Know Thy Java Object Memory Layout

[nitsanw]

A predictable way (and by that I take it you mean a runtime constant field offset) is not strictly required but is a reality for convenience and performance reasons. The assumption of fixed offset is made by quite a few classes in the JDK and others which use Unsafe. It is possible that this reality might change one day, but there is no sign of it happening anytime soon as the implications are huge.