this == other is faster than other instanceof JsonNull. That is relevant because code would mostly work with the JsonNull.INSTANCE singleton. But caring about that might be premature optimization.

Well, this == other would only be faster when comparing against JsonNull.INSTANCE, right? Whereas I think you might just as often be comparing against instances of other JsonElement classes. Anyway I found only 3 production classes in all of Google's source that did .equals(JsonNull.INSTANCE). Hundreds are doing .isJsonNull(), which sidesteps the question. (To be fair, I also didn't find many Set<JsonElement> or Map<JsonElement, cases either.)

On the whole the performance difference is likely to be absolutely negligible so we might as well simplify.