- Practically, Java would rather have to `return 3` when it detects a null child, effectively precomputing the penultimate level.

- Semantically, Java could no longer distinguish between an absent child and a child with no children.

Honestly, I have other variants that still don't use pooling but are less idiomatic; I find this exercise is begging the question hard. Any tools, however idiomatic, the language is giving you to reduce the effects of allocation seem to be off-limits for GCd languages. Whereas then e.g. C can just throw them all in a third-party pool library. And JIT languages are presumably allowed to fuse anything they want.