How to determine the actual memory usage of a data structure

Question

In data structures, I know that the size of the structure depends on the internal links from one section to another. Is there a way, aside from JProfiler to tell exactly how much memory is tied up in a particular structure?

For example, the class project this semester has to do with applying various structures to a song database. The projects have covered arrays, lists, unrolled lists and trees. What I would like to do is see how much memory is used. For example, a linked list has a memory requirement of 3N, but I would like to see how much space a node is taking up in my project.

JProfiler looks like it would work, but $500 is out of my price range, and I would like to use it for all the structures covered this semester, not the three applied so far.

Answer 1

Use VisualVM. It comes with the JDK and allows you to take a snapshot of the heap.

Answer 2

Use System.getRuntime().freeMemory().

Run the garbage collector; Check the free memory; create one of your objects; check the free memory; subtract.

Answer 3

Most accurate way is probably to use a profiler - use Google to find free ones.

Alternatively, you can try the technique in Do you know your data size?.

Answer 4

I have honestly never used this before, but there is a getObjectSize method as part of Instrumentation interface which may be able to provide the answers you want without a third party application. It may not be easier then VisualVM, as Amir suggested, but it never hurts to read about it.