There are various articles out there on the interwebs that try to empirically estimate the overhead of a java.lang.Object in particular JVM implementations. For example, I've seen the size overhead of a bare Object estimated at 8 bytes in some JVMs.
What I would like to know is whether a typical JVM implementation of the extends relationship introduces incremental size overhead at every level of the class hierarchy. In other words, suppose you have a class hierarchy with N levels of subclasses. Is the overhead of the in-memory representation of a class instance O(1) or O(N)?
I imagine it is O(1) because although the size of some of the hidden fluffy stuff you need to be a Java Object (vtable, chain of classes) will grow as the inheritance hierarchy grows, they grow per-class, not per-instance, and the JVM implementation can store constant-size pointers to these entities in a constant-size header attached to every Object.
So in theory, the overhead directly attached to the in-memory representation of any Java object should be O(1) for inheritance depth N. Does anyone know if it's true in practice?
When in doubt, look at the source (well, a source; each JVM is free to choose how to do it, as the standard does not mandate any internal representation). So I had a look, and found the following comment within the implementation of JDK 7-u60's hotspot JVM:
// A Klass is the part of the klassOop that provides:
// 1: language level class object (method dictionary etc.)
// 2: provide vm dispatch behavior for the object
// Both functions are combined into one C++ class. The toplevel class "Klass"
// implements purpose 1 whereas all subclasses provide extra virtual functions
// for purpose 2.
// One reason for the oop/klass dichotomy in the implementation is
// that we don't want a C++ vtbl pointer in every object. Thus,
// normal oops don't have any virtual functions. Instead, they
// forward all "virtual" functions to their klass, which does have
// a vtbl and does the C++ dispatch depending on the object's
The way I read it, this means that, for this (very popular) implementation, object instances only store a pointer to their class. The cost, per-instance of a class, of that class having a longer or shorter inheritance chain is effectively 0. The classes themselves do take up space in memory though (but only once per class). Run-time efficiency of deep inheritance chains is another matter.