> The cost of each new field is rarely considered

Most developers, in Java and in most other languages, do not consider the cost of every field, but I can tell you that people who need micro-optimisations certainly do care, and in Java's standard library, a layout is very much a concern (except, as always, you want to optimise what really matters; there's no point in optimising something that is unlikely to be a hot spot in a real program). Sometimes, though, you want to intentionally spread out the layout to avoid cache line sharing when concurrency is involved. You will find such examples in the standard library, too.